Article updated on 24 November 2020.
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Anyone who has accidentally put metal in a microwave oven knows that metal and microwaves are not friends. However, microwaves can be used in targeted, specific ways for the heat treatment of metals.
Microwaves generate internal friction and heat via dielectric or magnetic losses in the microwave field, processes known as “suscepting” or “coupling”. In the case of metals, microwaves can cause eddy currents and exterior heating, but are mostly reflected. However, metals in powder form can be heated effectively by microwave field as a result of their large surface area and low electrical connectivity. Metal powders also contain oxides, water and other compounds on their surface. These intricate surface chemistries expedite the heating of metal powders via microwaves.
There are numerous techniques that permit the use of microwaves in metal processing, even though metals reflect or do not suscept well. One effective technique is the combination of radiant heat and microwaves. Radiant heat can be added with conventional sources or by the use of microwave susceptors, which are materials capable of efficiently converting microwave energy into heat.
Below are a number of ways that microwaves can be used as a heat source in heat treatment processes.
Heat Treatment for Improving Mechanical Properties
Heat treatment is commonly used to boost the mechanical strength and hardness of a material. Convention heat treatments, performed by convection or radiation, require the transferring of heat from the surface of a target material to the material’s interior. A microwave heat source, on the other hand, could heat a target material more evenly.
Using heat insulator and susceptor layers, microwave ovens can be used to effectively increase mechanical strength and hardness using less energy than conventional methods, according to a 2014 study in the journal Procedia Engineering. Study researchers also found the microwave furnace heating test samples more evenly than conventional furnaces.
Microwave Powder Metallurgy Sintering
During sintering, a metal powder compact behaves more like a bulk metal: The compact becomes conductive as the protective exterior layers break down, and connections among particles increase. This leads to the metal shedding its dielectric behaviour, which stops total sintering with pure microwaves.
However, a combination heating process, such as one that uses a susceptor assist, can complement the later stages of sintering. A different approach is to customize the grain boundaries and retain dielectric heating behaviour.
Microwave Binder Removal
A common method used to produce part shapes, powder compaction calls for a binder to supply green-part strength and handleability. The binder must be taken out before parts start to densify during sintering. This typically is achieved through thermal processing. For many ceramic materials, the part is heated gradually in a furnace to burn the binder out of the part. Powder metallurgy parts are heated in an atmosphere-controlled furnace, causing the binders to be volatilized and cleared out.
In each case, binder removal is a process that requires significant amounts of time and energy, particularly for parts with substantial cross section. Heat must be transported from the exterior of the part into the middle of the part. If the part is heated too rapidly, binder can volatilize quickly creating pockets of gas that grow, possibly cracking the part. Furthermore, if excessive heat is used on the part exterior, it can start to densify prior to the binder being eliminated from the middle of the part, trapping the binder and possibly leading to significant defects.
A combination of electric or gas and microwave heating in an atmosphere-controlled environment can be used to address this problem by heating the powder compact evenly.
Metal Surface Treatments
A number of metal surface treatments, such as aluminizing and boronizing, have been performed at a laboratory scale with packed-bed cementation or chemical vapor deposition via microwave heating. However, more research is necessary to compare techniques, scale up strategies and perform cost-benefit analyses.
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