Microwave Sintering of Refractory Metals/alloys: W, Mo, Re, W-Cu, W-Ni-Cu and W-Ni-Fe Alloys

Abstract

Refractory metals and alloys are well known for their high mechanical properties which make them useful for wide range of high temperature applications. However, owing to the refractoriness of these metals and alloys, it is very difficult to consolidate them under moderate conditions. Conventional P/M processing is a viable sintering technique for these refractory metals. One of the constraints in conventional sintering is long residence time which results in undesirable microstructural coarsening. This problem gets further aggravated when using smaller (submicron and nano) precursor powder sizes. Furthermore, conventional heating is mostly radiative, which leads to non-uniform heating in large components. This review article describes recent research findings about how these refractory metals and alloys (W, Mo, Re, W-Cu, W-NiCu and W-Ni-Fe) have been successfully consolidated using microwave sintering. A comparative study with conventional data has been made. In most cases, microwave sintering resulted in an overall reduction of sintering time of up to 80%. This sintering time reduction prevents grain growth substantially providing finer microstructure and as a result better mechanical properties have been observed.

Microwave Heating, Isothermal Sintering, and Mechanical Properties of Powder Metallurgy Titanium and Titanium Alloys

“…[5][6][7][8][9] Magnetic and refractory powder metals appear to respond well to MW radiation compared with other powder metals which often show mild or weak responses. [10][11][12][13] Using the Heisenberg model, Tanaka et al [14] have recently shown that MW radiation can heat magnetic metal oxides to temperatures well above their Curie temperatures while it is essentially ineffective for non-magnetic oxides. Their modeling identified that such fast heating ''is caused by nonresonant response of electron spins in the unfilled 3d shell to the wave magnetic field,'' [14] and the effect ''persists above the Curie temperature T c because each electron spin is able to respond to the alternating magnetic field of MWs even above T c .''…”

A Comparative Spectrophotometric Study Using Ferrozine and 1,10-Ortho-phenanthroline to Evaluate the Iron Redox Ratio (Fe2+/ΣFe) in Glass Prepared by Microwave Heating

“…4,5 Many authors have demonstrated microwave material processing to develop materials with improved or new properties. [6][7][8][9][10] Improved properties such as micro-hardness and low OH content in borosilicate glasses have been identified elsewhere. 11,12 Improved chemical durability and low contamination from the crucible wall has also been observed in borosilicate and phosphate glass.…”

Microwave sintering of metal powders

“…at 1500°C, respectively, indicating that the microwave sintered sample has much smaller grain size than in conventional heating. 15 …”

“…Some refractory metals such as w, re, Mo, and w-based heavy alloys were microwave sintered at much lower temperatures and sintering times than normally used in a conventional process. 15 Figure 12.6 exhibits microstructures of microwave sintered nano-w powders which have been doped either with Y 2 O 3 or with HfO 2 as grain growth inhibitors. 16,17 It is to be noted that microwave sintering at 1400°C for 20 min produced submicrometer size microstructures and densities in the order of 95+%.…”