Darryl Albright

EMERGING TRENDS IN CORROSION PROTECTION OF MAGNESIUM DIE-CASTINGS

Due to the improved corrosion resistance of magnesium alloys brought forward by the development of the high purity grades 20 years ago, the use of die cast magnesium components has moved towards more and more demanding applications. The corrosion resistance is sufficient so that magnesium is used in the under-body environment of vehicles without additional general corrosion protection, although special attention must be given to galvanic corrosion protection. There has recently been an increasing interest in both semi-exterior and exterior automotive parts. Since magnesium alloys are not compatible with standard automotive phosphate treatments, the parts need to be pre-coated prior to assembly. For such components, the challenges are to find efficient, environmentally friendly pre-treatments and robust coating systems that can maintain their integrity through the assembly process. The present paper reviews some of the established and newly developed methods for corrosion protection and finishing of die cast magnesium automotive parts.

Friction Stir Welding of Magnesium Die Castings

Friction stir welding (FSW), being a solid-state process, is an attractive method for joining magnesium die castings. In this study, FSW of AZ91D and AM50A plates was performed both on the individual alloys and to join them together. The welds were sound and free from defects, except for small surface cracks in AM50A; a fine microstructure characterized the weld zones. The tensile properties of specimens transverse to the weld zone were measured, as were the corrosion properties. The tensile properties were somewhat lower than the base metal, with the largest percentage decrease found in the elongation of AM50A, perhaps due to the surface cracking. The corrosion resistance of the weld zone was relatively poor, most likely due to iron contamination from wearing of the tool. Further optimization of the FSW tool design and process parameters must take place to improve the reliability of FSW for magnesium die castings.

A Review of the Life Cycle Environmental Performance of Automotive Magnesium

The use of aluminum and magnesium provides cost-effective weight savings in car components. Its increased use, as a structural and body material will bring about further weight savings. Environmentally this translates into fuel savings and lower emissions. Substituting light metals as magnesium and aluminum for steel in the primary structure of a car opens up room for design innovation and changes in production processes tailored to the particular needs of automotive industry. Additionally sophisticated recycling techniques are needed, i.e. recycling of the light weight metals from ELVs must be efficient in terms of amount recovered and material properties. The aim of this paper is to review various Life Cycle Analyses (LCAs) conducted over the last years for environmental assessment of automotive parts made of light metals. Use of SF 6 and recycling effects are highlighted. The advantage of using magnesium and aluminum is obvious when using the car. Savings of energy throughout the use-phase pays back for the high energy consumption at production. Additionally light metals are considered well suited for recycling and will contribute to further improved environmental performance in the second life cycle. Due to upcoming legislation, increased awareness and investigated alternatives, use of SF 6 is assumed to be phased out in the future. This implies a major reduction in greenhouse gas emissions for parts made of magnesium and makes magnesium even more favorable as a material in automotive applications.

Die Cast Magnesium Alloys for High Temperature Applications

The development of creep resistant alloys for automotive drive train components has proven to be metallurgically challenging. This paper discusses the principles of high temperature alloy development, featuring metallurgical, microstructural and processing aspects of some alloys, relative to their high temperature performance. The creep resistant alloys within the Mg-Al base system obtain their creep resistance by a relatively low content of Al, and addition of elements that form stable intermetallic phases within the grain mantle. Various third elements affect the high temperature performance differently. The results demonstrate that rare earth elements (RE) show a remarkable potential as the third element(s).