Material Processing

In order to produce an article for any application out of a particular material there are several steps that may be required. The first step is usually to obtain the raw materials from our environment. This may involve discovering where these raw materials are located (often achieved with knowledge of geology) and developing processes to extract them from these locations (e.g. mining the ores, drilling for oil etc.). Otherwise, it may be possible to find sources of material suitable for recycling or reprocessing. Once these raw materials have been obtained they may need to undergo some initial processing to get them into a usable form. This may be some form of extractive metallurgy, chemical synthesis or some other chemical process. It may also be necessary to mix different raw materials to achieve a certain composition (e.g. alloying in metals) that is appropriate for or optimized to an application. The application will usually require that the material be in a particular shape and a suitable shaping process or combination of process must be employed to achieve this. Often, it may be possible to produce a shape out of a material with any one of the many different shaping processes. However, there is usually one particular process that either results in particular benefits in terms of the properties of the material or the article that is produced or meets some other important criteria - such as low cost - that it is selected over the other options. Finally, it may be necessary or beneficial to process the article further, once it has been formed, in order to optimise the properties of the material.

Firstly, this chapter will present the various chemical processes that may be necessary to produce suitable materials from the raw materials in our environment. The different methods for shaping these materials will then be presented. Finally, the processes used to optimise the properties of the materials will be discussed.

Powder processes are used in the production of metallic and ceramic parts. The use of metal powders is commonly referred to as Powder Metallurgy (P/M).

There are 4 main stages to producing products with, they are: Powder Mixing, Compaction, Sintering and final finishing.

A metal or ceramic powder is prepared, then compacted into a desired shape. This part is then heated in a furnace causing the powders to weld together forming a solid part. The part is then final processed by final shaping, minor smoothing, or drilling.

Using Powders to produce parts is viable when you require a high volume of simple parts that need to be cost efficient. All though casting can also do this, P/M offers near net shape products. This means that the part that comes out of the process needs little or no finishing done to it.

Ceramics lend themselves well to powder processing as they are very hard and brittle, thus a near net shape is highly beneficial.

Mixing is mainly done to add waxes for the compaction, binders to temporarily strengthen the compacts and sometimes to get the right chemistry.

As most suppliers recommend lubricant for idea compaction, mixing is a very important process, so a homogenous mixture is required. Optimum mixing occurs with turbulent mixing and at low centrifugal forces.

Along with ensuring a homogenous mix, the mixing process also provides some milling of the powders. As we all know you can put more tennis balls in an area than beach balls, thus increasing the surface area of the balls. The same is true with powders, more surface area, the better the final product is.

Compaction is the process of squishing the powders into the desired shape with enough force so as to hold its shape. This is called a green body, as it still has moisture in it and needs to be Sintered. Same basic concept as pottery, the plate or cup is considered "green" until it is fired

There are 2 categories of pressing: Isostatic and Axial.

Sintering is simply the furnace heating of a compacted powder object, also known as a green body to form a solid part. The powders can be either metallic or ceramic. They can be in elemental form, as an alloy, or mixture of both. Most sintering processes are done in a protective atmosphere, such as nitrogen or hydrogen mixed gas, to avoid degradation of the green bodies, and at a temperature lower than the melting point, approximately 60~90% of the main elements meting point. The specific atmosphere and temperature is dependent upon the material being processed.

If the material being sintered is an alloy, it is possible that one or more of the constitutes has a melting point lower than the sintering temperature, thus causing a small amount of liquid to form. This is called Liquid Phase Sintering. Caution needs to be taken when choosing a temperature as too much liquid will result in the deformation of the part. This is referred to as slumping.

The mechanism of sintering is the diffusion of the atoms across the particle boundaries of compacted powders. As the atoms diffuse, all voids are filled and the material forms one solid part. As the voids between particles are no longer present, the part increases in density, and experiences shrinkage. However, due to the nature of this process, only 93%-98% theoretical density can be achieved, thus further mechanical processing is needed to obtain 100% dense material.

The resultant microscopic structure resembles the starting green compact. The starting particle boundaries eventually turn into the final grain boundaries.

As the voids between the powder particles are filled during the sintering process, the gases need to be expelled from the compact. These gases are; air trapped between powders and gasses from additives added during the mixing and/or compaction process. These gases are expelled through capillaries formed by the particle boundaries. If the compacts are heated too fast, these capillaries can be “pinched” off and if these gasses are not expelled, the part will have defects such as warpage, porosity, or even holes.

A typical industrial sintering process is done on a traveling grate furnace with a 2 stages of heating. The green bodies are placed on a conveyor which travels into the furnace which has a positive pressure protective atmosphere blown onto the conveyor belt. The parts travel into the first temperature zone to vaporize and wax and degas. The second temperature zone is to do the actual sintering of the material. After the appropriate sinter time, the parts travel through a cooling zone to allow the parts to be handled, or to lock properties for continued processing. Degas and sinter times vary based on material.

Date: 2023-04-10     hits: 474    Return