Microstructure and Mechanical Properties of Friction Stir Spot-Welded IF/DP Dissimilar Steel Joints

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THE alarming increase in global pollution has a major impact on the dynamics of the automotive industry worldwide. Government regulations, consumer need, and competitive pressures demand reduced emission, improved crash protection, and affordability. A wide range of solutions are being developed to meet these requirements, which include the incorporation of lightweight high-strength materials, optimized product design, and application of innovative manufacturing processes. In this context, attributes such as cost, performance, manufacturability, and recyclability of steel ensures its place as the principal engineering alloy for the auto body. The advanced high-strength steel (AHSS) family are inherently strengthened through innovative alloy design and thermo-mechanical processing in advanced rolling mills. These steels do compete with other lightweight materials like Al, Mg alloys, and polymers. The AHSSs typically exhibit high yield strength, low yield to tensile strength ratio, good RAJARSHI SARKAR, Doctoral Fellow, and TAPAN KUMAR PAL, Professor, are with the Welding Technology Centre, Metallurgical and Material Engineering Department, Jadavpur University, Kolkata 700032, West Bengal, India. Contact e-mail: rajarshi.sarkar @gmail.com SHILADITYA SENGUPTA, formerly with the Welding Technology Centre, Metallurgical and Material Engineering Department, Jadavpur University, is now Graduate Research Assistant with the Metallurgical Engineering Department, Missouri University of Science and Technology, 1870 Miner Circle, Rolla, MO 65409. MAHADEV SHOME, Head, is with the Material Characterization & Joining Group, R & D, Tata Steel, Jamshedpur 831001, Jharkhand, India. Manuscript submitted January 29, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A

formability, high work hardening rate, and high strain energy absorption capabilities. These features enable the use of thinner sections in an automotive assembly, reducing the weight of the vehicle while absorbing significant deformation energy. The steel industry has developed a broad range of AHSS with unique properties to meet the diverse performance requirements of different vehicle components.[1] Auto manufacturers using steel intensive designs have aligned themselves with a light-weighting strategy that uses increasing AHSS content in their vehicles. It is envisioned that the optimized vehicle design in terms of performance and economy can only be achieved by a multi-material approach, i.e., use of different materials for different components to utilize the materials’ functionalities to the maximum possible extent.[2,3] For example, AHSS is used in components designed for crash and strength requirements such as chassis beams and pillars, while formable grades like interstitial-free (IF) steels are used for bonnets, roofs, and closures.[4] There are several parts like closures, pillars, and cross members where structural strength and shape factor are both essential, which is best provided by dissimilar strength steel combinations. The advantages of the multi-m

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