Use of Friction Stir Processing for Improving Heat-Affected Zone Liquation Cracking Resistance of a Cast Magnesium Alloy

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FRICTION stir processing (FSP) is a powerful technique for local microstructural modification. During FSP, the material is subjected to severe plastic deformation at high strain rates and at high homologous temperatures.[1,2] As a result, the material in the processed region dynamically recrystallizes and develops a very fine grain size. Secondly, any compositional inhomogeneities in the material get evened out. Similarly, any coarse non-deformable second-phases in the material get thoroughly fragmented and redistributed. These phenomena have been widely investigated in several different alloy systems.[3,4] The microstructural changes induced by FSP usually have a strong positive influence on the mechanical properties of the material.[3,4] Considerable improvements in corrosion and wear resistance have also been reported due to FSP in some alloy systems.[3,4] Further, FSP has recently been shown to significantly improve the heat-affected zone (HAZ) liquation cracking resistance of a few nickel-base superalloys[5,6] as well as an Al-Cu-Mg alloy AA 2024.[7] However, in a previous study by the present authors,[8] FSP was found to aggravate the problem of HAZ liquation cracking in an Al-Cu alloy AA 2219 by increasing the amount of liquating Al2Cu (h) phase and by increasing the population of grain boundary h particles. It is therefore

G.M. KARTHIK, G.D. JANAKI RAM, and RAVI SANKAR KOTTADA are with the Department of Metallurgical and Materials Engineering, Materials Joining Laboratory, Indian Institute of Technology Madras, Chennai 600 036, India Contact e-mail: [email protected] Manuscript submitted April 26, 2017.


necessary to assess the benefits of FSP on a case-to-case basis. It is meaningful to consider ‘‘FSP pretreatment + fusion welding’’ only in cases where friction stir welding (FSW) is not feasible. Rule et al.[6] explained in detail why in some cases a two-step ‘‘FSP pretreatment + fusion welding’’ solution is more practical than a one-step ‘‘friction stir welding’’ solution. Here, it is pertinent to note some of the limitations of FSW. First, FSW is essentially a shop-floor process. It is rarely considered or used in field welding applications. Secondly, FSW involves very high process forces and necessitates strong backing and rigid clamping. It is therefore not practicable for producing intricate assemblies and enclosed shells or containers. Thirdly, FSW is not well-suited for certain joint configurations (for example, edge or corner joints). If FSW is not feasible for any such reasons in a given application, one might consider using FSP as a pretreatment to fusion welding for overcoming specific weldability problems such as HAZ liquation cracking. It may be noted that, unlike FSW, FSP can be carried out remotely in a shop and is not restricted by the nuances of actual fabrication. Magnesium alloy AZ91 is widely used in die castings because of its good combination of strength, castability, and corrosion resistance.[9] During solidification of alloy AZ91, as prima