Application of response surface methodology for weld strength prediction in FSSWed TRIP steel joints
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RESEARCH PAPER
Application of response surface methodology for weld strength prediction in FSSWed TRIP steel joints Ali Ebrahimpour 1
&
Amir Mostafapour 2 & Mohamad Reza Nakhaei 3
Received: 15 May 2020 / Accepted: 30 September 2020 # International Institute of Welding 2020
Abstract The effect of friction stir spot welding (FSSW) parameters on microstructure and mechanical properties of transformationinduced plasticity (TRIP) steel joints was investigated. The microstructure of the base metal and welding zone (stir zone, thermomechanically affected zone, and heat-affected zone) was examined using OM, SEM, EBSD, and XRD. The results of fractography of joints show that the microstructure and size of bonding ligament are the main parameters affected on the weld strength. The influence of FSSW parameters such as rotational speed, dwell time, and plunge depth on the strength of the TRIP steel joints was investigated using response surface methodology and considering bonding ligament of joints. The results of analysis of variance (ANOVA) show that the mathematical model was in good agreement with actual amounts of joints strength. Analysis of variance also determined that the plunge depth was a more effective parameter than other parameters. By optimization of the parameters, it was found that the maximum strength (9.42 kN) can be achieved by choosing of the rotational speed at 1218 rpm, dwell time at 1.65 s, and plunge depth at 0.19 mm. Keywords TRIP steel . Friction stir spot welding . Strength . RSM . SEM . EBSD
1 Introduction Transformation-induced plasticity (TRIP) steel is one of the advanced high-strength steels (AHSS), which has been faced by increasing demand for various industries, especially car factories in the recent decades. The main reason for this increasing request is the good combination of high strength and formability of this type of steel. These excellent properties are because of the existence of retained austenite in microstructures of TRIP steels, which transforms to martensite during deformation [1]. In fact, Recommended for publication by Commission III - Resistance Welding, Solid State Welding, and Allied Joining Process * Ali Ebrahimpour [email protected] Amir Mostafapour [email protected] 1
Mianeh Technical and Engineering Faculty, University of Tabriz, Tabriz, Iran
2
Mechanical Engineering Faculty, University of Tabriz, Tabriz, Iran
3
Faculty of Mechanics and Energy, Shahid Beheshti University, Tehran, Iran
by applying strain, austenite in the microstructure is transformed to martensite, and this phenomenon increases the strength and strain at break, simultaneously. TRIP steel has a multiphase microstructure containing ferrite, bainite, retained austenite, and martensite. To achieve this microstructure, a two-step heat treatment (intercritical annealing (IA) and bainitic isothermal transformation (BIT)) is done. In the IA, the cold-rolled steel is heated between Ac1 and Ac3 to dissolve the primitive microstructure. In the BIT, the metal is quenched in a about 3
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