Today, regulations on vehicle emissions and fuel efficiency are strengthening due to environmental issues such as fossil fuel depletion and certified emission reduction. Therefore, it is important to improve the fuel efficiency of the automotive industry. Improvement in fuel economy of automobiles includes improving engine efficiency, decreasing driving resistance, lightweight materials and transmission technology. The use of lightweight materials is the simplest and most effective technology.

In this study, we used magnesium alloy and ultra-high strength steel of light weight vehicle. Magnesium alloy is AZ31B of Mg-Al-Zn, and ultra-high strength steel is SGAFC 590DP steel. Welding methods of dissimilar materials include arc welding, resistance spot welding, laser beam welding, adhesive bonding and friction stir welding (FSW). Arc welding, resistance spot welding, adhesive bonding, and laser welding are not suitable process due to different melting points, process times, and characteristic of magnesium alloy. Therefore, FSW is the most suitable welding method. FSW is performed at low temperature below the melting point and there are few welding defects. Hybrid FSW is the process of adding a heat source in FSW. This process enhances the flow of the two materials and improves the mechanical properties.

In order to investigate the characteristics of hybrid FSW, welding characteristics according to preheating position and TIG preheating intensity are investigated.

Prior to welding, thermal conduction numerical analysis and thermo-elasto-plastic numerical analysis were performed using an Ansys program. The heat distribution and thermal history were clarified by thermal numerical analysis and finally the intermetallic compounds(IMC) were predicted. IMC is important factors in welding because IMC influences the mechanical properties of welding. IMC of AZ31B and SGAFC 590DP steel are FeAl3 and Fe2Al5. Since this compound causes brittleness at welded parts, it is necessary to make it less than 10 μm.

As a result of numerical analysis, the amount of IMC is estimated to be about 1.4 ~ 1.9 μm in FSW and about 1.9 ~ 2.6 μm in hybrid FSW. This is a value that satisfies the amount below 10μm. As a result of experimental, the highest tensile strength is 161.93 MPa with travel speed 0.6mm/s in FSW. In hybrid FSW, the highest tensile strength is 172.80 MPa with preheating position 30mm and current intensity 20A. The amount of IMC is estimated to be about 1.3 ~ 1.9 μm in FSW and about 1.9 ~ 2.3 μm in hybrid FSW. This value is also satisfied the about below 10μm. Experimental results are validated extensively with corresponding numerical results. A fair aggrement is acheived in between experimental and numerical results.