MECHANICAL BEHAVIOR OF FRICTION STIR PROCESSED Mg ALLOY

Abstract

Microstructure having mixture of coarse and fine grained microstructure (bimodal or layered) in Al and few wrought Mg alloys were reported to exhibit superior mechanical properties when compared to the homogeneously fine grained microstructure but such studies were not carried out in as-cast Mg alloys. Motivated by the gap in the literature, layered microstructure containing fine grain and as-cast microstructure was generated using friction stir processing (FSP) and its effect on mechanical behavior was studied in this work. Preliminary studies were carried out on the tensile and fracture toughness behavior of as-cast material in solutionized (T4) and aged (T6) conditions. Then, layered microstructure with the mixture of fine grain and as-cast microstructure was obtained by multipass friction stir processing (FSP) using three tools with different pin lengths. Change in the as-cast microstructure after FSP, as well as effect of the tool pin length on the FSPed microstructure were analyzed before studying the effect of layered microstructure on the mechanical behavior of the material. Finally, a constitutive equation was developed for the superplasticity of FSPed AZ91 Mg alloy and its parameters are correlated with the microstructural evolution happened during the superplastic deformation. In the preliminary study on as-cast material, the alloy was heat treated to 416ºC for 48 hours and air quenched to get T4 condition (solutionized). Further, T4 solution treated alloy was heat treated at 167ºC for 24 hours and air quenched, to get T6 condition (solutionized and aged). The tensile test and fracture toughness test of AZ91 alloy in as-cast, T4 and T6 conditions were carried out. The tensile tests showed improvement in strength from approximately 80 MPa in cast alloy to 132 MPa in T4 condition and the corresponding improvement in ductility from 0.8% to 4.7%. After aging (T6 condition) the strength remains almost same, 130 MPa, but the ductility reduced to 2%. The tensile fractographs of as-cast and T6 samples exhibited cleavage facets and river patterns while T4 samples exhibited quasi-cleavage fracture morphology. The fracture toughness was measured using J-integral method. The JⅠC value increased from 1.7 kJ/m2 in as-cast material to 4.2 kJ/m2 after solution treatment but reduced to 3.2 kJ/m2 after aging. Crack bridging due to second phase particles was observed in the fatigue pre-crack regions. Precipitate/matrix interface resists crack growth at the crack front. This resistance force decides the fracture toughness of the samples. vi After studying the mechanical behavior of cast material, multipass FSP was performed using tools with different pin lengths. Three tools: PL4 with 4 mm pin length, PL5 with 5 mm pin length and PL7 with 7 mm pin length, were used. The effect of tool pin length on the microstructure and tensile properties were studied. Three zones were observed from the processed regions. ZONE Ⅰ was only affected by the tool shoulder, ZONE Ⅱ was influenced by both the shoulder and the pin while ZONE Ⅲ was affected only by the tool pin. ZONE Ⅰ and ZONE Ⅱ were present in the processed region of the samples FSPed using all the three tools. Semi-circular ring patterns were observed at ZONE Ⅲ in samples processed using tools PL5 and PL7. ZONE Ⅲ was absent in the material processed by tool PL4. There were no differences in the phases formed and their distribution after process using the three tools. The tensile strength increased from 290 MPa in tensile sample processed by tool PL4 to 340 MPa in tensile sample processed by tool PL7 and the ductility remains nearly same in all the cases. The results confirmed that the variations in the properties with increasing thickness of the processed region was insignificant. It ensures that the properties of layered microstructure (in subsequent chapters) can be analyzed considering similarity in the processed region using three tools with different pin length. Three kinds of layered microstructures was generated through the thickness of the material, namely, HFG, SFG and FFG by multipass FSP. Its effect on room temperature mechanical behavior were studied. Tensile test, notch fracture toughness test and high cycle fatigue test were conducted to understand the effect of layered microstructure on mechanical properties. The tensile properties, namely, yield strength, tensile strength and percentage elongation of AC material were found to be 92 MPa, 100 MPa and 0.8% respectively and the corresponding values for FFG were found to improve to 242 MPa, 327 MPa and 4.7%. For HFG and SFG, these values were found to follow the rule of mixture. Grain size strengthening as well as solid solution strengthening have contributed in improvement of the strength after FSP. The fracture morphology was intragranular cleavage in as-cast region and intergranular in FSPed region. The apparent fracture toughness (KQ) values of single edge notch bend (SENB) samples without precrack were compared, and the results showed improvement from 6.2 MPa√m in AC to 12.3 MPa√m in FFG material. SFG material showed pop-in phenomenon as crack originated from the as-cast region was arrested by the tougher FSPed region on both the surfaces. Number cycles to failure during high cycle fatigue also found to increase with the increase in FSPed region. vii To study the effect of layered microstructures (HFG, SFG and SFG) on the superplastic behavior high temperature tensile tests were carried out at 350 °C using three different initial strain rates i.e. 5×10−3 s−1,1×10−3 s−1 and 5×10−4 s−1. The FFG material showed superplasticity at all strain rates and highest ductility of 680 % was achieved at the strain rate of 5×10−4 s−1. The AC and HFG material displayed very low elongation while SFG material exhibited superplasticity of 388 %. The superplastic behaviour in SFG was due to increase in the fraction of fine grain microstructure and modification of as-cast microstructure on both the surfaces. Microstructure and texture studies revealed that grain boundary sliding accommodated by grain boundary migration and grain rotation were responsible for superplasticity in FSPed region. The constitutive equation for the superplasticity of full thickness fine grained material was developed using tensile data of the high temperature deformation performed at temperatures (150 °C, 250 °C and 350 °C) and at three different strain rates (5 ⅹ 10-3 s-1, 1 ⅹ 10-3 s-1 and 5 ⅹ 10-4 s-1). Maximum elongation of 818 % was observed at 250 ⁰C and at a strain rate of 1 ⅹ 10-3 s-1. Strain hardening was observed in the flow behavior of samples tested at 250 ⁰C and 350 ⁰C. Variation in parameters of the constitutive equation with increasing strain were observed. The strain hardening observed at 250 ⁰C, at a strain rate of 5 ⅹ 10-4 s-1 and at 350 ⁰C, at the strain rates of 1 ⅹ 10-3 s-1 and 5 ⅹ 10-4 s-1 was attributed to dynamic grain growth caused by GBS. Thus the variation in material parameters with increasing strain was due to the non-steady flow caused by dynamic grain growth during superplastic deformation in FSPed AZ91 Mg alloy