Microstructural Evaluation and Mechanical Properties of an Al-Zn-Mg-Cu-Alloy after Addition of Nickel under RRA Conditions

The effects of nickel in improving the mechanical properties and microstructural of Al-Zn-Mg-Cu alloys produced by semi-direct chill casting were investigated. Aluminium alloys were homogenized at different temperatures conditions, which aged at 120°C for 24 h (T6 temper), and retrogressed at 180°C for 30 min and then re-aged at 120°C for 24 h (RRA). The results of the microstructural analyses showed that with adding nickel to aluminium alloy, nickel-rich dispersoid particles were formed, such as Al7Cu4Ni, Al4Ni3, Al75Ni10Fe15, Al3Ni2, and Al50Mg48Ni7. Intermetallics compounds within the matrix alloy led to dispersion and fine-grain mechanisms which prevent the recrystallization and grain growth. Enhancement of mechanical properties of the alloys study is obtained through the precipitation hardening of alloying elements of the base alloy besides Ni-bearing dispersoid particles. The microstructure of these alloys were examined through optical and scanning electron microscopy along with energy dispersive X-ray and X-ray diffraction.

Synthesis and Densification of Tungsten-Brass Composite by Mechanical Alloying

Fabrication of full-density W-brass composites is very difficult to achieve because of evaporation of zinc, insolubility of W and brass and compacts expansion. In this study, to achieve full-density W-brass composites, mechanical alloying (MA) and activated sintering process were utilized. Mechanical coating of W with Ni using high energy planetary ball mill was carried out. The milling was divided into two stages: to alloy and modify the surface of W with Ni for enhanced activation. The microstructure of the milled powders and sintered compacts, elemental composition and phases present were studied by using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) respectively. As-received powder compacts was also sintered under the same condition for comparison purpose. The effects of milling time on the microstructure, sinterability and the hardness of the composites were investigated. It was observed that the samples produced from 8 h milled powder had the highest relative sintered density (98% TD) and microhardness (234 Hv). On the other hand, the samples from the as-received powders expanded and had a relative sintered density of (67% TD) and microhardness as low as 24 Hv. The significance of this study is the possibility of producing W-brass composites as a cheaper alternative to W-Cu composites.

The Effects of Sintering Temperature on the Densification of Mechanically Alloyed W-Brass Composites

Prealloyed (PA) and premixed (PM) W-brass with the composition of 60 wt% W, 1 wt% Ni and 39 wt% brass was sintered at the temperature of 800&degC, 920&degC and 1000&degC each. As a result of difficulties in the densification of W-Cu and W-Cu alloys, mechanical alloying (MA) and activated sintering were combined. The powders were mechanically alloyed for 13 hours to produce nanosized W grains embedded in brass. The microstructure and properties of these composites with increase in sintering temperature has been studied. Both prealloyed and premixed composites sintered at 800&degC (solid state sintering) and 920&degC (sub-solidus state sintering) have lower sintered densities and hardness. The densification rate in the premixed composites was observed to be higher than that of the prealloyed composites. Their densification and properties increased with the increase in the sintering temperature. Premixed composite sintered at 1000&degC had 91.0% sintered density, 180 Hv microhardness against 76.0% and 133 Hv respectively for prealloyed composite at the same temperature. The values of electrical conductivity in both prealloyed and premixed composites increased with increase in temperature.

Sintering Behavior and Microstructure Evolution of Mechanically Alloyed W-Bronze Composite Powders by Two-step Ball Milling Process

The miscibility of W in Sn and Cu is extremely poor. Sintering of W-bronze composites to their full density is proved to be difficult. To tackle this problem, the ball milling process of the W-bronze powder mixture proposed in this study was split into two steps. In the first step, the softness of Sn powder was exploited to modify the surface morphology of W particles. In the second step, Cu was added to the ball milled mixture. To achieve this goal, four 50 wt% W-bronze compact sets of different powder precursors and activator additions were produced. The sintering process was performed at 1150 ℃. The two-step ball milled powder yielded sintered compacts of homogeneous microstructure of fine polygonal W grains dispersed in bronze matrix. They showed the highest hardness and densification levels. The sintered density, microstructure, hardness and densification mechanisms of the sintered compacts were investigated, examined and evaluated using different metallographic, microscopic and measurement facilities.