Effect of DLC Coating on Limiting Drawing Ratio of AZ31 Magnesium Alloy Sheet

Magnesium alloys have several advantages that make them attractive for use in structural applications such as a superlight weight, high specific strength, ease of recycling and electromagnetic shielding capability. On the other hand, several disadvantages are associated with manufacturing using magnesium alloys such as the fact that it is practically impossible to apply conventional metal-forming techniques at room temperature without producing defects. Much effort, therefore, has recently been applied to improving the formability of magnesium-based alloys in order that they may realize their full potential. In this study, the methodology of coating the surface of a magnesium alloy with a diamond-like carbon (DLC) coating to increase the surface lubrication performance and for achieve deep drawability is discussed. The DLC acts as a lubricant during the forming process, thus reducing the deformation of the workpieces. Friction tests and deep-drawing tests were carried out to estimate the effect of the DLC coating by the formability of the magnesium alloy. We have compared the properties of non-lubricated blanks with those lubricated a DLC coating and the lubricants and MoS₂. The coefficient of friction of the DLC-coated blank was approximately 0.16. Those of the non-lubricated blank and the blanks coated with GM100 and MoS₂ were 0.41, 0.26, and 0.17 respectively. Thus, the DLC coating provides a similar or superior level of lubrication when compared with a conventional lubricant. The deep-drawing test was performed to obtain the limiting drawing ratio (LDR). For the DLC-coated blank, deep-drawing was possible at drawing ratio (DR) 2.2. On the other hand, the non-lubricated blank fractured at DR2.0. In the case of GM100 and MoS₂, deep-drawing was successful at DR2.2 and DR2.3, respectively. In both friction and deep-drawing tests, the performance of DLC-coated workpieces was compared with that of workpieces coated with the conventional lubricants. The deep-drawing tests were carried out at 200 °C, and the formability of each blank was measured. It was concluded that the use of DLC-coated blanks produces an improvement in the formability of magnesium alloys compared with the use of conventional lubrication techniques.