Short Fatigue Crack Growth and Durability Modeling of Resistance Spot Welded 5754 and 6111

Abstract

The growing demand for more fuel-efficient vehicles to reduce energy consumption and air pollution is a challenge for the automotive industry. Aluminum alloys are of interest due to their favorable cost, density, strength, and production methods, leading to improved fuel efficiency and exhaust emissions when such alloys are used to reduce the weight of automobiles and trucks. The alloys 5754 (Al-3Mg) and 6111 (Al-1Mg-1Si) are used as inner and outer automotive body panels, respectively, and overlapping sheets are frequently joined together using resistance spot welding. Resistance spot welding (RSW) is one of the most widespread and practical joining techniques in the automotive industry for sheet metal components. Despite its widespread use, little is known about the propagation behavior of short cracks within the spot welds. This investigation provides insight into the factors affecting propagation of physically short cracks within the spot weld regime and applies that information to develop a new approach for modeling joint durability. Short crack propagation rates in the parent sheet materials were found to be similar for the two aluminum alloys. However, crack propagation rates in the RSW regions were observed to be as much as 100x faster than the parent alloys. This was determined to be due to the interactions of cracks with pores that form in the weld fusion zone. A unified, dislocation based short crack model was modified to account for this effect. The short crack data was incorporated into an analytical master fatigue life relationship for predicting the lifetime of spot-welded components using a structural stress methodology. The durability model consists of finite element analysis calculations of structural stresses, adjusted short fatigue growth relationships and a calibration procedure based on fatigue lifetime data of aluminum RSW joints. This methodology was validated by application to experimentally determined fatigue lives of lap-shear joints in a wide variety of weld and sheet geometries in both alloys. Implementation into a full scale vehicle model can aid in the optimization of the size, location and number of RSW joints and enable weld process optimization via Integrated Computational Materials Engineering (ICME)