Materials Science Forum Vols. 828-829

Abstract: Hot tearing is the one of the biggest problems when casting aluminium alloys. Although there has been much research work in this field, it is still not clear what causes hot tearing and how it propagates. Furthermore, the effect of solidified structure on the hot tearing has not been clear. Therefore, this study has been carried out to correlate the solidified structure and hot tearing. Al-2.0 wt% Cu alloy was used for test alloy. To change the solidified structure, some amount of refiner was added to the molten alloy. A permanent mould, which has been developed by us, can form hot tearing intentionally in the center region of a solidified shell. The cross section of a solidified shell was metallographically investigated and solidified structure and hot tearing were characterized. The length of hot tear decreased with increasing the amount of refiner. This may indicate that it is difficult for hot tear with fine equiaxed grains to propagate. In addition to this, the residual liquid around the hot tear was moved to the cracking due to negative pressure and hot tearing was partially healed.

Abstract: Plastic optical components and lenses produced in mass quantities are usually manufactured using high-precision plastic injection technology. For that, high-precision plastic moulds with aluminium optical inserts made with extremely high dimension accuracy and high optical surface quality are used. Ultra-high precision single-point diamond turning have been successfully used in shaping optical mould inserts from various aluminium grades such as traditional 6061. However, extreme care should be taking when selecting machining parameters in order to produce optically valid surfaces before premature tool wear takes place especially when the machined optical materials has inadequate machining database. The current experimental study looks at the effect of cutting conditions on optical surfaces made from aluminium. The study embarks on helping establish some diamond machining database that helps engineers select the most favourable cutting parameters. The papers reports on the accuracy and surface finish quality received on an optical surface made on mould inserts from a newly developed aluminium alloy. Rapidly solidified aluminium (RSA) grades have been developed recently to address the various problems encountered when being cut by single-point diamond turning operation. The material is characterised by its extremely fine grained microstructure which helps extend the tool life and produce optical surfaces with nanometric surface finish. It is found the RSA grades can be successfully used to replace traditional optical aluminium grades when making optical surfaces. Surface finishes of as low as 10 nanometres and form accuracy of less than one micron can be achieved on RSA.

Abstract: Surface quality as well as internal quality of cast products of aluminum alloys are strongly affected by the process of initial solidification. Control of solidified structure in this region is therefore quite important. In order to understand the growth of solidified grain, crystallographic characterization has been performed using EBSD (Electron Backscattered Diffraction) in this study. Al-6 mass%Si alloy was cast at 750°C on the chill plate. Longitudinal cross section of solidified shell was analyzed. In the region of initial solidification, many small crystals nucleated on the mold surface. The crystallographic orientations of these grains were random. It is normally found that an unfavorable grain was eliminated by a favorable grain. However, occasionally, we have found that an unfavorable grain enlarged its size. In this case, dendrite, the growth direction of which was far from the heat flow direction, gradually changed its crystallographic orientation from unfavorable one to favorable one. The grain enlarged its size by multiplication of dendrite arms. Crystallographic orientation of dendrite changed little by little when it branched. This kind of phenomena may take place in unsteady condition, such as initial solidification region.

Abstract: It is reported actual volume fraction of the primary phase in alloys is larger than the equilibrium value. Larger volume fraction of the primary phase may cause shrinkage cavities and surface or internal cracks. Although control of the solidified structure is important for the quality of cast products, this problem has not been fully elucidated. Taking these results into account, this study has been carried out in order to comprehend the phenomenon of larger volume fraction of primary phase. The Sn-Pb alloy has been used as a test alloy to examine the relation between supercooling for nucleation and the volume fraction of primary phase. Actually, the volume fraction of the primary phase in Sn-Pb alloy is larger than that of the lever rule. It was also observed that the volume fraction of β-Sn decreases with decreasing the supercooling in the early stage of solidification. In the final stage of solidification, however, the effect of supercooling on volume fraction of primary phase is small. Futhermore, when the supercooling was low, the volume fraction of primary phase slowly increased.

Abstract: Mg especially in the molten state is well known for its high affinity to O₂. When O₂ content of the atmosphere is larger than 4%, molten Mg will burn! To avoid this, melt protection is necessary. At present mostly SF₆ is used during primary production and processing of Mg and its alloys. Unfortunately SF₆ is a very potent greenhouse gas that is > 23,000 times more effective than CO₂. This also affects life cycle considerations e.g. for the use of Mg alloys in transportation. However, other protective gases like SO₂ or fluorinated hydrocarbons like HFC134a, Novec 612, or AMCover (=HFC134a) have been suggested to replace SF₆. Additionally fluxes mixed from different salts may be used again as well to protect molten Mg. But fluxes and feasible replacements of SF₆ have also disadvantages. Moreover SF₆ and other fluorinated hydrocarbons are under discussion especially in Europe. There is an existing EU legislation that will ban SF₆ from 2018 and there are similar discussions regarding all other fluorinated hydrocarbons. Due to this, new innovative ways have to be found or old methods have to be renewed to allow Mg industries further safe processing of molten magnesium. This contribution will report the state of the art in protecting molten Mg and alternatives to the use of SF₆.

Abstract: Steering wheel and headlamp housing scrap are chosen as targets for the development of a refining technology for highly contaminated magnesium melt, because they contain abundant foreign materials, which have high potential to form non-metallic inclusions and to deteriorate the corrosion resistance of recycled alloys. A sequential melt treatment technology has been investigated for refining old scrap. The headlamp housing scrap composed of AZ91D magnesium alloy covered with phosphating and Al-Si deposition layers was successfully refined by repeating gas bubbling without flux addition, whereas prefiltering and fluxing were necessary to refine the steering wheel scrap composed of AM50A magnesium alloy containing 1% polyurethane. However, the corrosion rates of the recovered ingots from the headlamp housing were relatively higher than those from the steering wheel, likely due to the greater concentration of iron with the increased amount of flux.

Abstract: The novel melt conditioned twin roll casting (MCTRC) process, in which the melt is conditioned by intensive shearing prior to twin roll casting, has allowed magnesium sheets to be produced with a fine and uniform microstructure and substantially reduced segregations across the sheet thickness. It is thus possible to eliminate the extensive downstream processing via repetitive hot rolling, which is required after conventional twin roll casting, and to produce sheets to the required thickness for forming. The present work was conducted to study the feasibility of producing magnesium sheets ready for stamping by the MCTRC process, focusing on the development of microstructures and textures. An AZ31 magnesium alloy was used in the investigation and MCTRC experiments were carried out to produce sheets of 6 mm and 2.5 mm in thickness respectively. After MCTRC, the 6 mm sheet was processed following the conventional procedures via homogenization, hot rolling and annealing, whereas the 2.5 sheet was only homogenized. Experimental results showed that: 1) the as-cast microstructures for both sheets were similalr in terms of grain size and distribution and their texture intensity and components were also similar, being dominated by basal components with a small fraction of primatic components; 2) downstream processing by hot rolling substantially intensified the basal textures for the 6 mm sheet; 3) the 2.5 mm sheet subjected only to homogenization after casting showed a grain structure similar to that obtained after repetitive hot rolling and annealing with substantially weakened textures. Mechanisms of texture formation and development during MCTRC and downstream processing are discussed in the paper.

Abstract: The difficulties and issues associated with the economics of the process and die life in casting aluminium alloys, as experienced by the high pressure die casting industry, were reasons behind undertaking this research project. The use of a tungsten alloy able to withstand high temperature process conditions without the welding problems experienced by standard die construction materials, such as H13, was examined in an extensive series of casting trials. The importance of operating dies at elevated temperatures to minimize heat checking has been demonstrated previously, both through theoretical thermal modelling and experimentation. This paper describes both aspects of die life extension and possibilities to reduce the amount of alloy material used in the cast part feed system, including overflows. CSIR intends using the results of this research for further development and application of high temperature die construction materials in high pressure die casting processes of light metal alloys.

Abstract: Al-Cu-Mg-Ag alloy 2139 is a wrought alloy that is heat-treatable and used in aerospace constructions. This is mainly due to the addition of silver in the alloy. Hot-tearing is a problem for casting wrought alloys with conventional liquid casting techniques. The risk is reduced by using rheo-high pressure die casting (R-HPDC) to allow the alloy to be used for near-net shape forming of components. This study investigates the segregation characteristics of R-HPDC alloy 2139. The effects of segregation on the age-hardening response as well as the tensile properties are studied. The chemical composition differences across the casting were found to aid in correlation of results. It is found that segregation occurs in three dimensions, namely from the surface to the centre of the casting, as well as across the width and length of the casting.

Abstract: The investment casting technology is one of the major methods to produce the parts of the titanium due to its low production cost. However, the high activity of titanium melt gives rise to the requirement of high chemical stability of shell materials, to avoid or decrease the interfacial reaction between the mould and the melt. In this paper, a novel BaZrO₃ – coated Al₂O₃ shell was first introduced to the investment casting of titanium alloy. The grain size and baking temperature on the properties of the novel mould were investigated, and then the Ti6Al4V and TiNi alloys were successfully casted by means of this shell. The alloy-mould interaction was discussed. The results showed that the mould achieve high mechanical properties when the content of coarse powder was 50% after sintering 4 hours under 1500°C, and the BaZrO₃ coating exhibited an effective barrier to avoid the direct contact between the mould base material and the melt, the thickness of reaction layer of TiNi alloy was about 8 μm, and 17 μm to Ti6Al4V alloy, no refractory particles and elemental diffusion were observed inside the metal. This may imply that BaZrO₃ is a promising candidate material for the investment casting of titanium alloy.