Gas Metal Arc Welding (GMAW) Process Optimization of 1.5 mm Uncoated Dual Phase 780 (DP780) Joint for Automotive Body Structural Applications
With the increasing demand for safety, energy saving and emission reduction, Advanced High Strength Steels (AHSS) have become very attractive steels for automobile makers. The usage of AHSS steels is projected to grow significantly in the next 5–10 years with new safety and fuel economy regulations. These new steels have significant manufacturing challenges, particularly for welding and stamping. Welding of AHSS remains one of the technical challenges in the successful application of AHSS in automobile structures due to heat-affected zone (HAZ) at the weld joint. In this study Gas Metal Arc Welding (GMAW) of a lap joint configuration consisting of 1.5 mm uncoated DP780 to itself was investigated. The objective of the study was to understand the wire feed rate (WFR) and torch (or robot) speed (TS) influence on lap joint tensile strength (static and fatigue). A two factor, two level, full factorial design of experiment (DOE) was conducted to understand the wire feed and torch speed influence on tensile and fatigue strength of the welded joints. In order to understand the curvature effect, center point was also considered in the experiment. Based on the statistical analysis both factors are significant on static tensile strength and two way interaction between wire feed rate and torch speed was also significant on static tensile strength. Wire feed rate was the common significant factor on all three fatigue load conditions (1200 lbf, 1500 lbf and 1900 lbf). Metallurgical properties of the lap joints were evaluated using optical microscopy. Significant hardness drop of 25% was observed at the HAZ. To understand the influence of weld parameters and weld geometry on mechanical properties, correlation analysis was conducted among weld heat input parameters, weld geometry and mechanical properties (both static and fatigue loads).