Original Article Proc IMechE Part B: J Engineering Manufacture 1–14 Ó IMechE 2019 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/0954405419838379 journals.sagepub.com/home/pib Enhancement of joint strength in friction stir lap welding between AA6061 and AISI 304 by adding diffusive coating agents Omkar Mypati 1 , Abhijit Sadhu 1 , Suryakanta Sahu 2 , Debasish Mishra 2 , Surjya K Pal 1 , Shiv Brat Singh 3 and Prakash Srirangam 4 Abstract This study focuses on the usage of fly-ash and Al-12%Si as diffusive coating materials over AISI 304 steel welded with AA 6061 in friction stir welding process to improve the joint quality. Various mechanical and metallurgical studies have been carried out to evaluate the characteristics of the weld, and investigate and compare the effect of the two coating materi- als. Formation of intermetallic reaction layer has been found to be an influential parameter affecting the joint quality. Aluminium-rich intermetallic reaction layer has been observed in the bare welds, whereas the coated samples have been found to have ferrous-rich intermetallic reaction layer. Keywords Friction stir lap welding, dissimilar materials, coating, fly-ash, Al-12%Si Date received: 17 September 2018; accepted: 25 February 2019 Introduction Friction stir welding (FSW) was developed with the aim to join aluminium alloys, but, at present, it is being used to join a wide range of materials such as magne- sium, copper, lead, zinc, steel, titanium and thermo- plastics. 1–3 It is a popular technique among other solid state welding methods like ultrasonic welding and fric- tion welding, because of the lesser heat input during the welding process, and formation of superior welds. The process has also proved its potential to be far bet- ter than the traditional fusion welding processes, such as metal inert gas (MIG) welding and tungsten inter gas (TIG) welding, because of the welds having higher joint strength and fatigue life. 4–6 A promising advan- tage of FSW is its ability to join dissimilar material combinations yielding high joint strength and superior metallurgical properties. 1,2,4,5 Various combinations of dissimilar materials have been attempted such as alumi- nium to steel, magnesium to steel and aluminium to copper. 7–9 Out of these, combination of aluminium to steel in a lap joint configuration has been observed as the highly explored one. 9–11 In joining of aluminium to steel by using FSW, the heat generation in the process has been observed to be one of the crucial factors which determines the material mixing and metallurgical bonding. 12,13 The heat generation in the process results in the formation of intermetallic compounds (IMCs). 14–16 These welds have aluminium (Al)- and ferrous (Fe)-rich IMCs which tend to increase the joint strength of the weld. 17–19 However, it has also been observed that the presence of thick IMCs, that is, beyond a thickness of 10 mm, adversely affects the joint strength. 20 Various studies have reported the use of coatings which have caused reduction in the formation of thick IMCs. However, in some cases, the tool has been observed causing an abra- sion to the coated steel surface during joining with alu- minium, and results in the formation of IMCs, which are often referred as intermetallic reaction layer (IRL). During FSW, the coating material has been observed 1 Department of Mechanical Engineering, Indian Institute of Technology Kharagpur, India 2 Advanced Technology Development Centre, Indian Institute of Technology Kharagpur, India 3 Department of Metallurgical & Materials Engineering, Indian Institute of Technology Kharagpur, India 4 Warwick Manufacturing Group, The University of Warwick, Coventry, UK Corresponding author: Surjya K Pal, Department of Mechanical Engineering, Indian Institute of Technology Kharagpur, 721302, West Bengal, India. Email: skpal@mech.iitkgp.ac.in