Weldability of Advanced High-Strength Steel Drawn Arc Stud Welding

摘要

Designed experiment is used to statistically quantify fastener welding characteristics of bare boron steel, Usibor ® , and HC500C against mild steel This paper presents the findings of a weldability study of drawn arc stud welding of various advanced high-strength steels (AHHS) including two grades of boron steel, and one grade of dual-phase steel of various thicknesses and coatings from several automakers, and benchmarked against mild steel. A wide top (or large flange) stud of 6-mm-diameter ISO metric thread (M6) was used in the study. A drawn-arc stud welding process was accomplished by shorting the stud to the workpiece with a pilot current, lifting the stud away from the workpiece to draw a small arc, increasing the current to a high level, holding the stud at the lifted position for a given duration to melt the stud end and the workpiece, and plunging the stud back to the workpiece to complete the weld. Instead of poking for a nine-box weld lobe from trial and error, a robot was used to comb a 3-D weld parameter space of arc current, arc time, and lift height, with parameter grid of 380―1200 welds for each AHHS grade. A design of experiment (DOE) approach maps out the relationship between DOE inputs of control variables, and DOE outputs of weld quality statistically. Objective and subjective weld quality were measured, including destructive conical bend for weld strength, dimple, sag, melt-through, head melting, cracking, excessive expulsion, and backside marking. Among the 3496 welds visually classified, photographed from both sides and destructively tested, it was found that not all AHHS behave the same in drawn arc welding. Advanced high-strength steel of different types, thicknesses, and coatings exhibited different welding characteristics. Mild steel as a baseline had the classic C or kidney-shaped operating lobe, and the best weldability characterized by the largest lobe size and tolerance to lift. Uncoated boron steel of 1.2- and 1.4-mm thickness had excellent weldability at lower lift, with deteriorating performance at higher lift. It is best welded at hot and fast settings. ArcelorMittal's boron steel Usibor ® of 1.4-mm thickness had marginal weldability. It is best welded at slow and cool settings. At 1.0-mm thickness, it could not pull 70% nugget at optimum settings. HC500C of 0.8-mm thickness had unacceptable weldability but shows potential in very hot and fast settings.