On the Prediction of Hot Tearing in Al-to-Steel Welding by Friction Melt Bonding
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RODUCTION
THE tight regulations regarding the greenhouse gas emissions generated by the transportation convinced the industry to reduce the weight of vehicles’ structures. An efficient combination of the properties of aluminum and advanced high-strength steels (AHSS) is regarded as a promising solution to reduce the weight of cars.[1] However, the dissimilar welding of aluminum and steel is challenging due to the metallurgical incompatibility of the two materials.[2] At first, the challenges are to treat the large differences in melting temperatures and thermal expansion coefficients of both materials. In addition, the formation of a reacting brittle intermetallic layer (IML) results in poor mechanical properties of the assembly.[3,4] Most of the joining techniques lead to the formation of this brittle intermetallic layer (IML). This is the case for most welding techniques such as friction stir welding,[4–6] friction welding,[7] laser welding,[8] and arc welding.[2] According to Tanaka et al.,[4] the thickness of the IML determines the toughness of the joint. There is a significant increase in fracture toughness for IML thicknesses below 1 lm. Friction melt bonding (FMB) has recently been developed to join sheets of dissimilar materials in a lap-joint configuration.[9,10] This process is adapted to weld materials showing large differences in melting temperature (i.e., aluminum and steel). In this process,
N. JIMENEZ-MENA, P.J. JACQUES, and A. SIMAR are with the iMMC-IMAP, Universite´ catholique de Louvain, 1348, Louvainla-Neuve, Belgium. Contact e-mail: [email protected] J.M. DREZET is with the E´cole polytechnique fe´de´rale de Lausanne, IMX, 1015, Lausanne, Switzerland. Manuscript submitted July 19, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
sketched in Figure 1(a), the steel plate is heated up by a rotating cylindrical tool pressed against its upper surface. The generated heat is not large enough to melt steel, but it locally melts the aluminum in contact with its bottom surface. The tight contact between the molten aluminum and the steel surface leads to some reactivity and the formation of the IML. No protective atmosphere is needed since the molten Al is confined within the assembly and is not in contact with the atmosphere. In previous studies, van der Rest et al.[9] and Crucifix et al.[11] observed hot tears in the re-solidified aluminum after FMB. Such hot tears were located in the molten pool close to the aluminum–steel interface. This location corresponds to the last re-solidified aluminum. van der Rest et al.[9] highlighted the influence of the aluminum composition on the formation of hot tears when welded to ultra-low-carbon (ULC) steel. They observed that the commercially pure aluminum alloy AA1050 was free of hot tears, while age-hardenable AA2024 led to hot tearing. Crucifix et al.[11] considered the thermal cycles when welding AA2024 to ULC steel. They observed that the number of hot tears increased as the welding speed increased. They suggested that the size of the molten pool might be used as a cr
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