Question

What safety precautions we need to take in the Powder Metallurgy lab? Please specify and explain

Answer 1

Powder metallurgy is the manufacturing science of producing solid parts of desired geometry and material from powders. Commonly known as powder metallurgy, it may also be referred to as powder processing considering that non-metal powders can be involved. Powders are compacted into a certain geometry then heated, (sintered), to solidify the part. The manufacturing advantages and disadvantages, as well as the applications for parts produced by this method, are discussed latter in the design and applications of powder metallurgy sectionPowder metallurgy is the manufacturing science of producing solid parts of desired geometry and material from powders. Commonly known as powder metallurgy, it may also be referred to as powder processing considering that non-metal powders can be involved. Powders are compacted into a certain geometry then heated, (sintered), to solidify the part. The manufacturing advantages and disadvantages, as well as the applications for parts produced by this method, are discussed latter in the design and applications of powder metallurgy section. The first consideration in powder metallurgy is the powders used for the manufacturing process. Several different measures are used to quantify the properties of a certain powder. Powders can be pure elements or alloys. A powder might be a mixture of different kinds of powders. It could be a combination of elemental powders, alloy powders, or both elemental and alloy powders together. Material and the method of powder production are critical factors in determining the properties of a powder. It should always be remembered, when working with powders, that the powder itself may be a potential hazard. Some powders may be flammable and/or present health risks to workers. Safety precautions should always be taken when handling or storing powders. Also, be sure to follow any regulations regarding the handling, storage, or disposal of powders. Powder selection and processing will depend on cost, desired purity and mechanical properties of finished product. Environmental control is critical in proper storage and handling of powders. Contamination of powder can result in powder degradation and should be avoided. Remember, high surface areas cause powders to react readily with outside materials. This can have various results, oxidation for example, caused by oxygen present in the air.

1. Powder Properties

Powders are finely divided solid particulates. The size and shape of individual particulates is important. Characteristics of a powder can be quantified in several ways. These characteristics are necessary to understand when selecting a powder for an operation, since powder properties will affect processing factors.

2. Size and Distribution

Size of particles is a factor that will affect processing of metal powders. In manufacturing practice, powders are commonly measured using a series of screens with different sized openings. Each screen is a wire mesh with openings ideally of the same size. Screens for powder measurement are designated according to the number of openings per linear inch, (i.e. 30, 100). Openings per linear inch are the same in the 2 dimensions of the screen's surface, therefore the number of openings per square inch is the square of the linear number. A screen with a linear measurement of 100 has 1002 or 10,000 openings per square inch. When determining the size of an opening, the size of the screen's wire must also be considered. Mesh opening size, (MS), can be determined by
MS = 1/MC - WS, where MC is the mesh count, (openings per linear inch), and WS is the thickness of the wire.

3. Particle Structure

The structure, or shape, of particles is a major factor in a powder processing operation. Material and method of powder production are the main variables determining powder shape. Particles of a certain powder may have similar shapes but no particle shapes are exactly the same. Hence, there will exist a shape distribution within a powder. Different types of powders combined together may also have significant differences in particle shape, which will show in the shape distribution.

Particle shape plays a large roll in powder density and flow characteristics, it is also a major factor in pressing and sintering. There are several types of basic powder particle shapes. These are ideal shapes, particles in reality are imperfect and may exhibit characteristics of more than one shape type. Two ways used, in manufacturing analysis, to quantify the shape of particles are the shape index and the aspect ratio. Shape index relates the particle's surface area to the particle's volume and compares that to a sphere, which has the lowest shape index. Aspect ratio is the ratio between a powder particle's greatest dimension and its smallest. A perfect sphere will have an aspect ratio of 1, a rounded particle may have an aspect ratio of 1.5, while acicular or flakey particles may easily have aspect ratios of 3, 5 or 10.

4. Porosity

Pores, or space, within the powder is in a large part determined by particle shape, (and size), since the shape dictates how particles will contact each other. Spaces that exist between particles of a powder and are open to the outside are called open pores. These spaces expose external surfaces of powder particle's.

5. Powder Chemistry

Chemical composition of the powder should be known. Powder chemistry is a major factor in manufacturing process design. Individual particles must be examined. Metal powders can be a pure element, a mixture of different elemental particles, or the particles themselves can be an alloy material, called pre-alloyed. Powder metallurgy is unique in its ability to alloy or combine materials that otherwise could not be produced by any other method. Often different materials are powdered and combined as powders. In this case individual powder particles will be all of one type of material or another. The particles will have different characteristics as well. The alloying of metal powders of different materials occurs latter during the sintering of the part. Sometimes nonmetallic powders, particularly ceramic powders, are mixed and combined with other materials.

6. Surface Area

Surface is measured by considering the combined surface area of all the particles and relating that to the volume of powder, (usually cm2/g). Powders have an extremely high surface area to volume ratio. Surface area to volume increases as particle size decreases. Particle shape also is a factor in surface area. The higher the surface area, the more activity that will occur during sintering. There are advantages and disadvantages to increased surface area of powders. Increased surface area will increase the area for oxides and other surface films to develop. Also agglomeration, or the sticking together of powder particles, tends to occur in smaller sized particles. Smaller particles are advantageous in that they provide more uniform material distribution in the manufacturing process and better mechanical properties in the product.

7. Bulk. Density

Bulk density is the density of the powder in its loose, uncompressed form. A container of known volume is filled with the powder. The powder is then weighed and the density is determined, usually expressed in g/cm3. Major factors determining bulk density of a powder are particle size, particle size distribution and particle shape.

8. Compressability indicates the relative ability for a powder to compress. For example, lubricants can improve compressability.
Compactability is not the same as compressability. Compactability is a measurement of the structural strength of a compacted, unsintered powder, (called a green compact). Binders, for example, can increase the compactability of a powder.
Sinterability is the ability of powder particles of the green compact, to bond together when heated during the sintering process. Sinterability is dependent to a large extent on surface characteristics of the powder particles.