A. Upadhyaya

Processing strategy for consolidating tungsten heavy alloys for ordnance applications

The environmental concern over the use of depleted uranium (DU) alloys as kinetic-energy (KE) penetrator for high strain rate applications has focussed the interest in tungsten alloys. However, in general, tungsten-based alloys exhibit about 10% lower performance than DU at high strain rate. This paper provides an update on some of the processing strategies adopted for fulfilling this objective.

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-Ni-Cu 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 of pure copper powder with varying particle size and porosity.

In recent years, microwave processing of metal/alloy powders have gained considerable potential in the field of material synthesis. Microwave heating is recognized for its various advantages such as: time and energy saving, rapid heating rates, considerably reduced processing cycle time and temperature, fine microstructures and improved mechanical properties, better product performance, etc. Microwave material interaction for materials having bound charge are well established, but for highly conductive materials like metals, there is not much information available to interpret the mechanism of microwave heating and subsequent sintering of metallic materials. The present study describes how the thermal profile of electrically conductive powder metal like copper changes with particle size and also with porosity content; in other words, initial green density when the material is exposed to 2.45 GHz microwave radiation in a multimode microwave furnace.

Effect of conventional and microwave sintering on the properties of yttria alumina garnet-dispersed austenitic stainless steel

The current study examines the effect of heating mode, temperature, and varying yttria alumina garnet (YAG) addition (5 and 10 wt pct) on the densification and properties of austenitic (316L) stainless steel. The straight 316L stainless steel and 316L-YAG composites were heated in a radiatively heated (conventional) and 2.45 GHz microwave sintering furnace. The compacts were consolidated through solid state as well as supersolidus sintering at 1200 °C and 1400 °C, respectively. Both 316L and 316L-YAG compacts couple with microwaves and heat to the sintering temperature rapidly (∼45 °C/min). The overall processing time was reduced by about 90 pct through microwave sintering. As compared to conventional sintering, compacts sintered in microwaves exhibit higher densification and finer microstructure but no corresponding improvement in mechanical properties and wear resistance. This has been correlated to elongated, irregular pore structure in microwave-sintered compacts.

Electrochemical behavior of sintered oxide dispersion strengthened stainless steels

The electrochemical behavior of powder metallurgy (P/M) oxide dispersion strengthened stainless steels (SS) (316L and 434L) have been compared with standard 430 and 316 wrought samples in 0.05 mol/l sulfuric acid. The effects of sintering temperature and yttria addition on the electrochemical behavior have been studied. The behaviour of the dispersion strengthened SS was comparable to that of the straight P/M samples. The straight P/M samples sintered at 1400 °C exhibited better corrosion resistance compared to the samples sintered at 1250 °C and this has been correlated to sintered densities. The P/M austenitic SS were superior to the P/M ferritic SS. Pitting resistance, studied by cyclic polarization experiments in 3.56 wt.% NaCl, of the P/M samples were comparable to the wrought samples. The addition of Y2O3 did not affect the pitting resistance.

Gravitational effects during liquid phase sintering

Abstract Liquid phase sintering (LPS) is routinely used to consolidate dense components. However, there are several problems during LPS that have their origin in gravitational effects. Under terrestrial conditions, to ensure structural stability and net-shaping, this technique is generally limited to high solid contents. In addition, there are several unresolved issues pertaining to the microstructural evolution and compact reshaping during sintering. Experimental conditions present during microgravity processing have allowed liquid phase sintering over a wide range of solid–liquid ratios than possible on earth. A rare opportunity was provided to conduct extensive liquid phase sintering experiments on tungsten heavy alloys aboard the space shuttle Columbia as a part of the International Microgravity Lab (IML-2) and Microgravity Space Lab (MSL-1 and MSL-1R). Results from these experiments show novel behavior associated with microgravity both in microstructural as well as macrostructural evolution.

Microwave sintering of W-18Cu and W-7Ni-3Cu alloys.

Microwave processing is emerging as an innovative and highly effective material processing method offering many advantages over conventional methods, especially for sintering applications. It is recognized for its various advantages, such as: time and energy saving, rapid heating rates, considerably reduced processing cycle time and temperature, fine microstructures and improved mechanical properties which lead to better product performance. Major constraints in conventional sintering of refractory material such as tungsten and its alloys are high sintering temperatures and long soaking times which cause abnormal grain growth and lead to poor mechanical properties. They get further aggravated at smaller (submicron and nano) tungsten powder sizes. This study describes recent research findings; W-18Cu and W-7Ni-3Cu alloys have been successfully consolidated using microwave heating which resulted in an overall reduction of sintering time of up to 80%. The microwave sintered samples exhibited finer microstructure and superior mechanical properties when compared with the conventional samples.

Effect of Fe on the constitution of Cu–W alloys at 1200 °C

The phase equilibria of the Cu–W–Fe system were studied experimentally, except for the Cu–Fe limit region. The isothermal section at 1200 °C was determined from analysis of the phases formed after thermal treatment. Forced sedimentation and immersion techniques were used for a reliable sampling of the liquid involved in the liquid/solid phase equilibria. Equilibrium results were complemented by diffusion couple-type experiments. From the Cu–W side, the phase fields formed with increasing Fe content were: {Cu-based liquid+b.c.c. W}, {Cu-based liquid+b.c.c. W+Fe7W6}, {Cu-based liquid+Fe7W6}, {Cu-based liquid+Fe7W6+b.c.c. Fe} and {Cu-based liquid+b.c.c. Fe}. In the ternary system the homogeneity ranges of the Cu-based liquid, b.c.c. W and Fe7W6 are very similar to those of the binary phases.

Effect of heating mode on sinterability of carbonyl iron compacts

Abstract In recent years, microwave processing has gained wide acceptance as a novel method for sintering metal powders. As compared to conventional sintering, microwave sintering provides rapid and volumetric heating involving conversion of electromagnetic energy into thermal energy within the material. This results in finer microstructures, thereby providing improved mechanical properties and quality of the products. This study examines the dependence of densification, microstructure and mechanical properties on the heating mode of Fe–2%Cu and Fe–2%Cu–0·8%C. The powdered compacts were sintered in conventional (radiation mode) and microwave (2·45 GHz, multimode) furnaces at 1120°C in 90N2–10H2 atmosphere, and comparative analysis of the properties was investigated.

Comparative study of densification and microstructural development in W–18Cu composites using microwave and conventional heating

Abstract Tungsten based composites such as W–Cu have been widely used as electrical contacts, especially in heavy duty applications and as spark erosion electrodes. The lack of solubility between tungsten and copper makes it very difficult to achieve full densification through liquid phase sintering. Higher sintering temperatures or longer holding times always help to improve the densification but Cu may leach out from the skeleton which leads to Cu segregation and results in non-homogeneous microstructure and poor product performance. Microwave heating has been increasingly gaining popularity in the field of sintering of particulate materials. As compared to conventional heating, microwave heating is more rapid resulting in substantial reduction in the overall sintering time. In addition to the energy efficiency, the faster heating rate achieved in microwave furnaces minimises microstructural coarsening and improves homogeneity. This study examines the effect of heating mode (conventional and microwave) and temperature on the consolidation of specially prepared commercial W–Cu powder. Near theoretical density has been achieved under optimum conditions in microwave sintering. The bulk hardness and electrical conductivity of the samples sintered by two methods have been determined and the data compared.