Multi Response Optimization of Friction Stir Processing Parameters on Cryo-Rolled AZ31B Alloys
Authors: Velukkudi Santhanam Senthil Kumar , Gnana Anbalagan Jeffrin Michael , Karibeeran Shanmuga Sundaram , Manickam Dhanashekar , Sankar Ramaiyan 
Published: February 2021

Friction stir processing (FSP) method is a solid-state technique used for microstructural alteration and enhancing mechanical properties of sheet metals and as-cast materials. Aluminium, brass, copper, steel, tin, nickel, magnesium and titanium are the widely used materials in friction stir processing. Even though magnesium has low density compared to aluminium, only few reports are made on magnesium. Two stage of process was carried out on the experiment to obtain fine grain refinement and improved strength. First, Cryo-rolling processing on 6mm thickness AZ31B alloy at constant roller power, roller rotation speed, strength coefficient and strain exponent. AZ31B alloy is dipped in liquid nitrogen for certain period and rolled in it’s cold state. Number of passes into roller was same for 9 samples. Cryo-rolled AZ31B is used as sample for the second stage i.e., Friction stir processing. FSPed material produce refined grain structure, micro-structurally modified cast alloys by alloying specific elements, and improvement in material strength. Based on Process parameters the properties of the material alters. Friction stir processing was performed on cryo-rolled AZ31B magnesium alloy with various processing parameters. The effect of process parameters (tool pin geometry, tool rotational speed and tool traverse speed) on two responses namely ultimate tensile strength and micro-hardness values were measured. The tool used for Friction stir processing is H13 high carbon steel with hardness upto 60 HRC. Tool pin geometry used for Friction stir processing are square, cylinder and tapered. The processed materials are cut using wire cut EDM as per ASTM standards to measure the ultimate tensile strength and hardness. Universal tester and Vickers hardness tester were used to measure the tensile strength and hardness of the Friction stir processed sample. Most of the research has been published on cryo-rolled and FSP experiments separately. In this work, a combination of these two process is developed for improved tensile strength, hardness, and ultrafine grain refinement.

A multi-response optimization was performed using grey relation analysis (GRA) to find out the optimum combination of the process parameters for maximum ultimate tensile strength hardness. Analysis of variance (ANOVA) and F-test were performed to determine the most significant parameters at a 95% confidence level. The corrosion test was made on Friction stir processed cryo-rolled AZ31B alloy for every process parameters. Salt spray test was done as per ASTM standard to find the corrosion rate. The corrosion rate for Friction stir processed cryo-rolled material is less (at optimal condition). The microstructure analysis was done on the samples using a Scanning Electron Microscopy. For clear view of grains the material is subjected to polishing and etching. The etchant used on the material is Picral + Acetic acid + Hydrogen peroxide. Fine grain size was obtained on the Friction Stir processed Cryo-rolled AZ31B magnesium alloy at optimal condition.