Materials Science Forum Vols. 488-489

Abstract: The present paper describes the microstructural evolution of AZ31 alloy along the process chain, from the melt treatment, the direct chill casting up to the extrusion process. Each step in the process chain will be considered in order to design the appropriate properties of the endproducts. The research has been done in industrial scale at ARC Leichtmetallkompetenzzentrum Ranshofen GmbH (LKR), in order to analyse the grain size dependency of AZ31 alloy on different casting conditions. The casting trials with and without grain refinement were carried out at the vertical direct chill casting machine MAGNUMCAST®. After the extrusion process metallographic analysis and mechanical tests were carried out.

Abstract: In order to achieve further reductions in the weight of the body-in-white, efforts are being made in the field of car design to replace structural elements by magnesium components. In addition to the use of extrusion and sheet metal processes, die casting in particular is an especially important method of producing thin-walled, highly integrative components, because of the very good casting properties of magnesium. Integrating of die casted components into a vehicle structure calls for joining techniques which offer the maximum utilization of the materials of the joined parts under operating loads. This article discusses the necessity for material-specific joining techniques for future magnesium body structures. Using the example of aluminium/magnesium joints, the article describes the benefits of different joining techniques with regard to their efficient use of material in the case of both quasi-static and dynamic loads.

Abstract: This study seeks to analyze the presence method and quantity of pore during hybrid laser-tungsten inert gas arc (TIG) Welding of Magnesium Alloy AZ31B by radiography, optical microscopy and electron probe microanalysis (EMPA). At the same time, it identifies both the mechanism of pore formation and a remedy for this problem. The experimental results indicate that lacking of shielding gas for laser beam is the dominant cause of macro-porosity formation during the hybrid of laser-TIG welding of magnesium Alloys AZ31B plate, and hydrogen is not main cause to form large pores. A favorable weld without porosity can be obtained by appending lateral shielding gas for laser beam.

Abstract: In modern car concepts the aspect of lightweight constructions becomes more and more important. Lightweight materials, as aluminum and magnesium, get in the spotlight, thereby. Particularly because of the enormous potential for lightweight constructions industrial interests in magnesium wrought- and casting materials have increased in recent years. Against this background new alternative methods in the range of joining techniques are necessary which consider the specific mechanical-technological properties, such as limited deformability at room temperature and high corrosion-affinity of magnesium. The present article discusses the integration of heating principles in a mechanical joining process of magnesium components without an additional pre-punch operation. In this connection, feasibilities and limits of the considered joining techniques are shown and a concept for thermal support is presented.

Abstract: This paper presents results of recent investigations on the weldability of several wrought (AZ31, AZ61) and cast magnesium-based alloys (AZ91) by laser-TIG welding process. The investigations showed that magnesium alloys can be easily welded by laser-TIG welding. The grain of the fusion zone was finer than that of in base metal. The width of the heat-affected zone welded by laser-TIG welding process was obviously narrower than that of welded by TIG. Besides, with the Al content of magnesium alloys increasing, the width of the heat-affected zone (HAZ) was increased，as well as the content of β phase(Mg17Al12). The hardness in the fusion zone (FZ) and in HAZ of AZ61 and AZ91 has a large change to the base metal due to the existing of β phase, while no change relative for AZ31. It results from above discussing that laser-TIG welding is an excellent welding process for magnesium alloys.

Abstract: Thixomolding® is a new process of net shape parts for magnesium alloys in one step, and it is suited to manufacture the thin wall products, especially to the electronic component shells. In this paper, the sample of mobile phone housing is fabricated by 220Tons Thixomolding® machine made by Japan Steel Works. The effects of barrel temperature, screw rotate rate and mold temperature on microstructure were studied. The relationship between process parameters and microstructure was also discussed in this paper.

Abstract: 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.

Abstract: Nd, Y and Ca containing Mg-Zn-Zr alloys are produced by electromagnetic direct-chilling casting process, and extruded at a temperature of 643K with two extrusion ratios of 38:1 and 22:1, respectively. The grain size is markedly reduced from 80µm in as-cast alloy to 2~5µm in as-extruded alloy due to dynamic recrystallization, and lamellar eutectics at grain boundaries in as-cast alloy are broken up and fine precipitates in the matrix come forth during hot extrusions. Mechanical properties of the alloys are measured by tensile test from room temperature to 523K. Nd, Y and Ca are favorable to the strength of the hot-extruded alloy, especially the elevated-temperature strength, which is above 200MPa in ultimate tensile strength at 523K.

Abstract: The plastic forming of magnesium alloy is difficult, but the semi-solid material forming is a good method solved this problem. The mechanical model of the semi-solid materials was treated as that of the continuous porous materials in the high solid volume fraction. The upper bound theory applied for semi-solid metal plastic forming process was developed. The velocity discontinuities exist not only in the tangential component but also in normal component for the kinematically admissible displacement increment filed. The latter one was responsible for a change in solid volume fraction when the material passes the discontinuity. An upper bound analytical model and theoretical method of plastic forming process for semi-solid material has been proposed. The calculating formulas of deformed power were derived. It is theoretical basement to apply further for the practice technology analysis such as the plastic forming of magnesium alloy.

Abstract: Most magnesium alloy components available for automobile are made through die casting. In this paper, PAM-CASTTM, commercial die casting simulation software based on the finite difference method, is employed to simulate the low-pressure die casting process of magnesium wheel hub. The temperature field and velocity field during filling and solidification stages are analyzed; the evolution of temperature distribution and liquid fraction was numerically studied. Then, the potential defects including the gas entrapments in the middle of the spokes, shrinkages between the rim and the spokes are predicted. The cooling performance of mould during casting is also investigated. Via analyzing the shrinkage defects generated under various cooling conditions, the cooling system set in the side mould is found to be more effective for enhancing the cooling capacity at the rim/spoke junction areas. With this cooling system, the hot spots at the junctions are obviously reduced and product quality is improved.