Effect of Processing Parameters on the Mechanical and Structure Properties of 93W–4.9Ni–2.1Fe Tungsten Heavy Alloy

Abstract

The objective of this experimental study is to investigate the effect of the main compaction and sintering parameters on the micro-structure and the mechanical properties of the liquid phase sintered 93%wt.W–4.9%wt.Ni–2.1%wt.Fe tungsten heavy alloy aiming at determining the optimum values of these parameters. Elemental powders were mixed using planetary mixer for 5 hours to ensure suitable homogeneity. Uni-axial compaction was applied to obtain standard tensile and impact specimens using compaction pressures ranging from 50MPa to 300MPa. Vacuum liquid phase sintering was carried out under different temperatures from 1460ᵒC up to 1500ᵒC and sintering time from 30 minutes up to 120 minutes. The effect of these parameters was characterized in terms of density, hardness, impact resistance and tensile properties. Microstructure variations, notably grain size, matrix volume fraction and contiguity were measured and used to explain the effect of sintering temperature and time on the properties. The obtained results indicated that optimum hardness and impact resistance can be obtained at a compaction pressure of about 200 MPa. As the sintering temperature increases, the grain size and volume fraction of matrix increase, while the contiguity decreases. As a result of these micro-structural changes, strength and hardness decrease. On the other hand, ductility and impact resistance increase with sintering temperature to some maximum at 1480ᵒC. At further increase of this temperature, grain growth becomes the dominant factor leading to a sensible decrease in all properties. The effect of sintering time on mechanical properties isrelated to grain growth and pore coarsening mechanisms. It is generally observed that strength and ductility decrease after sintering over 90 minutes at 1480ᵒC. It was also noticed that the ductility is more sensitive to sintering time and is reduced sharply with prolonged holding at the sintering temperature. It can be concluded that the mechanical properties of tungsten heavy alloys are sensitive to the processing cycle and are adversely affected by residual porosity. Moreover, tungsten grain size plays an important role in dictating the failure mode during tensile testing. If tungsten grain size is large interface failure is predominant. However, as the grain size decreases, fracture mode changes gradually from interface failure to matrix failure and then to tungsten grain cleavage failure.