Low-pressure die casting of magnesium alloy AM50: Response to process parameters

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Abstract

Low-pressure die casting (LPDC) process has been successfully used to produce sound magnesium alloy AM50 castings. The influence of process parameters: filling time, pressure holding time, die temperature, holding pressure and casting temperature, on the mechanical properties, microstructure and density of LPDC castings were studied. The optimal process parameters for LPDC casting have been experimentally determined as follows: filling time 15–22 s, pressure holding time 8–12 s, die temperature 390–410 °C, casting temperature 705–710 °C and holding pressure 0.08 MPa. The mechanical properties of the experimental part under the optimal process conditions are: yield strength (YS) 57.5 MPa, ultimate tensile strength (UTS) 193.8 Mpa and elongation 9.1%.

Introduction

Magnesium alloys represent unique structural materials combining high specific strength with good castability. Magnesium castings are increasingly used in the automotive and electronic industries (Luo, 2002, Mordike and Ebert, 2001). Although magnesium alloys are mainly used as high-pressure die casting (HPDC) components, future automotive applications require higher mechanical properties, especially the ductility and fatigue strength, which can not be met by HPDC due to the inherent high porosity generated in the HPDC process. There have been many reported studies on the microstructure and mechanical properties of HPDC of AM50 magnesium alloy. Micro- and macro-porosity are reported as the most important defects that seriously reduce the mechanical properties and fatigue life of the HPDC AM50 castings (Zhou et al., 2005, Lee et al., 2005a, Han et al., 2005, Chadha et al., 2004, Gokhale et al., 2004, Regener and Dietze, 2003, Donlon et al., 1995). Most of pores are gas-pores, caused by entrapped gas during the high-speed die fill process, and the porosity distribution is heavily influenced by wall thickness (Zhou et al., 2005, Han et al., 2005, Chadha et al., 2004, Regener and Dietze, 2003). The process parameters have effects on the mechanical properties of HPDC AM50 castings, and it was reported (Lee et al., 2005b, Lee et al., 2004, Peete and Winkler, 1993) that casting die temperatures, pressure and gate velocity have the most significant influence on the soundness of the castings.

Many casting processes can be potentially used for structural cast magnesium components. Low-pressure die casting (LPDC) is one of the most promising processes. A low-pressure die casting machine usually includes a pressurized melt furnace located below the die table with a feeding tube running from the furnace to the bottom of the die. A schematic diagram of a typical LPDC machine is shown in Fig. 1. The process is an application of Pascal's pressure theory. The surface of molten metal in the furnace is pressed by a dry protective gas at relatively low-pressure so that to overcome the difference of metallic pressure between the die and the surface of the molten metal. Molten metal is then forced to rise through the riser tube, feeder and gating system, and consequently feeds the die cavity. When the die cavity is full, the exerting pressure is increased to pressurize the casting and improve the feeding of shrinkage during solidification. Once the casting is completely solidified, the external pressure is released and the molten metal not yet solidified in the feeder and the riser tube flows back down to the furnace by the action of gravity.

LPDC has been well established in the aluminum casting industry, and commercial equipments are readily available, while this is not yet the case for magnesium alloys. Producing magnesium alloy parts using low-pressure casting process has the potential advantages of low porosity and semi-automatic production, thus better casting quality and high productivity (Westengen and Holta, 1989). In spite of the many potential advantages, the low-pressure casting process has not yet been fully appreciated and used in magnesium production. While there are many publications dealing with the relationship between process parameters and mechanical properties on HPDC AM50 (Peete and Winkler, 1993, Lee et al., 2005b, Lee et al., 2004), very few on LPDC magnesium castings (Gertsman et al., 2005).

In the present paper, the effects of LPDC process parameters on the microstructure and mechanical properties of magnesium alloy AM50 are examined. Optimal process conditions of LPDC are experimentally determined. The relationships between microstructure and mechanical properties of LPDC AM50 alloy are also studied.

Section snippets

Process parameters

There are many process parameters in LPDC that are very important to casting quality, such as filling pressure, fill speed, holding pressure, pressure holding time, casting temperature, die temperature, etc. In the present study, the effect of fill speed, pressure holding time, casting temperature and die temperature were studied on the LPDC casting as shown in Fig. 2.

A crucial part of the LPDC operation is the control of the exerting pressure in the crucible to ensure a laminar flow of molten

Effect of filling time

The effect of filling time on the mechanical properties and density of LPDC AM50 alloy is shown in Fig. 7. With increasing filling time, the mechanical properties, i.e., yield strength (YS), ultimate tensile strength (UTS) and elongation, and the density of the AM50 castings tend to decrease. From 15 to 22 s, the mechanical properties show only slight reduction. From 22 to 25 s, however, the mechanical properties drop considerably. The density vs. filling time shows similar trend as the

LPDC vs. gravity casting/HPDC: microstructure and mechanical properties

The microstructure of LPDC AM50 is different from that of gravity casting, as shown in Fig. 8, Fig. 9. Compared to the gravity casting, there are fewer and finer Mg17Al12 and Al8Mn5 (Gertsman et al., 2005) particles in the LPDC AM50, but with coarser grains (Fig. 10, Fig. 13, Fig. 16, Fig. 18). Defects such as entrapped gas porosity, defects bands, shrinkage porosity, oxides and carbonate films are significantly reduced in the LPDC samples. These defects are known to reduce the mechanical

Conclusion

  • (1)

    High-integrity AM50 castings can be produced by using LPDC process, with higher ultimate tensile strength (UTS) and elongation due to reduced shrinkage/porosity, but lower yield strength (YS) due to coarser grains, compared to HPDC. The tensile properties (YS, UTS and elongation) of the LPDC AM50 castings are all improved compared to the gravity permanent mold castings.

  • (2)

    With increasing filling time, the grains become coarser, leading to lower mechanical properties (YS, UTS and elongation) and

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