Welding Journal | October 2017

present tensile strengths of 840 and 650 MPa and total elongation of 2.7 and ...... LIU ([email protected]) are with the Colorado School of Mines, Golden, Colo.
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WELDING RESEARCH

Microstructure and Mechanical Behavior of Induction­Assisted Laser Welded AHS Steels The method combining laser welding and induction heating at high temperatures was performed

BY M. S. F. LIMA, D. GONZALES, AND S. LIU

ABSTRACT The present study proposes an innovative method of laser welding at high tempera­ tures, where the rapid solidification and preheating were performed consecutively in the same setup. The 22MnB5 hot pressed, advanced high­strength steel (AHSS) parts were heated to a given temperature and held for a given time to ensure an isothermal condi­ tion, after which, laser welding was performed. The welded part was then maintained at a high temperature for a sufficient time to develop a bainitic structure. A steel part was welded both in ambient and high­temperature conditions. The welds made using the same laser parameters, an induction heating to 445° or 529°C, and isothermal treatment of 10 min produced a bainite plus retained austenite microstructure. The hardness was greatly reduced when using the high­temperature welding method, and the hardness profiles were flat compared to the room temperature welded sample. The tensile behav­ ior of the room temperature welded coupons presents high tensile strength (1200 MPa) and negligible maximum elongation (1.3%). Alternatively, the high­temperature coupons present tensile strengths of 840 and 650 MPa and total elongation of 2.7 and 3.5%, for the conditions 445° and 529°C, respectively. Therefore, the high temperature coupons demonstrated higher toughness compared to the room temperature coupons.

KEYWORDS • Laser Beam Welding • Advanced High­Strength Steels (AHSS) • Austempering • Induction Heating

Introduction Over the last decade, transportation industries have produced a strong competition between steel and low density metals as a result of increasing requirements of passenger safety, vehicle performance, and fuel economy (Ref. 1). The response of the steel industry to the new challenges is a quest for the rapid development of high-performance alloys, namely advanced high-strength steels (AHSS) (Ref. 2). These steels are characterized by improved formability and impact toughness during crashing compared to conventional steel grades. The category of AHSS covers the following generic types: dual phase (DP), transformation-induced plasticity (TRIP), com-

plex phase (CP), and martensitic steels (MART). All these above-mentioned AHSS have been used in critical safety locations of automobile structures to absorb energy from impacts. Highstrength steels with high-energy absorption will better manage dynamic loading occurring during car crashes or collisions (Ref. 1). DP and TRIP steels with ultimate tensile strength (UTS) exceeding 1000 MPa have been shaped by cold or hot forming processes for these applications. Hot stamping is an innovative process by which AHSS is more efficiently formed into complex shapes than with traditional cold stamping (Ref. 3). The process involves the heating of the steel blanks until they are malleable, followed by deformation

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and rapid cooling in a specially designed die, creating a transformed and hardened material. The ability to efficiently combine strength and complexity allows hot forming to produce parts in one relatively lightweight part that would typically require a thicker and heavier part. Press hardened blanks therefore currently represent one of the most advanced lightweight solutions for the car body structure that simultaneously allows improved crash performance and passenger safety requirements. The hot stamping process currently exists in two main variants: the direct and the indirect hot stamping method (Ref. 4). In the direct hot stamping process, a blank is heated up in a furnace, transferred to the press, and subsequently formed and quenched in the closed tool. The indirect hot stamping