Magnesium alloy is one of the lightest structural metals because of their high specific strength and stiffness. Due to the increasing demand for light weight metals in automotive applications, the use of magnesium alloys are increasing rapidly. Mg alloy has poor formability at room temperature because of the limited number of slip planes in the Hexagonal close-pack (HCP) crystal structure. Warm forming is one of the effective ways of improving formability of Mg alloys. Mg alloy is an ideal material for Inner door panels because of lower density and collision safety property. In the past, due to lack of availability and limitations of manufacturing methods Mg alloy had limited use in automotive applications. Recent developments in die-casting and superplastic forming of Magnesium alloys have caught the interest of automotive companies.
Properties of Magnesium alloy
The important properties that must be considered for Mg alloy are Forming, Joining, Machining, Corrosion and Life cycle.
Forming: Magnesium has poor flow characteristics at room temperature, which affects the formability for sheet applications. Magnesium has Hexagonal close packed crystal structure; the active slip systems at lower temperatures are limited because of the basal planes. The critical resolved shear stress and distribution of basal planes play an important role in determining formability at low temperatures.
By increasing the temperature (warm working) twinning reduces, the basal and non-basal plane slip system increases and improves fromability. As the working temperature increases formability and production rate improve, while maintaining the temperature for economy and surface finish. Optimum combination of strain rate and temperature increases the formability of the given magnesium sheet material.
Limit drawing ratio (LDR) is an important index to assess
deep drawing formability of sheets. Forming temperature and
heating duration of blanks have apparent effects on LDR in
warm deep drawing experiments of magnesium alloy sheets.
Joining: Inner door panels made from Magnesium alloys are joined with other parts made with a dissimilar metal to form a multi-material structure. Usually, Magnesium alloys are welded with aluminum and steel. Fusion welding has a tendency to generate porosity and part distortion, so other alternative joining techniques like soldering, brazing and adhesive bonding can be implemented. Drilling holes for mechanical fastening induces stress. Difference in physical and chemical properties of dissimilar joint creates a challenge for mechanical bolted assembly. Due to the low electronegative potential of magnesium it is susceptible to galvanic corrosion.
Due to the physical properties of Magnesium, its welding requires well controlled and low power input. Magnesium alloys have high affinity to react with atmospheric Oxygen; hence it requires shielding gases to protect the liquid weld form reacting with the environment. Welding techniques can be adapted to satisfy requirements for arc-welding, electromagnetic welding, resistance spot welding, friction stir welding, laser and electron beam welding.
Corrosion:
Material selection:
Magnesium alloys being the lightest structural metal, are of current interest to automotive applications.
The density of magnesium is approximately 77% lower than that of steel and 35% lower than that of aluminum.
Magnesium AZ31 alloys:
Magnesium alloy is identified by ASTM and SAE standards, the first part of the designation indicated the two principle alloying elements, the second part indicated the percentages. Magnesium alloys are divided into AZ-series alloys with addition Zinc, and AM-series with addition of Manganese respectively. The addition of Zn to the Mg-Al alloy system reduces the solid solubility of Aluminum in Magnesium, increase the amount of precipitate phase after aging and causes moderate increase in strength.
When strength is concerned Die-cast magnesium alloy AZ 91 has the same yield strength and ductility as aluminum. For extrusion alloys AZ80 aluminum provides comparable tensile strength as aluminum alloys but less ductile. Magnesium sheet metal such as AZ31 alloys offers slightly lower strength but high ductility than commonly used aluminum alloys. Magnesium alloys are slightly heavier than plastics components but are much stiffer due to higher elastic modulus.
Superplastic Gas Blow Forming:
Superplasticity is used to indicate the ductility that materials exhibit under certain deforming conditions, superplastic forming uses the plasticity of alloys to form sheet components. Such materials are formed at high temperatures, typically half the melting temperatures specific strain rate of flow stresses. High dimensional accuracy can be achieved without spring back. Complex, light and structurally strong thin sheets components can be formed. Typically superplastic forming consists of a one sided tool, gas pressure is applied on one side of the sheet to force it into forming cavity (Fig a).
A thin sheet is clamped into a furnace, induction coils in the tool heat the work piece to half the melting temperature and argon gas is blown with high pressure on to one side of the sheet metal. High flow stress lower than the mechanical yield stress deforms the sheet metal towards the cavity. If argon is blown at very high pressure, high strain and stress are caused at imperfections and the sheet metal will burst. If the pressure is too low the work piece will not deform. A pressure time cycle that produces an optimum strain rate must be maintained. Additionally, friction between the forming material and the die is to be taken into consideration; lubricant oil can be used to reduce the friction, fasteners in the tool are coated with a lubricant to allow easy ejection of the formed part exposed to elevated temperatures. Die material is made with heat resistant nickel-chromium cast steel.
The mechanical properties of the finished product are very good, the hardness of the material is absent and the spring back is nearly zero. The surface finish is excellent, eliminating the need for finishing operations.
Disadvantage of this process is slow forming rate which affects the cycle time. This process is well suited for low volume production.
Environmental issues:
The usage of Magnesium is continuously increasing to achieve high fuel economy due to environmental concerns. The new Corporate Average Fuel Economy (CAFE) program has restrictions of the gasoline usage in cars to create less atmospheric pollutants. Magnesium is replacing most of the heavy metals to enable the cars to produce more miles per gallon while enabling the size to be maintained.
Argon occurs naturally in the environment, it has no adverse effect on the environment. It dissipates rapidly in well ventilated areas. Argon has not shown any effect on animals or plants, it does not contain any ozone depletion chemicals or water pollutants. Argon might cause super-saturation of air leading to serious suffocation in confined areas.
Magnesium is likely to ignite through machining process, magnesium chips have the tendency to ignite but if the chips are large the risk is reduced. The machine must be cleaned regularly and the chips must be managed and stored safely and away from moisture. Solid magnesium is very difficult to ignite. It has relatively high thermal conductivity, dissipating localized heat. For ignition to occur the material must exceed solidus temperature. Small chips and swarf have very small volume to conduct heat and thus results in ignition. The machinist is required to be constantly monitor the operation with a magnesium fire arresting kit.
Conclusion:
