Microwave energy has been in use for a variety of applications for over 50 years. Some of the early applications include communication, navigation and drying of food items. At present, industrial uses of microwaves include wood processing, vulcanisation of rubber, meat tempering, and medical therapy. In the past two decades, the remarkable success of domestic microwave ovens has revolutionised home cooking.
The use of microwaves in ceramic processing is a relatively recent development. They can be applied effectively and efficiently to heat and sinter ceramic objects. The most recent development in microwave applications is in sintering of metal powders, a surprising application, in view of the fact that bulk metals reflect microwaves. However, reflection by a metal occurs only if it is in a solid, nonporous form and is exposed to microwaves at room temperature. Metal in the form of powder will absorb microwaves at room temperature and will be heated very effectively and rapidly. This technology can be used to sinter various powder metal components, and has produced useful products ranging from small cylinders, rods, gears and automotive components in 30-90 min.
Microwave Heating of Metals
Microwave heating and sintering is fundamentally different from the conventional sintering, which involves radiant/resistance heating followed by transfer of thermal energy via conduction to the inside of the body being processed. Microwave heating is a volumetric heating involving conversion of electromagnetic energy into thermal energy, which is instantaneous, rapid and highly efficient.
The microwave part of the electromagnetic spectrum corresponds to frequencies between 300 MHz and 300 GHz. However, most research and industrial activities involve microwaves only at 2.45 GHz and 915 MHz frequencies. Based on their microwave interaction, most materials can be classified into one of three categories - opaque, transparent and absorbers. Bulk metals are opaque to microwave and are good reflectors - this property is used in radar detection. However, powdered metals are very good absorbers of microwaves and heat up effectively, with heating rates as high as 100°C min-1. Most other materials are either transparent or absorb microwaves to varying degrees at ambient temperature. The degree of microwave absorption, and consequently of heating, changes dramatically with temperature.
Microwave vs. Conventional Heating
The use of microwave energy for materials processing has major potential, and real advantages over conventional heating. These include:
• Time and energy savings
• Rapid heating rates
• Considerably reduced processing time and temperature
• Fine microstructures and hence improved mechanical properties and better product performance
• Lower environmental impact.
Which Metals can be Microwave Sintered?
Until recently, microwave heating has been applied to sinter only oxide ceramics and semi-metals like carbides and nitrides. However, our research reveals that in powdered form, virtually all metals, alloys, and intermetallics will couple and heat efficiently and effectively in a microwave field, and their green parts will produce highly sintered bodies with improved mechanical properties. For example, in our exploratory experiments we tried two common commercial steel compositions, namely Fe-Ni-C (FN208) and Fe-Cu-C (FC208). These formed highly sintered bodies in a total cycle time of about 90 min at temperature range of 1100-1300°C with a soaking time of 5-30 min in forming gas (a mixture of N2 and H2) atmosphere. Mechanical properties such as the modulus of rupture (MOR) and hardness of microwave processed samples were significantly higher than the conventional samples - in the case of FN208, the MOR was 60% higher. The densities of microwave processed samples were close to the theoretical densities, and the net shape of the green body was preserved without significant dimensional changes.
Which Metals have been Microwave Sintered?
Many commercial powder-metal components of various alloy compositions, including iron and steel, copper, aluminum, nickel, molybdenum, cobalt, tungsten, tungsten carbide, tin, and their alloys have been sintered using microwaves, producing essentially fully dense bodies. Figure 1 illustrates some of the metallurgical parts processed using microwave technology. The biggest commercial steel component that has been fully sintered in our system so far is an automotive gear of 10 cm in diameter and about 2.5 cm in height.
Figure 1. Metallic parts produced by microwave sintering such as gears cylinders, rods and discs.
Microwave Sintering Devices
A typical microwave sintering apparatus operates at a 2.45 GHz frequency with power output in the range of 1-6 kW. The sintering chamber consists of ceramic insulation housing (batch system) or an alumina tube insulated with ceramic insulation from outside, figure 2. The primary function of the insulation is to preserve the heat generated in the workpiece. The temperatures are monitored by optical pyrometers, IR sensors and/or sheathed thermocouples placed close to the surface of the sample. The system is equipped with appropriate equipment to provide the desired sintering atmosphere, such as H2, N2, Ar, etc, and is capable of achieving temperatures up to 1600°C.
Figure 2. Schematic of a microwave sintering furnace.
The technology can be easily commercialised by scaling up the existing microwave system or designing a continuous system capable of sintering parts of various shapes and sizes.
Potential for Microwave Sintering of Metals
The implications of microwave sintering of metals are obvious in the field of powder metal technology. Metal powders are used in a diverse range of products and applications in various industries, including the automotive industry, aerospace, and heavy machinery. The challenging demands for new and improved processes and materials of high integrity for advanced engineering applications require innovation and new technologies. Finer microstructures and near-theoretical densities in special powder metal components are still elusive and widely desired. Increasing cost is also a concern of the industry. Microwave processing offers a new method to meet these demands of producing fine microstructures and better properties, and potentially at lower cost.
Why does Microwave Sintering Produce better Properties compared to Conventional Processing?
There are two main reasons why the microwave process yields better mechanical properties, especially in the case of powder metals - it produces a finer grain size, and the shape of the porosity, if any, is quite different than in a conventional part. In microwave-processed powder metal components, we have observed round-edged porosities producing higher ductility and toughness.
Microwave Sintering Mechanisms
So far, there has been little effort devoted to understanding the mechanisms and the science behind microwave sintering of metals. However, it is obvious that the microwave-metal interactions are more complex than those working actively in the field had expected. There are many factors that contribute significantly to the total microwave heating of powdered metals. The sample size and shape, the distribution of the microwave energy inside the cavity, and the magnetic field of the electromagnetic radiation are all important in the heating and sintering of powder metals. This research is just at the early stages, and it will be a long time before the exact mechanisms are elucidated.