R.S. Chen

Correlation of recalescence with grain refinement of magnesium alloys

The grain refinement of Mg-Al based alloys with carbon inoculation was investigated by a computer-aided cooling curve analysis (CA-CCA) system. The results show that carbon inoculation decreases the main parameters of the recalescence regime during the initial stage of solidification. These parameters include the recalescence undercooling (Δθrec), duration of recalescence (trec), and liquid peak parameter (LPP) which is firstly introduced into magnesium alloys. The resultant grain size decreases with increasing nucleation temperature (θn) and decreasing values of Δθrec, trec and LPP.

Thixoformability of a Mg–Gd–Y magnesium alloy and its mechanical properties

Abstract A new magnesium alloy with composition of Mg–8·57Gd–3·72Y–0·54Zr (wt-%, GW94) amenable to semisolid forming is presented. A quantitative investigation of its thixoformability in terms of metallurgical parameters in the semisolid state is performed based on Pandat thermodynamic calculation. The optimised working window for thixoforming is from 581 to 605°C, where the volume fraction of liquid does not change significantly with temperature. The alloy is then successfully thixoformed at 600°C, and a typical thixotropic microstructure is obtained. It is shown that significant improvement of mechanical properties is achieved in the thixoformed (TF) GW94 alloy compared to its permanent mould casting counterpart. This is attributed to an obvious decrease in the amount of porosity and fine distribution of the brittle Mg24(Gd,Y)5 particles in the thixoforming process.

Microstructure and mechanical properties of melt-conditioned high-pressure die-cast Mg-Al-Ca alloy

A new shape casting process, melt-conditioned high-pressure die-casting (MC-HPDC) was developed. In this process, liquid metal was conditioned under intensively forced convection provided by melt conditioning with advanced shear technology (MCAST) unit before being transferred to a conventional cold chamber high-pressure die-casting (HPDC) machine for shape casting. The effect of melt conditioning was investigated, which was carried out both above and below the liquidus of the alloy, on the microstructure and properties of a Mg-Al-Ca alloy (AZ91D+2%Ca (mass fraction), named as AZX912). The results show that many coarse externally-solidified crystals (ESCs) can be observed in the centre of conventional HPDC samples, and hot tearing occurs at the inter-dendritic region because of the lack of feeding. With the melting conditioning, the MC-HPDC samples not only have considerably refined size of ESCs but also have significantly reduced cast defects, thus provide superior mechanical properties to conventional HPDC castings. The solidification behaviour of the alloy under different processing routes was also discussed.

Influences of Tensile Temperature on the Mechanical Properties of Rolled Mg-Zn-Gd Sheets with Non-Basal Texture

Mg-2.0Zn-xGd sheets with non-basal texture were fabricated by common rolling process, which showed excellent ductility and formability at room temperature. In this paper, tensile tests were carried out at moderate temperature along the rolling direction and transverse direction to evaluate the influences of tensile temperature on mechanical properties and formability of the sheet. The microstructural evolution during tensile deformation was also investigated to analysis deformation mechanisms. The results showed that the elongation of the sheets increased from 57% at 373K to 253% at 573°C along the rolling direction, while the yield strength decreased with the increase of tensile temperature. The microstructure observation indicated that twining was one of the deformation modes and no dynamic recrytallization took place during deformation at 373K. With temperature increasing up to 473K, dynamic recrystallization took place and led to finer microstructure. This suggests that the formability of the Mg-Zn-Gd sheets with high ductility at room temperature could be further improved by increasing temperature up to 473K, which could refine the microstructure leading to higher strength during second forming process.

An Extruded and Peak Aged Mg-5Gd-3Y-1Zn-Zr Alloy with High Strength

This article presents an extruded Mg-5Gd-3Y-1Zn-Zr alloy produced by conventional ingot metallurgy, exhibiting high-strength and excellent ductility at room and elevated temperatures, with a low density of 1.84g/cm3. The 0.2% yield strength (YS), ultimate tensile strength (UTS)and elongation-to-failure (EL)at room temperature, of samples in T5 condition, can reach 324MPa, 382MPa and 9.3%, respectively. At 250°C, the UTS can be still higher than 317MPa, with EL of 21%. The excellent mechanical properties were attributed to the high density of the static aging prismatic precipitates in the matrix, dynamic β phase precipitated on the dynamic recrystallization grain boundaries, and bimodal grain size distribution with fine recrystallized grains and large deformed grains with intense basal texture and LPSO phase inside the grain.

Microstructure evolution and anisotropy in Mg-7Gd-5Y-1Nd-0.5Zr alloy (EW75) fabricated by normal and cross-rolling

Abstract Microstructure and texture evolution of an Mg–7Gd–5Y–1Nd–0·5Zr (EW75) extruded alloy after normal rolling (NR) or cross-rolling (CR) up to three passes with 20% reduction per pass were investigated. The results showed that microstructures with finer grains and weaker textures were developed due to the strain state change during cross-rolling between passes in comparison with the conventional normal rolling. It is suggested that, fewer twins and more secondary particles benefit the dynamic recrystallisation and weaken the texture. Tensile tests conducted at room temperature showed that the material subjected to cross-rolling (CR) after three passes resulted in an enhanced ductility and almost isotropic strength, which mainly attributed to the finer grain structure and texture modification with the (0002) poles distributing more homogeneously.