Haiping Cao

Determining effect of slurry process parameters on semisolid A356 alloy microstructures produced by RheoMetal process

Abstract The RheoMetal process, based on the rapid slurry forming (RSF) technology, is a new rheocasting process enabling the production of semisolid slurry of high quality and with high efficiency. The RSF technology is based on an enthalpy exchange between two alloy systems rather than applying external cooling as is done in many other rheocasting processes. In this work, several process parameters important for the RSF technology have been investigated on aluminium A356 alloy slurry microstructures. During slurry preparation, the thermal history for the different parameters studied was also recorded. It was found that the process parameters tested (amount of solid addition, melt superheat, rotation speed) have an effect on the thermal history during processing and on the microstructures formed after slurry formation.

Effect of injection velocity on porosity formation in rheocast Al component using RheoMetal process

Abstract Rheocasting is on its way to establish as a cost efficient method for producing high integrity aluminium components. There is still, however, a need to learn more about the possibilities and limitations of this relatively new process in industrial use. This especially relates to ingate design and machine process parameters, e.g. shot profile, die temperature and intensification pressure, for obtaining sound quality. A component was produced by rheocasting at varying phase II velocities in a normal high pressure die casting machine supplied with a RheoMetal station. The component quality was investigated in the most massive section of the casting, using X-ray and metallographic techniques. The pore content and its distribution were found to strongly depend on the injection velocity where a low velocity is preferred due to the low risk for air entrapment and the higher fraction solid obtained at completed die filling, thereby reducing the solidification shrinkage.

The effect of Si Content on the Mechanical Properties of Rheocast Al Components Using the RHEOMETAL Process

The RHEOMETALTM process is a commercially used semi-solid process for production of high integrity cast components. The process differs from most other semi-solid casting processes in that temperature control is not necessary during processing and large amount of slurry with required solid fraction can be quickly produced. The simplicity of this process has led to a large commercial interest during the last year. This work is based on an investigation regarding the variation of as-cast mechanical properties for secondary Al-Si based alloys (~2.5 % Cu) with a Si content varying from 4.55 to 8.90 % using the RHEOMETALTM process. The purpose was to find the most suitable Al-Si alloy for rheocasting, in comparison with the common HPDC-alloy A380 (EN-AC46000). It was found that lower Si containing alloys exhibited better elongation but slightly lower yield strength. The alloy containing 5.39 wt% Si showed the highest ultimate tensile strength in this investigation. The lower Si containing alloys also demonstrated better feedability in the rheocasting process, which is contrary to what normally is found for normal liquid casting processes. Based on the results in this investigation it is recommended to use an alloy containing about 5-7 wt% Si for rheocasting purposes.

On some characteristics of microstructure and defects in die-cast magnesium components

Some commercial Mg components have been die cast at various casting conditions. The influence of process parameter settings on the microstructural features as well as one kind of defect called segregation band defect were investigated. It was found that more pre-solidified crystals were formed in the condition of lower casting temperature, and more broken dendrites and spherical crystals were formed under higher injection speed. The distribution characters of pre-solidified crystals were also presented and discussed in this paper. Concerning the band defects formed during die casting, a variety of segregation band morphologies as well as the influence of various casting conditions the performances on bands was presented. It was found out that intensification pressure has the strongest effect on the performance of segregation bands inside the castings.

The Effect of Mould Constraints on the 0.2% Proof Stress of As-Cast Mg-Al Alloys

When a casting solidifies and cools down in a metallic mould, stresses and strains develop as a result of uneven temperature distributions, metal shrinkage and mould constraints. At ejection from the mould, the casting usually springs back slightly when the elastic part of the strain is released. However, due to the low flow stress at high temperatures it is likely that the casting also has experienced some plastic deformations, meaning that the dislocation density has increased. This paper discusses how the existence of remaining plastic deformations affects the initial flow behavior and the yield strength during tensile testing of as cast Mg-Al alloys with different degrees of mould constraints.

Mechanisms of segregation band defect formation in pressure die-cast magnesium components

Segregation bands which normally follow the outer contours of a casting are common in commercial magnesium alloy pressure die castings. Several models have been proposed in the literatures which attempt to explain mechanisms behind the formation of this type of segregation bands. However, it is difficult to explain some phenomena which occur in real die cast components. In this paper, a new theory concerning the formation of one common and detrimental segregation band defect (Type I) has been proposed, which is based on a coupled analysis of heat flow and volume changes during solidification. The formation of this type bands was related to a pressure drop in the liquid and resulting flow of segregated liquid from the surrounding two-phase regions. Mechanism on the formation of the other type segregation band (Type II or under surface band) is also proposed. The sudden increase of cooling rate at the moment of applying intensification pressure is believe to has main contribution to the formation of this type of bands.