grinding mills - common types

grinding mills - common types

In many industries the final product, or the raw material at somestage of the manufacturing process, is in powdered form and in consequence the rapid and cheap preparation of powdered materials is a matter of considerable economic importance.

In some cases the powdered material may be prepared directly; for example by precipitation from solution, a process which is used in the preparation of certain types of pigments and drugs, or by the vacuum drying of a fine spray of the material, a process which is widely adopted for the preparation of milk powder, soluble coffee extracts and similar products. Such processes are, however, of limited applicability and in by far the greatest number of industrial applications the powdered materials are prepared by the reduction, in some form of mill, of the grain size of the material having an initial size larger than that required in the final product. These processes for the reduction of the particle size of a granular material are known as milling or grinding and it appears that these names are used interchangeably, there being no accepted technical differentiation between the two.

Examples of the first two classes occur in mineral dressing, in which size reduction is used to liberate the desired ore from the gangue and also to reduce the ore to a form in which it presents a large surface to the leaching reagents.

Under the third heading may be classed many medicinal and pharmaceutical products, foodstuffs, fertilizers, insecticides, etc., and under the fourth heading falls the size reduction of mineral ores, etc.; these materials often being reduced to particles of moderate size for ease in handling, storing and loading into trucks and into the holds of ships.

The quantity of powder to be subjected to such processes of size reduction varies widely according to the industries involved, for example in the pharmaceutical industries the quantities involved per annum, can be measured in terms of a few tons, or in the case of certain drugs, possibly a few pounds; whereas in the cement industry the quantities involved run into tens of millions of tons; the British cement industry alone having produced, in round figures, 12 million tons of Portland Cement.

For the preparation of small quantities of powder many types of mill are available but, even so, the ball mill is frequently used. For the grinding of the largest quantities of material however, the ball, tube or rod mill is used almost exclusively, since these are the only types of mill which possess throughput capacity of the required magnitude.

The great range of sizes covered by industrial ball mills is well exemplified by Fig. 1.1 and Fig. 1.2. In the first illustration is shown a laboratory batch mill of about 1-litre capacity, whilst in Fig. 1.2 is shown a tube mill used in the cement industry the tube having a diameter of about 8 ft and length of about 45 ft.

In Fig. 1.3 is shown a large ball mill, designed for the dry grinding of limestone, dolomite, quartz, refractory and similar materials; this type of mill being made in a series of sizes having diameters ranging from about 26 in. to 108 in., with the corresponding lengths of drum ranging from about 15 in. to 55 in.

At this point it is perhaps of value to study the nomenclature used in connection with the mills under consideration, but it must be emphasized that the lines of demarcation between the types to which the names are applied are not very definite.

The term ball mill is usually applied to a mill in which the grinding media are bodies of spherical form (balls) and in which the length of the mill is of the same order as the diameter of the mill body; in rough figures the length is, say, one to three times the diameter of the mill.

The tube mill is a mill in which the grinding bodies are spherical but in which the length of the mill body is greater in proportion to the diameter than is the case of the ball mill; in fact the length to diameter ratio is often of the order of ten to one.

The rod mill is a mill in which the grinding bodies are circular rods instead of balls, and, in order to avoid tangling of the rods, the length to diameter ratio of such mills is usually within the range of about 15 to 1 and 5 to 1.

It will be noticed that the differentiation between ball mill and the tube mill arises only from the different length to diameter ratios involved, and not from any difference in fundamental principles. The rod mill, however, differs in principle in that the grinding bodies are rods instead of spheres whilst a pebble mill is a ball mill in which the grinding bodies are of natural stone or of ceramic material.

As the name implies, in the batch mills, Fig. 1.4a, the charge of powder to be ground is loaded into the mill in a batch and, after the grinding process is completed, is removed in a batch. Clearly such a mode of operation can only be applied to mills of small or moderate sizes; say to mills of up to about 7 ft diameter by about 7 ft long.

In the grate discharge mill, Fig. 1.4b, a diaphragm in the form of a grating confines the ball charge to one end of the mill and the space between the diaphragm and the other end of the mill houses a scoop for the removal of the ground material. The raw material is fed in through a hollow trunnion at the entrance end of the mill and during grinding traverses the ball charge; after which it passes through the grating and is picked up and removed by the discharge scoop or is discharged through peripheral ports. In this connection, it is relevant to mention that scoops are sometimes referred to as lifters in the literature. In the present work, the use of the term lifter will be confined to the description of a certain form of mill liner construction, fitted with lifter bars in order to promote the tumbling of the charge, which will be described in a later section.

In the trunnion overflow mill, Fig. 1.4c the raw material is fed in through a hollow trunnion at one end of the milland the ground product overflows at the other end. In this case, therefore, the grating and discharge scoop are eliminated.

A variant of the grate discharge mill is shown in Fig. 1.4d, in which the discharge scoop is eliminated by the provision of peripheral discharge ports, with a suitable dust hood, at the exit end of the mill.

Within the classes of mills enumerated above there are a number of variations; for example there occur in practice mills in which the shell is divided into a number of chambers by means of perforated diaphragms and it is arranged that the mean diameter of the balls in the various chambers shall decrease towards the discharge end of the mill; such an arrangement being shown in Fig. 1.6. The reason for this distribution of ball size is that, for optimum grinding conditions, the ratio of ball diameter to particle diameter should be approximately constant. In consequence smaller balls should be used for the later stages of the grinding process, where the powder is finer, and by the adoption of a number of chambers in each of which the mean ball diameter is suitably chosen an approximation is made towards the desired constancy in the ratio of the ball size to the particle size.

The problem of the optimum distribution of ball size within a mill will be dealt with more fully in a later chapter, but at this point it is relevant to mention a mill in which the segregation of the balls is brought about by an ingenious method; especially as the mill carries a distinctive name, even though no principles which place it outside the classification given previously are involved.

The Hardinge mill, Fig. 1.7, uses spheres as a grinding agent but the body is of cylindro-conical form and usually has a length to diameter ratio intermediate between those associated with the ball mill and the tube mill. The reason for this form of construction is that it is found that, during, the operation of the mill, the largest balls accumulate at the large end of the cone and the smallest balls at the small end; there being a continuous gradation of size along the cone. If then the raw material is fed in at the large end of the mill and the ground product removed at the smaller end, the powder in its progression through the mill is ground by progressively small balls and in consequence the theoretical ideal of a constant ratio between ball size and particle size during grinding is, to some extent, attained.

The type of ball mill illustrated in Fig. 1.3, incorporates a peripheral discharge through line screens lining the cylindrical part of the mill. Heavy perforated plates protect the screens from injury and act as a lining for the tumbling charge; sometimes also the fine screen is further protected by coarse screens mounted directly inside it. This type of mill, which is often known as the Krupp mill, is of interest since it represents a very early type of mill which, with modifications, has retained its popularity. The Krupp mill is particularly suited to the grinding of soft materials since the rate of wear of the perforated liners is then not excessive. At this point it will perhaps be useful to discussthe factors upon which the choice between a ball a tube or a rod mill depends.

When a mill is used as a batch mill, the capacity of the mill is clearly limited to the quantity which can be handled manually; furthermore the mill is, as far as useful work is concerned, idle during the time required for loading and unloading the machine: the load factor thus being adversely affected. Clearly then, there will be a considerable gain in throughput, a saving in handling costs and improved load factor, if the mill operation is made continuous by feeding the material into the mill through one trunnion and withdrawing it either through the other trunnion or through discharge ports at the exit end of the mill body.

Since, however, the flow of powder through the mill is now continuous, it is necessary that the mill body is of such a length that the powder is in the mill for a time sufficiently long for the grinding to be carried to the required degree of fineness. This, in general, demands a mill body of considerable length, or continuous circulation with a classifier, and it is increased length which gives rise to the tube mill.

In the metallurgical industries very large tonnages have to be handled and, furthermore, an excess of fine material is undesirable since it often complicates subsequent treatment processes. In such applications a single-stage tube mill in circuit with a product classifier, by means of which the material which has reached optimum fineness is removed for transport to the subsequent processing and the oversize is returned to the mill for further grinding, is an obvious solution. Once continuous feed and a long mill body have been accepted, however, the overall grinding efficiency of the mill may be improved by fairly simple modifications.

As has already been mentioned; for optimum grinding conditions there is a fairly definite ratio of ball size to particle size and so the most efficient grinding process cannot be attained when a product with a large size range is present in the mill. If, however, a tube mill is divided into a number of compartments and the mean ball size of the grinding media decreases in each succeeding compartment; then the optimum ratio between ball size and particle size is more nearly maintained, and a better overall performance of the mill is achieved; this giving rise to the compartment mill shown in Fig. 1.6. The tube mill has the further advantage that, to some extent, the grinding characteristics of the mill are under control; for example, an increase in the size of the balls in the final chamber will reduce the rate of grinding of the finer fractions but will leave the rate of grinding of the coarser fractions sensibly unchanged and so the amount of coarse material in the final product will be reduced without any excessive overall increase in fineness.

The principal field of application of the rod mill is probably as an intermediate stage between the crushing plant and the ball mills, in the metallurgical industries. Thus, material between about 1-in. and 2-in. size may be reduced to about 6 mesh for feeding to the ball mills. Rod mills are, however, being used in closed circuit with a classifier to produce a product of less than about 48-mesh size, but such applications are unusual.

ball mill: operating principles, components, uses, advantages and

ball mill: operating principles, components, uses, advantages and

A ball mill also known as pebble mill or tumbling mill is a milling machine that consists of a hallow cylinder containing balls; mounted on a metallic frame such that it can be rotated along its longitudinal axis. The balls which could be of different diameter occupy 30 50 % of the mill volume and its size depends on the feed and mill size. The large balls tend to break down the coarse feed materials and the smaller balls help to form fine product by reducing void spaces between the balls. Ball mills grind material by impact and attrition.

Several types of ball mills exist. They differ to an extent in their operating principle. They also differ in their maximum capacity of the milling vessel, ranging from 0.010 liters for planetary ball mills, mixer mills, or vibration ball mills to several 100 liters for horizontal rolling ball mills.

Im grateful for the information about using a ball mill for pharmaceutical products as it produces very fine powder. My friend is working for a pharmaceutical company and this is a good article to share with her. Its good to know that ball mills are suitable for milling toxic materials since they can be used in a completely enclosed for. Thanks for the tips!

milling process, defects, equipment

milling process, defects, equipment

Milling is the most common form of machining, a material removal process, which can create a variety of features on a part by cutting away the unwanted material. The milling process requires a milling machine, workpiece, fixture, and cutter. The workpiece is a piece of pre-shaped material that is secured to the fixture, which itself is attached to a platform inside the milling machine. The cutter is a cutting tool with sharp teeth that is also secured in the milling machine and rotates at high speeds. By feeding the workpiece into the rotating cutter, material is cut away from this workpiece in the form of small chips to create the desired shape. Milling is typically used to produce parts that are not axially symmetric and have many features, such as holes, slots, pockets, and even three dimensional surface contours. Parts that are fabricated completely through milling often include components that are used in limited quantities, perhaps for prototypes, such as custom designed fasteners or brackets. Another application of milling is the fabrication of tooling for other processes. For example, three-dimensional molds are typically milled. Milling is also commonly used as a secondary process to add or refine features on parts that were manufactured using a different process. Due to the high tolerances and surface finishes that milling can offer, it is ideal for adding precision features to a part whose basic shape has already been formed.

The time required to produce a given quantity of parts includes the initial setup time and the cycle time for each part. The setup time is composed of the time to setup the milling machine, plan the tool movements (whether performed manually or by machine), and install the fixture device into the milling machine. The cycle time can be divided into the following four times:

Following the milling process cycle, there is no post processing that is required. However, secondary processes may be used to improve the surface finish of the part if it is required. The scrap material, in the form of small material chips cut from the workpiece, is propelled away from the workpiece by the motion of the cutter and the spraying of lubricant. Therefore, no process cycle step is required to remove the scrap material, which can be collected and discarded after the production. Cutting parameters In milling, the speed and motion of the cutting tool is specified through several parameters. These parameters are selected for each operation based upon the workpiece material, tool material, tool size, and more.

During the process cycle, a variety of operations may be performed to the workpiece to yield the desired part shape. The following operations are each defined by the type of cutter used and the path of that cutter to remove material from the workpiece.

Milling machines can be found in a variety of sizes and designs, yet they still possess the same main components that enable the workpiece to be moved in three directions relative to the tool. These components include the following:

The above components of the milling machine can be oriented either vertically or horizontally, creating two very distinct forms of milling machine. A horizontal milling machine uses a cutter that is mounted on a horizontal shaft, called an arbor, above the workpiece. For this reason, horizontal milling is sometimes referred to as arbor milling. The arbor is supported on one side by an overarm, which is connected to the column, and on the other side by the spindle. The spindle is driven by a motor and therefore rotates the arbor. During milling, the cutter rotates along a horizontal axis and the side of the cutter removes material from the workpiece. A vertical milling machine, on the other hand, orients the cutter vertically. The cutter is secured inside a piece called a collet, which is then attached to the vertically oriented spindle. The spindle is located inside the milling head, which is attached to the column. The milling operations performed on a vertical milling machine remove material by using both the bottom and sides of the cutter. Milling machines can also be classified by the type of control that is used. A manual milling machine requires the operator to control the motion of the cutter during the milling operation. The operator adjusts the position of the cutter by using hand cranks that move the table, saddle, and knee. Milling machines are also able to be computer controlled, in which case they are referred to as a computer numerical control (CNC) milling machine. CNC milling machines move the workpiece and cutter based on commands that are preprogrammed and offer very high precision. The programs that are written are often called G-codes or NC-codes. Many CNC milling machines also contain another axis of motion besides the standard X-Y-Z motion. The angle of the spindle and cutter can be changed, allowing for even more complex shapes to be milled.

The tooling that is required for milling is a sharp cutter that will be rotated by the spindle. The cutter is a cylindrical tool with sharp teeth spaced around the exterior. The spaces between the teeth are called flutes and allow the material chips to move away from the workpiece. The teeth may be straight along the side of the cutter, but are more commonly arranged in a helix. The helix angle reduces the load on the teeth by distributing the forces. Also, the number of teeth on a cutter varies. A larger number of teeth will provide a better surface finish. The cutters that can be used for milling operations are highly diverse, thus allowing for the formation of a variety of features. While these cutters differ greatly in diameter, length, and by the shape of the cut they will form, they also differ based upon their orientation, whether they will be used horizontally or vertically. A cutter that will be used in a horizontal milling machine will have the teeth extend along the entire length of the tool. The interior of the tool will be hollow so that it can be mounted onto the arbor. With this basic form, there are still many different types of cutters that can be used in horizontal milling, including those listed below.

Another operation known as a straddle milling is also possible with a horizontal milling machine. This form of milling refers to the use of multiple cutters attached to the arbor and used simultaneously. Straddle milling can be used to form a complex feature with a single cut. For vertical milling machines, the cutters take a very different form. The cutter teeth cover only a portion of the tool, while the remaining length is a smooth surface, called the shank. The shank is the section of the cutter that is secured inside the collet, for attachment to the spindle. Also, many vertical cutters are designed to cut using both the sides and the bottom of the cutter. Listed below are several common vertical cutters.

All cutters that are used in milling can be found in a variety of materials, which will determine the cutter's properties and the workpiece materials for which it is best suited. These properties include the cutter's hardness, toughness, and resistance to wear. The most common cutter materials that are used include the following:

The material of the cutter is chosen based upon a number of factors, including the material of the workpiece, cost, and tool life. Tool life is an important characteristic that is considered when selecting a cutter, as it greatly affects the manufacturing costs. A short tool life will not only require additional tools to be purchased, but will also require time to change the tool each time it becomes too worn. The cutters listed above often have the teeth coated with a different material to provide additional wear resistance, thus extending the life of the tool. Tool wear can also be reduced by spraying a lubricant and/or coolant on the cutter and workpiece during milling. This fluid is used to reduce the temperature of the cutter, which can get quite hot during milling, and reduce the friction at the interface between the cutter and the workpiece, thus increasing the tool life. Also, by spraying a fluid during milling, higher feed rates can be used, the surface finish can be improved, and the material chips can be pushed away. Typical cutting fluids include mineral, synthetic, and water soluble oils.

In milling, the raw form of the material is a piece of stock from which the workpieces are cut. This stock is available in a variety of shapes such as flat sheets, solid bars (rectangular, cylindrical, hexagonal, etc.), hollow tubes (rectangular, cylindrical, etc.), and shaped beams (I-beams, L-beams, T-beams, etc.). Custom extrusions or existing parts such as castings or forgings are also sometimes used.

When selecting a material, several factors must be considered, including the cost, strength, resistance to wear, and machinability. The machinability of a material is difficult to quantify, but can be said to posses the following characteristics:

The material cost is determined by the quantity of material stock that is required and the unit price of that stock. The amount of stock is determined by the workpiece size, stock size, method of cutting the stock, and the production quantity. The unit price of the material stock is affected by the material and the workpiece shape. Also, any cost attributed to cutting the workpieces from the stock also contributes to the total material cost.

The production cost is a result of the total production time and the hourly rate. The production time includes the setup time, load time, cut time, idle time, and tool replacement time. Decreasing any of these time components will reduce cost. The setup time and load time are dependent upon the skill of the operator. The cut time, however, is dependent upon many factors that affect the cut length and feed rate. The cut length can be shortened by optimizing the number of operations that are required and reducing the feature size if possible. The feed rate is affected by the operation type, workpiece material, tool material, tool size, and various cutting parameters such as the axial depth of cut. Lastly, the tool replacement time is a direct result of the number of tool replacements which is discussed regarding the tooling cost.

The tooling cost for machining is determined by the total number of cutting tools required and the unit price for each tool. The quantity of tools depends upon the number of unique tools required by the various operations to be performed and the amount of wear that each of those tools experience. If the tool wear exceeds the lifetime of a tool, then a replacement tool must be purchased. The lifetime of a tool is dependant upon the tool material, cutting parameters such as cutting speed, and the total cut time. The unit price of a tool is affected by the tool type, size, and material.

microstructure and mechanical properties of aluminium-graphene composite powders produced by mechanical milling | mechanics of advanced materials and modern processes | full text

microstructure and mechanical properties of aluminium-graphene composite powders produced by mechanical milling | mechanics of advanced materials and modern processes | full text

The SEM observation shows that aluminium particles are firstly flattened into flakes, and then fractured/ rewelded into equiaxed particles as the ball milling progresses. The crystalline size is decreased and the lattice strain is increased during the ball milling, which are also intensified by the added GNSs. The hardness of the composite is increased by 115.1% with the incorporation of 1.0 vol. % GNSs.

The local stress induced by the hard GNSs accelerates the milling process. The X-Ray diffraction patterns show that the intensity ratio of (111) to (200) can reflect the preferred orientation of the particle mixture, and the evolution of I(111)/I(200) agrees well with the observed results using SEM. The increased hardness is mainly attributed to the refined microstructure and Orowan strengthening.

Aluminium matrix composite (AMC) has found wide application in the fields of aerospace, automobile, military, transportation and building, due to its attractive properties such as light weight, corrosion resistance and superior ductility (Bodunrin et al. 2015). Graphene is a very promising reinforcing phase in AMC because of its outstanding properties, including high mechanical strength, modulus, thermal and electrical conductivity (Stankovich et al. 2006; Novoselov et al. 2012; Zhu et al. 2010; Niteesh Kumar et al. 2017; Shin et al. 2015). Bartolucci et al. (2011) are among the pioneer researchers and introduced graphene into AMCs using ball milling in 2011. Graphene is normally added into the matrix in the form of graphene nanosheets (GNSs) with several to tens of layers (Asgharzadeh and Sedigh 2017; Prez-Bustamante et al. 2014; Nieto et al. 2016). Up to 5wt.% GNSs were incorporated into AA2124 alloy, and it was found that the hardness of the composite was increased by 102%; the wear rate decreased 25% with 9% reduction in coefficient of friction (El-Ghazaly et al. 2017). A wet method was utilised to mix aluminium with graphene in the study of Asgharzadeh et al. (2017), which showed that the yield strength and hardness were both enhanced. The possible strengthening mechanism for the GNSs reinforced AMCs were reported to be grain refinement, Orowan strengthening, stress/load transfer and increased dislocation density (Nieto et al. 2016). The strengthening effect of GNSs also highly depends on the uniform dispersion of GNSs among the metal grains. Mechanical milling involves the cold welding, fracturing and rewelding of particles, which is an effect way to uniformly disperse GNSs into aluminium matrix (Nieto et al. 2016; Hu et al. 2016; Suryanarayana and Al-Aqeeli 2013). In the literature, it is noted that Al-Si alloy is widely used in the fields of aerospace and automobile due to its high specific strength, good corrosion resistance and castability (Mazahery and Shabani 2012). However, this alloy is restricted in certain tribological applications owing to the low hardness and wear-resistance. GNSs are in the right place to improve the hardness and tribological behaviour, as GNSs are potential to boost the mechanical properties and are also tribology-favoured (Nieto et al. 2016).

The research on GNSs reinforced Al-Si alloy is still quite limited. The current study focuses on the synthesis and characterisation of GNSs reinforced A355 Al-Si alloy matrix composites. The effect of GNSs on the morphological and microstructural evolution of the composite powder has been investigated during the mechanical milling. The preferred orientation, lattice strain, crystalline size and micro hardness have been studied as well.

The morphologies of the starting materials are shown in Fig.1. The as-received aluminium powder is generally in spherical shape. Commercial A355 Al-Si alloy (Si: 4.6%, Cu: 0.8%, Mg: 0.51% and Fe: 0.15%) powder with an average particle size of 30m was supplied by Haoxi Nanotechnology. The GNSs are characterised with 15nm in thickness and ~5.0m in lateral size, which were bought from XFNANO Materials Tech. as shown in Fig.1(b) and (c).

The SEM morphologies of the as-received (a) aluminium powder. The morphology of the as-received GNSs observed using (b) SEM and (c) TEM. The inset shows a high-resolution micrograph of the lattice of the GNSs, with indicated number of layers

The powder mixture of Al alloy and 1.0 vol.% GNSs was milled in a planetary ball mill, which was carried out in a 500ml stainless steel jar. The confined powders were firstly ball milled at 180rpm for 0.5h for pre-mixing, and then at 250rpm for the following 20h under argon atmosphere. Samples were taken out at 2, 5, 10, 15 and 20h to investigate the effect of ball milling on the microstructure of the powder mixture. In a typical milling campaign, 300g of 5mm stainless steel balls was used with a ball to powder ratio of 15:1 in mass. Stearic acid (2wt.%) flakes were added to work as a process control agent. To avoid the overheating and sticking of the powder mixture, every 5min ball milling was followed with15 min rest in every milling cycle. Pure A355 powder was ball milled under the same conditions for reference.

The ball milled powders at different times were observed on the JEOL JSM-7500FA microscope with an acceleration voltage of 5kV. X-Ray diffraction (XRD) patterns were acquired using GBC MMA XRD diffractometer with Cu-K radiation from 25 to 85. The step size and scanning rate were 0.02 and 1.5 /min respectively. The crystalline size and lattice strain were analysed using William-Hall theory as follows (Wagih 2014):

The ball milled powders were cold pressed at 350MPa and then vacuum hot pressed under 50MPa at 500C for 60min to produce 20mm disks. The disks were degasified to remove the stearic acid at 400C for 2h before the hot pressing. The produced disks were grinded using abrasive papers and polished before the following characterisation. Vickers hardness was measured on a TIME TH715 micro-hardness tester under 9.8N with a dwell time of 10s. At least ten readings were taken for each sample to obtain the average value. Raman tests were conducted on a WITec alpha 300R confocal Raman microscope (532nm laser) to examine the distribution of GNSs. TEM samples were prepared using a FEI Helios nanoLab G3 CX dual beam microscope and then observed on a JEOL JEM-2011microscope.

The SEM micrographs of the ball milled Al alloy and Al-GNSs composite powders are shown in Figs.2 and 3 respectively. Aluminium is a relatively ductile phase in the ball milling system, while Si and GNSs particles are relatively brittle. The ductile aluminium particles are repeatedly flattened, cold welded, fractured and rewelded in the ball milling process (Suryanarayana and Al-Aqeeli 2013). As shown in Fig.2, the starting aluminium particles are in spherical shape with about 30m in diameter and gradually flatten into flakes from 2 to 10h. The lateral size of the aluminium flakes reaches around 80m at 10h. The flakes are fractured into smaller pieces at 15h as shown in Fig. 2(e) and rewelded into equiaxed particles at 20h. The relatively hard phase, Si, could accelerate the fracturing and rewelding, and is embedded into the aluminium matrix (Suryanarayana and Al-Aqeeli 2013). The presence of GNSs can further intensify the localised stress, and thus accelerates the flattening of aluminium powders as shown in Fig.3 (a) to (d). Aluminium flakes with more than 120m in lateral size can been seen at 10h. When the plastically deformed aluminium flakes are work-hardened to a critical level, the localized stress induced by GNSs will promote the fracture and rewelding of powders. As shown in Fig. 3 (e) to (f), aluminium powders are fractured and rewelded into relatively equiaxed shape at 15h and further fractured into smaller particles at 20h. As a result, the size of the Al-GNSs mixture is less than 20m, which is smaller than the size of the Al alloy powder (around 25m) after 20h of ball milling. The GNSs tend to become occluded and trapped in the aluminium particles, and finally get uniformly dispersed inside the matrix (Suryanarayana and Al-Aqeeli 2013).

The XRD patterns of the ball milled Al alloy and Al-GNSs powder mixtures at different milling times are shown in Figs.4 and 5 respectively, revealing the microstructural evolution of the powder mixing during the ball milling. It is seen that the peak intensity of aluminium decreases with the increase of milling times up to 20h. There is no obvious change for the peaks of Si, indicating no significant structural change for this relatively hard phase. As the concentration of GNSs is only 1.0 vol.%, the peak of GNSs is not distinguishable in XRD observation. For FCC metals, it has been indicated that the intensity ratio of (111) to (200), I(111)/ I(200), can reflect the change in crystallographic orientation of particles in the ball milling process. While for BCC metals, I(110)/ I(200) is used (Razavi-Tousi and Szpunar 2015; Alizadeh et al. 2011). As shown in Fig.6, the I(111)/ I(200) firstly drops to a minimum value and then increases to the initial level. This process is faster for the Al-GNSs composite due to the aforementioned localised stress induced by the addition of GNSs. This could be explained by considering the anisotropy in the elastic modulus of a single aluminium crystal (Alizadeh et al. 2011). To be specific, the aluminium grains/particles tend to be deformed in the soft direction (111), which is perpendicular to the collision direction of milling balls. When the powder sample is prepared for the XRD analysis, the flattened flakes arrange themselves parallel to the sample holder. As a result, I(111) decreases and I(200) increases, which is the case for the powders from 2 to 10h. With further ball milling, the flattened particles are fractured and rewelded into equiaxed particles, which means the texture and the preferential orientation are eliminated from 15 to 20h. Meanwhile, the I(111)/I(200) recovers to the initial level. This evolution behaviour agrees very well with the SEM observation results as shown in Figs. 2 and 3, which show the morphological change of the powders.

It is also seen in Figs. 4 and 5 that peak broadening is caused as the milling process progresses, which indicates the refinement of crystalline grains and the generation of lattice strain. The mean crystalline size and lattice strain can be evaluated using the William-Hall theory, and are illustrated versus milling time in Figs.7 and 8 respectively. It is shown in Fig.7 that the crystalline size of aluminium decreases quickly during the initial 5h and decrease slowly in the following milling process. In addition, the crystalline size of Al-GNSs composite is smaller than that of Al alloy at the same milling time, which could be attributed to the intensified stresses by the GNSs. This also causes the increased lattice strain during the ball milling as shown in Fig.8. The GNSs accelerate the deformation of the crystalline lattice and promote the lattice strain rate of Al-GNSs composite.

Figure9 shows the TEM microstructure of the produced bulk samples. For the Al alloy sample, most of the coarse grains are in flake-shaped with an average grain size of about 1m. It is also noticed that there is a small portion of fine grains (around 100nm). This means that the microstructure of Al alloy sample is not uniform, which could be caused by the insufficient deformation during ball milling. The microstructure of the Al-GNSs composite presented in Fig. 9 (c) shows that the addition of 1vol.% GNSs dramatically reduce the grain size and the size of the grains is quite similar (app. 100nm). The grain refinement is firstly attributed to the intensified deformation during ball milling, which greatly reduces the grain size and gets the GNSs well dispersed. Highly deformed regions are marked in Fig. 9(b), where also feature the concentrated sites of dislocations. Secondly, the incorporation of the thin GNSs largely decreases the interplanar distance between GNSs, which could perform pinning effect and restrain the grain growth during hot pressing. It is challenging to directly observe the GNS in the bulk sample using TEM due to the ultrathin profile of the GNSs and the interference from the matrix. Raman spectroscopy is sensitive to carbonaceous materials and offers a reliable tool to probe the GNSs (Ferrari and Basko 2013). The Raman scanning results of G band are shown in Fig. 9 (c), in which the bright areas represent the presence of GNSs. It is seen that GNSs are well distributed among aluminium matrix.

TEM micrographs of the produced (a) Al alloy, and (b) Al-GNSs composite. Grains are indicated using dashed circles; highly deformed regions are marked using solid circles. c The Raman mapping of G band on the Al-GNSs composite

As shown in Fig.10, the hardness of the hot-pressed Al alloy averages at 81.2HV. The hardness of Al-GNSs composite is 115.1% higher and reaches 174.7HV. This can be understood by considering the presence of GNSs, grain refinement, Orowan strengthening mechanism, the effect of thermal mismatch. The presence of GNSs is difficult to deform and hinders the movement of dislocation (Prez-Bustamante et al. 2014; Liu et al. 2016). The grain size of the composite is refined and more uniform, which contributes to the increased dislocation and hardness as well. In this regard, Hall-Petch equation was derived to express the relationship between grain size and hardness as follows by combining with Tabors empirical relationshipH3. (Petch and Iron Steel 1953; Moon et al. 2008)

where H is the overall hardness, and H0 is the hardness of the matrix. k is a modified locking parameter, 0.068MPam-0.5 (Boostani et al. 2015). The HHallpetch is estimated to be 65.8HV by taking the grain size as 100nm for the composite.

where\( {H}_0^{\ast } \) is the intrinsic hardness of the matrix. HOrowan, HCTE, andHL represent the contribution from the Orowan strengthening, thermal mismatch mechanism and load-bearing effect, respectively.

where G and b represent the shear modulus of the aluminium matrix (26.2GPa (Khodabakhshi et al. 2017)) and Burgers vector (0.286nm (Khodabakhshi et al. 2017)) respectively; and is the interparticle spacing between the dispersoids (taken as 100nm). The HOrowan is calculated to be 39.7HV.

Another contributing mechanism is from the thermal mismatch between the GNSs (1106 /K) and the aluminium matrix (~23.6106 /K). The hardness increase HCTE could be expressed as (Khodabakhshi et al. 2017):

where is a proportional constant (~1.25), T is the temperature difference between the sintering temperature and the ambient temperature (475K). fv is the volume fraction of GNSs (1%). The particle size is selected to be 5m. It is estimated that the contribution from this mechanism is limited and only 2.7HV is obtained.

where m represents the yield strength of the matrix. m is not measured in this study, but normally falls in the range of 100300MPa for the aluminium alloy. The HL is estimated to be less than 1HV, which is neglectable.

Therefore, the hardness of the Al-GNSs composite is predicted to be 189.4HV by taking all these effects into account, which is higher than the measured value of 174.4HV. This could be owing to the presence of defects (such as pores), agglomeration of GNSs, and simplified expressions in Eqs. (2)(6).

The ductile aluminium particles are firstly flattened at the initial stage of the ball milling, and then fractured and rewelded into equiaxed particles. The addition of the GNSs accelerates the flattening and fracturing, and a smaller particle size is achieved for the composite powder.

The I(111)/I(200) firstly falls to a minimum value and then recovers to the initial level, indicating the creation and elimination of texture during the ball milling, which is consistent with the SEM observation results.

Bodunrin MO, Alaneme KK, Chown LH (2015) Aluminium matrix hybrid composites: a review of reinforcement philosophies; mechanical, corrosion and tribological characteristics. J Mater Res Technol 4(4):434445

Boostani AF, Yazdani S, Mousavian RT, Tahamtan S, Khosroshahi RA, Wei D, Brabazon D, Xu J, Zhang X, Jiang Z (2015) Strengthening mechanisms of graphene sheets in aluminium matrix nanocomposites. Mater Design 88:983989

El-Ghazaly A, Anis G, Salem HG (2017) Effect of graphene addition on the mechanical and tribological behavior of nanostructured AA2124 self-lubricating metal matrix composite. Compos Part A-A 95:325336

Khodabakhshi F, Arab SM, vec P, Gerlich AP (2017) Fabrication of a new Al-mg/graphene nanocomposite by multi-pass friction-stir processing: dispersion, microstructure, stability, and strengthening. Mater Charact 132(Supplement C):92107

Liu J, Khan U, Coleman J, Fernandez B, Rodriguez P, Naher S, Brabazon D (2016) Graphene oxide and graphene nanosheet reinforced aluminium matrix composites: powder synthesis and prepared composite characteristics. Mater Design 94:8794

Niteesh Kumar SJ, Keshavamurthy R, Haseebuddin MR, Koppad PG (2017) Mechanical properties of Aluminium-Graphene composite synthesized by powder metallurgy and hot extrusion. Trans Indian Inst Metals 70(3):605613

Prez-Bustamante R, Bolaos-Morales D, Bonilla-Martnez J, Estrada-Guel I, Martnez-Snchez R (2014) Microstructural and hardness behavior of graphene-nanoplatelets/aluminum composites synthesized by mechanical alloying. J Alloys Compd 615:S578S582

The authors acknowledge use of the facilities at the UOW Electron Microscopy Centre. We appreciate Dr. Monika Wyszomirska who performed the TEM sample preparation and helped interpret the observed results.

J Zhang carried out most of the experiments and wrote the manuscript. ZC, J Zhao and ZJ fixed this topic and helped on experimental design, discussion and manuscript revision. All authors read and approved the final manuscript.

Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Zhang, J., Chen, Z., Zhao, J. et al. Microstructure and mechanical properties of aluminium-graphene composite powders produced by mechanical milling. Mech Adv Mater Mod Process 4, 4 (2018). https://doi.org/10.1186/s40759-018-0037-5

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ball milling machine| mse supplies llc

ball milling machine| mse supplies llc

At MSE Supplies, we are experts at powder processing materials, ball milling equipment and accessories. When it comes to grinding media, our most popular materials include zirconia and alumina. Youll also find materials including tungsten carbide, stainless steel and agate. Grinding balls, otherwise known as milling media are available in a variety of sizes, ranging from kg to tons in measurement. For more reading on the application of milling media, written by a trusted MSE Supplies technical staff, please read our article here.

In addition to our milling media, we offer for sale milling and roller jars made specifically for use in planetary mills and roller mill machines. Roller milling is considered the most economical and common method of powder processing. Our jars are complete with gasket, lid and clamps. You can count on grinding jars from MSE Supplies to give you a secure and tight seal.

Nothing less than professional grade milling equipment is sold at MSE Supplies. We carry a wide variety of options for both planetary and roller mills machine equipment. Our materials and supplies for powder processing and mill grinding come with a guarantee for preciseness and quality for your research and development needs. We strive to provide our customers with many options for their mill grinding and powder processing projects and offer our expert services to answer any questions our customers may have, please contact us.

study of thermal and safety behaviour of nanoboron blended flash powder | springerlink

study of thermal and safety behaviour of nanoboron blended flash powder | springerlink

The flash powder composition consists of KNO3, Al and S that are used in major quantities in pyrotech industries. This mixture is sensitive to both shock and friction. As a result, recent research works were focused on reducing sensitivity by replacing the aluminium metal which is the root cause for this sensitiveness. From studies, it is identified that micron-sized boron is used to replace about 65.65% of aluminium. The boron used in this mixture has an activation energy of about 205 kJ mol1. Due to the high activation energy, the replacement is restricted up to 65.65%. In order to reduce the activation energy, the size of the boron particles are reduced by using wet ball milling method. Thus, the reduced boron particles are now employed in flash powder mixtures. The nanoboron blended mixtures are subjected to sensitivity tests, thermal behaviour tests, MIE tests, explosive pressure tests and finally sound performance tests. From the test results, it is concluded that the aluminium replacement with boron is increased up to 85% with simultaneous increase in safety also.

Azhagurajan A, Prakash L, Jeyasubramanian K, Jaya Christa ST. Measurement of MIE and LOC for flash powder mixture containing boron to authenticate safety in firework industries. Measurement. 2020;153:107435.

Risha GA, Evans BJ, Eric Boyer, Wehrman RB, Kuo KK. Nano-sized aluminum and boron-based solid fuel characterization in a hybrid rocket engine. In: 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit. Huntsville: Alabama; 2003.

Vinayan BP, Zhao-Karger Z, Diemant T, Chakravadhanula VSK, Schwarzburger NI, Cambaz MA, Behm RJ, Kubel C, Fichtner M. Performance study of magnesiumsulfur battery using a graphene based sulfur composite cathode electrode and a non-nucleophilic Mg electrolyte. R Soc Chem. 2016;8:3296306.

The authors thank the Science and Engineering Research Board (SERB), Government of India, for sanctioning the project (File No. EMR/2016/001715) as well as the management and the Principal of Mepco Schlenk Engineering College for motivating and providing support to conduct this research work.

Dr. Azhagurajan, principal investigator, was involved in design and preparation of the samples and also helped in data collection and experimentation process. Mr. Prakash, corresponding author, participated in sample preparation and carried out the experimentation process and also consolidated the results. Dr. Jeyasubramanian, co-investigator, helped in the experimentation process, analysing the results and manuscript preparation.

Arumugachamy, A., Lakshmana Pandian, P. & Kadarkaraithangam, J. Study of thermal and safety behaviour of nanoboron blended flash powder. J Therm Anal Calorim (2021). https://doi.org/10.1007/s10973-021-10891-3

high-energy ball milling - an overview | sciencedirect topics

high-energy ball milling - an overview | sciencedirect topics

High-energy ball milling is a ball milling process in which a powder mixture placed in a ball mill is subjected to high-energy collisions from the balls. High-energy ball milling, also called mechanical alloying, can successfully produce fine, uniform dispersions of oxide particles in nickel-base super alloys that cannot be made by conventional powder metallurgy methods. High-energy ball milling is a way of modifying the conditions in which chemical reactions usually take place, either by changing the reactivity of as-milled solids or by inducing chemical reactions during milling [20].

High-energy ball milling is a mechanical deformation process that is frequently used for producing nanocrystalline metals or alloys in powder form. This technique belongs to the comminution or attrition approach introduced in Chapter 1. In the high-energy ball milling process, coarse-grained structures undergo disassociation as the result of severe cyclic deformation induced by milling with stiff balls in a high-energy shaker mill [8,9]. This process has been successfully used to produce metals with minimum particle sizes from 4 to 26nm. The high-energy ball milling technique is simple and has high potential to scale up to produce tonnage quantities of materials [8]. However, a serious problem of this technique is the contamination from milling media (balls and vial) and/or atmosphere. Therefore, a number of improvements, including the usages of surfactants, alloy-coated milling media, and protective atmospheres, have been developed to alleviate the contamination problem [8].

The fine powder (in nano or submicron sizes) produced from ball milling can be consolidated to bulk form for large-scale applications such as hip implants and bone screws. Usually, the fine powders are compacted and sintered together via methods like hot isostatic pressing and explosive compaction under the temperatures or conditions that suppress grain growth and maintain nanocrystalline microstructure [8,10]. Bulk metallic materials produced by this approach have achieved the theoretical densities of nanocrystalline materials and greatly improved mechanical properties compared to their conventional, micron-grained counterparts.

High-energy ball milling is effective in getting well-dispersed slurry.79 The preparation procedure is summarized in Fig.24.2. First, commercially available PZT powders (APC 850) were high-energy ball milled to get the desired particle size. Secondly, a selected dispersant was added to the milled powders to get the surface-modified powders. The smaller the powder, the more important this procedure. Afterwards, PZT precursor solution was added to these surface-modified powders and mixed by further ball milling. Finally, the resultant uniform slurry was ready for further processing, such as spin coating, tape casting, screen printing and molding. The recipe for the slurry, including the concentration of xerogel solution and powder to solution mass ratio, depends on the further processing method employed. For our convenience, the recipes for the slurry were given four numbers with regard to the above two important parameters. For example, in 3025, the first two numbers represent the concentration of the xerogel solution9 in weight percent, i.e. 30wt%, and the last two numbers represent the mass ratio of the added PZT powder to xerogel solution, namely 2 to 5.

High-energy ball milling, also called mechanical attrition, can be used to reduce the grain size of materials from many micrometers to 220nm (see Mechanical Alloying). This is a result of the cold-working process creating large-angle grain boundaries. Most of the reduction in grain size occurs rapidly, but the process slows, and long times are required to reach the smallest sizes. This process has the advantage of being relatively inexpensive and can be easily scaled up to produce large quantities of material. Usually, to maximize the energy of collision, high-mass hard-steel or WC balls are used. Contamination by materials removed by the balls is a major concern. Severe mechanical deformation and plastic deformation at high strain rates (103104s1) occurs during the process. Initially, shear bands are formed consisting of a high density of dislocations. Later these dislocations annihilate and recombine as small-angle grain boundaries forming nanometer-sized grains. Finally, the orientation of these nanometer-sized grains is randomized.

The range of solubility of multicomponent systems is greatly increased by mechanical attrition. Mechanical attrition can also produce metastable materials. If the milling is done in the presence of O2 or N2, oxides or nitrides can be formed.

High-energy ball milling, a predominantly mechanical process, nevertheless results in significant structural and chemical changes in the material. Nonequilibrium synthesis of materials at low temperatures via ball milling is possible through a combination of multiple processes, which occur during milling. These processes include thermal shock, high-speed plastic deformation, mechanical grinding and fracturing, cold welding, and intimate mixing [9].

BNNTs were typically synthesized by the prolonged (approximately 150 h) high-energy milling of pure boron or h-BN powder using stainless-steel milling vessels and hardened steel balls in a pressurized (2.3 103 Torr) NH3 atmosphere. The milled material was then annealed at high temperature (>1000 C) in an N2 atmosphere for 10 h. It was found that large quantities of BNNTs can be synthesized using this method. The yield of the BNNTs depended on the duration of the milling treatment [11]. It was proposed that nanotube formation by this method was caused by two different mechanisms. The first mechanism being the nitridation of B nanoparticles in the NH3 atmosphere, which in turn served as nucleation sites for the formation of BNNTs. The second mechanism proposed was that the Fe (and other metals such as Cr and Ni) from the milling process was incorporated into the B powder during high-energy milling and that the metal particles then served as catalysts for BNNT growth [11]. In order for these two mechanisms to operate effectively, it is necessary that both the ball milling and annealing steps be carried out for long times. Other variations of this technique have been reported including the use of tungsten carbide (WC) balls, and a mixture of NiB and alumina [5,12]. Even though the yield of BNNTs can be very high using this method, the resultant nanotubes can suffer from contamination and structural defects. Figure 8.6 shows a micrograph of BNNTs synthesized using this technique.

Figure 8.6. Transmission electron microscope image of BNNTs synthesized by the ball milling process. The growth of the nanotubes from the milled material is clearly evident. The largest nanotube imaged has a bamboo-like morphology.

High energy ball milling can lead to glass formation from elemental powder mixtures as well as by amorphization of intermetallic compound powders. Solid state amorphization by high energy milling has been demonstrated in a number of Ti- and Zr-based and other alloy systems such as NiTi, CuTi, AlGeNb, SnNb, NiZr, CuZr, CoZr and FeZr. The process of ball milling is illustrated in Figure 3.56. Powder particles are severely deformed, fractured and mutually cold welded during collisions of the balls. The repeated fracturing and cold welding of powder particles result in the formation of a layered structure in which the layer thickness keeps decreasing with milling time. A part of the mechanical energy accumulates within these powder particles in the form of excess lattice defects which facilitate interdiffusion between the layers. The continuous reduction in the diffusion distance and the enhancement in the diffusivity with increasing milling time tend to bring about chemical homogeneity of the powder particles by enriching each layer with the other species being milled together. The sequence of the events that occur during milling can be followed by taking out samples from the ball mill at several intervals and by analysing these powder samples in respect of their chemical composition and structure. Let us describe one such experiment in which elemental powders of Zr and Al were milled in an attritor under an Ar atmosphere.

Elemental powders of Zr and Al of 99.5 purity, when milled in an attritor using 5 mm diameter balls of zirconia as the milling media and keeping the ball to powder weight ratio at 10:1, showed a progressive structural change as revealed in XRD patterns (Figure 3.57(a) and (b)). Diffraction peaks associated with the individual elemental species remained distinct upto 5 h of milling at a constant milling speed of 550 rpm. All particles and the balls appeared very shiny in the initial stages. With increasing milling time, the particles lost their lustre, the 111 and 200 peaks of fcc Al gradually shrunk and the three adjacent low-angle peaks of hcp -Zr, corresponding to1010, 0002 and1011, became broader. After about 15 h of milling, XRD showed only -Zr peaks which shifted towards the high angle side, implying a decrease in the lattice parameters resulting from the enrichment of the -Zr phase with Al. After 20 h of milling, all Bragg peaks except one broad peak close to the{1010} peak disappeared. Powders milled for 25 h showed an extra reflection corresponding to a lattice spacing of 5.4 nm, which matches closely to a superlattice reflection of a metastable D019 (Zr3Al) phase. On further milling, the powders transformed into an amorphous phase. The sequence of structural evolution could be described as -Zr + Al -Zr (Al) solid solution + Al nanocrystalline solid solution + localized amorphous phase Zr3Al (D019) + -Zr (Al) solid solution + amorphous phase bulk amorphous phase.

Figure 3.57. XRD patterns showing a progressive structural change for different times when elemental powders of Zr and Al of 99.5 purity were milled in an attritor using 5 mm diameter balls of zirconia with a ball to powder weight ratio of 10:1.

The mechanism of solid state amorphization during mechanical alloying has been studied on the basis of experimental observations made on several alloy systems. One of the probable mechanisms, based on local melting followed by rapid solidification, has not found acceptance as evidence of melting could not be seen in experiments. The example of ball milling of elemental Zr and Al powders has demonstrated that the amorphisation process is preceded by the enrichment of the -Zr phase to a level of approximately 15 at.% Al. The solute concentration progressively changes during milling. The various stages encountered in the course of amorphization can be explained in terms of schematic free energy versus concentration plots for the , the metastable D019, and the amorphous phases (Figure 3.58). With increasing degrees of Al enrichment, the free energy of the interface region gradually moves along the path 1-2 (Figure 3.58). Once the concentration crosses the point 2, it becomes thermodynamically feasible to nucleate the Zr3Al phase which has the metastable D019 structure. Although the equilibrium Zr3Al phase has the L12 structure, it has been shown (Mukhopadhyay et al. 1979) that the metastable D019 structure is kinetically favoured during the early stages of precipitation from the -phase. This is not unexpected as the hcp structure and the D019 structure (which is an ordered derivative of the former) follow a one-to-one lattice correspondence and exhibit perfect lattice registry.

Figure 3.58. Schematic free energy concentration plots in ZrAl system for the , the metastable D019 and the amorphous phases illustrating the various stages encountered in the course of amorphization.

With further Al enrichment, as the concentration crosses the point 3, nucleation of the amorphous phase becomes possible. It is to be emphasized that the change in composition occurs gradually from the interface to the core of the particles, with the result that the amorphous phase starts appearing at interfaces while the core remains crystalline. As the Al concentration in the powder particles crosses point 4, each particle can turn amorphous by a polymorphic process. The observed sequence of solid state amorphization in the case of ball milling of elemental Zr and Al powders suggests the occurrence of amorphization by a lattice instability mechanism which is brought about by solute enrichment of the -phase beyond a certain limit (point 4 in Figure 3.58).

The synthesis of materials by high-energy ball milling of powders was first developed by John Benjamin (1970) and his coworkers at the International Nickel Company in the late 1960s [42,43]. It was found that this method, called mechanical alloying, could successfully produce fine and uniform dispersions of oxide particles (Al2O3, Y2O3, ThO2) in nickel-base superalloys which could not be made by conventional powder metallurgy methods.

It is a ball milling process where a powder mixture placed in the ball mill is subjected to high-energy collision from the balls. Fig. I.7 shows the motions of the balls and the powder. Since the rotation directions of the bowl and balls are opposite, the centrifugal forces are alternately synchronized. Thus, friction resulted from the hardened milling balls and the powder mixture being ground alternately rolling on the inner wall of the bowl and striking the opposite wall. The impact energy of the milling balls in the normal direction attains a value of up to 40 times higher than that due to gravitational acceleration. Hence, the planetary ball mill can be used for high-speed milling [44].

During the high-energy ball milling process, the powder particles are subjected to high energetic impact. Microstructurally, the mechanical alloying process can be divided into four stages: (1) initial stage, (2) intermediate stage, (3) final stage, and (4) completion stage [44].

At the initial stage of ball milling, the powder particles are flattened by the compressive forces caused by the impact of the balls. Microforging leads to changes in the shapes of individual particles, or clusters of particles being repeatedly impacted by the balls with high kinetic energy. However, such deformation of the powders shows no net change in mass.

At the intermediate stage of the mechanical alloying process, a significant change occurs as compared to the initial stage. Cold welding becomes significant. The intimate mixture of the powder constituents decreases the diffusion distance to the micrometer range. Fracturing and cold welding are the dominant milling processes at this stage. Although some dissolution may take place, the chemical composition of the alloyed powder is still not homogeneous.

At the final stage of the mechanical alloying process, more refinement and reduction in particle size becomes evident. The microstructure of the particle also appears to be more homogeneous in microscopic scale than those at the initial and intermediate stages. True alloys may have already been formed.

At the completion stage of the mechanical alloying process, the powder particles possess an extremely deformed metastable structure. At this stage, the lamellae are no longer resolvable by optical microscopy. Further mechanical alloying beyond this stage cannot physically improve the dispersoid distribution. Real alloy with a composition similar to the starting constituents is thus formed [44].

MA in high-energy ball milling equipment is accomplished by processing an initial powder charge usually comprising a mixture of elemental, ceramic (e.g., yttria for ODS alloys), and master alloy powders, all supplied in strictly controlled size ranges. Master alloy powders are used in order to reduce in situ oxidation of highly reactive species, such as aluminum or titanium alloy additions during processing. The milling medium normally used in commercial systems is a charge of hardened steel balls, typically 2cm in diameter. The ball-to-powder weight ratio is chosen carefully for each mill and powder charge combination, but is typically around 10:1 for commercial systems. Given the enormous surface area, both of the initial powders and the fresh powder surfaces generated during MA processing, control of the milling atmosphere and its purity is essential to avoid undue alloy contamination. The principal protective atmospheres employed during commercial milling of MA powders are usually either argon or hydrogen and this protection generally extends both to pre- and post-MA powder handling. Both the purity of these gas atmospheres and the integrity of gas seals on the milling equipment are essential to control contamination, particularly when processing reactive species. For example, levels of oxide contamination in Ni3Al can double with just a few hours of milling in impure argon. Occasionally, however, deliberate doping of the milling environment has been used to facilitate alloying additions during processing.

The central event during MA is the ballpowderball collision within the milling medium during processing. It is repetition of these high-energy collisions which leads eventually to MA of the powder charge. Intimate mixing and eventual MA of the powder charge occurs in a series of identifiable, more or less discrete stages during processing (e.g., Gilman and Benjamin 1983). For ductileductile or ductilebrittle combinations of starting powders, MA initially proceeds by the flattening and work hardening of ductile powders and fragmenting of brittle constituents, which is followed by extensive cold welding between powder particles, formation of lamellar structures, and coarsening of the powder particle size distribution. Brittle powder fragments are trapped at cold weld interfaces between the evolving lamellas of the ductile constituents and thus, while continuing to comminute, become dispersed. With continued milling a balance, which is dependent on processing parameters and the composition of the constituents, is established between further cold welding and powder particle fracture, leading to relatively stable powder particle sizes.

This balance between welding and fracture is accompanied both by further decreases in lamella spacings and by folding and mixing-in of lamella fragments to produce microstructures typical of MA (Fig. 1). For ODS alloys, powder constituents are milled to the stage where light microscopy examination reveals that lamella spacings have decreased to below the resolution limit (1m). For typical levels of oxide addition (e.g., 0.5wt.% yttria) this criterion ensures average dispersoid interparticle spacings of <0.5m (Fig. 2). In other systems, milling can progress until true alloying occurs. Surprisingly, MA can also be achieved between essentially brittle powder constituents. The mechanisms by which this occurs are less well understood than in systems incorporating at least one ductile powder component. Nevertheless, granular as opposed to interlamellar mixtures of brittle powder constituents do evolve, typically with smaller, harder fragments progressively incorporated to a very fine scale within the less hard constituents, e.g., aluminanickel oxide. Moreover, MA of these brittle constituents can progress to true alloying, as has been demonstrated using lattice parameter measurements on Si28 at.% Ge progressively milled from constituent powders (Davis and Koch 1987).

Figure 1. Polished and etched metallographic section of ODM 751 FeCrAl alloy powders in the fully MA condition, showing the folded lamellar structures typical of material processed by high-energy ball milling (courtesy of D.M. Jaeger).

Figure 2. Transmission electron microscope image showing alignment of a fine-scale dispersion of oxide particles in extruded ODS alloy PM2000. The arrow shows the extrusion direction (courtesy of Y.L. Chen).

Milling of very ductile metals such as aluminum and tin has to be carefully controlled to avoid complete agglomeration of the ductile phase rather than the balance between cold welding and fracture that leads to MA. This is normally achieved by adding precise amounts of organic compounds termed process control agents (PCAs) to the milling environment. Typically waxes or solvents, these compounds that interfere with cold welding progressively break down during milling to become incorporated within the final MA powders (e.g., in aluminum alloys) as fine-scale distributions of carbides or oxides. Similar restrictions to the proclivity for cold welding in ductile powders can be achieved without use of PCAs by milling at low temperatures, e.g., below 100C for aluminum.

The processing equipment used to effect MA by high-energy ball milling of powders originated in mining and conventional powder metallurgy industries. The range of high-energy ball milling equipment divides, approximately, into two categories: small, high-energy laboratory devices, and larger facilities capable of milling commercial quantities of powder. The former category includes SPEX shaker mills and planetary ball mills. Both devices are capable of rapidly effecting MA, but in quantities of powders up to no more than a few tens of grams. SPEX mills vibrate at up to 1200rpm in three orthogonal directions to achieve ball velocities approaching 5ms1. Planetary mills incorporate a rotating base plate upon which are mounted counter-rotating, smaller-radius vials containing the ball/powder charge. The kinetic energy imparted to the ball charge in the planetary mill depends on the base plate and vial radii and angular velocities. Attritor or Szigvari ball mills, depending on their size, can be used either for laboratory or commercial ball milling applications and incorporate a rotating vertical shaft with attached horizontal impellors which stirs a container housing the ball and powder charge. These devices can process batches of up to several kilograms or more of powder through the significant differential movement the impellors generate between the ball and powder charge. Balls can either cascade or tumble when leaving the mill wall during attritor processing, depending on the ball charge and impellor velocity.

The largest commercial devices applied to MA are horizontal ball mills. When these devices exceed several meters in diameter they impart sufficient kinetic energy through ball impacts to effect MA and can process over 1000kg of powder per batch in larger units. Balls either cascade or tumble during processing in these mills depending on rotational speed (see Fig. 3). The time taken to achieve MA scales approximately inversely to the size of the milling equipment used. Hence, milling which might take minutes to accomplish in a SPEX mill could take hours in an attritor or days in a horizontal ball mill. All of these processing routes, however, have very low energy conversion efficiency, in that only a small fraction of the milling energy expended effects microstructural change contributing to the MA process.

Figure 3. Configuration of a horizontal ball mill, showing the release of the powder and ball charge (at angular position ) from the inner wall of the mill rotating with angular velocity (after Lu et al. 1995).

It is worth noting that during MA, powder particles also coat (condition) the ball milling medium, which means that, to avoid cross-contamination of commercial alloys, the repeat use of ball charges is restricted to compositionally similar batches of raw materials.

Mechanical means, such as high-energy ball milling, ultrasonic or jet milling, and others, can have powder prepared into nanoparticles. This is an example of a top-down approach, which is suitable for refractory metals or materials beyond the use of chemical reactions. The disadvantages include the difficulties in classification according to the particle size and serious surface contamination.

Bombarding a metal surface with high-energy balls makes it possible to turn the surface structure into nanoscale; this can improve the abrasion and corrosion resistance of the processed material. Meanwhile, the surface is identical to the bulk material, and thus it does not peel off like nanocoating material. The main mechanism of this method is to produce a large number of defects and dislocations, which further develop into dislocation walls, and thus cut the large crystals into nanocrystalline grains (Figure 5.11).

MCP is normally a dry, high-energy ball milling technique and has been employed to produce a variety of commercially useful and scientifically interesting materials. The formation of an amorphous phase by mechanical grinding of a Y-Co intermetallic compound in 1981 (Ermakov et al., 1981) and its formation in the Ni-Nb system by ball milling of blended elemental powder mixtures (Koch et al., 1983) brought about the recognition that this technique is a potential non-equilibrium processing technique. Beginning in the mid-1980s, a number of investigations have been carried out to synthesize a variety of equilibrium and non-equilibrium phases including supersaturated solid solutions, crystalline and quasicrystalline intermediate phases, and amorphous alloys. Additionally, it has been recognized that powder mixtures can be mechanically activated to induce chemical reactions, at room temperature or at least at much lower temperatures than normally required, to produce pure metals, nanocomposites and a variety of commercially useful materials. Efforts have also been under way since the early 1990s to understand the process fundamentals of MA through modeling studies. Because of all these special attributes, this simple but effective processing technique has been applied to metals, ceramics, polymers and composite materials. The attributes of mechanochemical processing are listed below. However, in the present chapter, the focus will be on the synthesis of nanocrystalline metal particles.

Inducement of chemical (displacement) reactions at low temperatures for (a) Mineral and Waste processing, (b) Metals refining, (c) Combustion reactions, and (d) Production of discrete ultrafine particles

Nanocrystalline materials are single- or multi-phase polycrystalline solids with a grain size of the order of a few nanometers (1nm=109m=10), typically 1100nm in at least one dimension. Since the grain sizes are so small, a significant volume of the microstructure in nanocrystalline materials is composed of interfaces, mainly grain boundaries. That is, a large volume fraction of the atoms resides in the grain boundaries. Consequently, nanocrystalline materials exhibit properties that are significantly different from, and often an improvement on, their conventional coarse-grained polycrystalline counterparts. Compared to the material with a more conventional grain size, that is, larger than a few micrometers, nanocrystalline materials show increased strength, high hardness, extremely high diffusion rates and consequently reduced sintering times for powder compaction, and improved deformation characteristics. Several excellent reviews are available giving details on different aspects of processings, properties, and applications of these materials (Gleiter, 1989; Suryanarayana, 1995a, 2005).

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