Literature Review
2.3 Effect of Process Parameters on FSWed Mg Alloy Joint
FSW process offers many advantages and scope for detailed investigation of welding Mg alloys is worthwhile. Effect of process parameters on the microstructural and mechanical properties have been observed by a few researchers. Yang et al. (2012) investigated the effects of rotation rates and observed that as the rotation rate increases the nugget shape varies from basin or ellipse shaped structure to a two-layer structure.
They also observed variation of tensile strength and fracture behaviour with rotation rate were accounted by the variation in texture of basal plane. Commin et al. (2009) investigated ratio of rotation speed to welding speed on AZ31 Mg alloy and its effect on grain structure, micro-hardness and tensile properties of the weld. Rose et al. (2012) studied the influence of welding speed of the FSWed AZ61 Mg alloy and tensile properties values are correlated with the microstructure and hardness of the joint. They found that the welding speed is the main cause for the formation of fine grain in the stir
Chapter 2 zone which leads to higher hardness and acceptable tensile properties. Padmanaban et al.
(2009) investigated on effect of shoulder diameter to weld AZ31B Mg alloy and found that the joint fabricated with 18 mm shoulder diameter (3 times of plate thickness) produced mechanically sound and metallurgically defect free welds. Montazerolghaem et al. (2015) implemented differential rotation speeds of pin and shoulder in the FSW process. They observed that appropriate selection of pin and the shoulder rotation speeds results in defect-free joints. Selvaraj et al. (2013) investigated on mechanism of material matrix movement to weld formation and relation between the weld parameters on temperature with weld quality in FSW process. Ganesh et al. (2015) investigated that tool rotational speed is important parameters to control the superplastic forming of friction stir welded joint.
Rose et al. (2011) studied the effect of axial force on tensile properties of AZ61A Mg alloy. They found that weld specimen with 5 kN axial force, (with minimum of 3 kN and maximum of 7 kN) exhibits superior tensile properties compared to other joints.
Chen et al. (2012) investigated the effect of initial grain size of base metal (1.06 and 6.14 µm) on mechanical properties of AMX602 Mg non-combustive alloy. They observed that the grains growth occurred in the stir zone of the fine-grained specimens after welding. However, grain refinement occurred in the coarse grained counterpart.
Rajakumar et al. (2013) did parametric study on FSW of AZ61A Mg alloy and developed an empirical relationship using response surface method to predict tensile properties. Harikrishna et al. (2010) used FSW process to weld ZM21 Mg alloy and studied the tensile strength and bending strength of the joints. They found that tensile strength was 75% of BM and welded joint bend up to 65°. It was also observed that grain size in the weld nugget found to increase with increase in base material thickness.
Padmanaban et al. (2010) compared three welding processes namely, gas tungsten arc welding (GTAW), FSW and laser beam welding (LBW). They found that LBW exhibit 14% higher strength compared to GTAW and 2% higher compared to FSW joint specimen. Yu et al. (2009) studied microstructural modification and mechanical properties improvement in friction stir zone of thixo-molded AE42 Mg alloy. They found that the grains became finer when welding speed increases and the stir zone hardness and tensile strength increases due to grain refinement. Microstructural and mechanical
Literature Review
properties of FSWed AZ31B Mg alloy added with cerium were studied by Sirong et al.
(2010). It was found that tensile properties added with cerium was more compare to without cerium and the micro hardness in the weld nugget slightly lower than that in the BM.
Chai et al. (2013) reported strain rate and tensile strength of friction stir processed (FSP) AZ91 Mg alloy. They observed that submerged FSP produces finer grains, enhanced superplastic ductility with reduced flow stress and higher optimum strain rate, due to the enhanced cooling rate compared with normal FSP specimen.
Balamurugan et al. (2013) investigated the effect of tool profile on mechanical and tribological properties of FSP AZ31B Mg alloy. They used a concave and stepped type shoulder tool for variation of grain size, corrosion rate, tensile properties, and tool wear.
Nia et al. (2013) examined the effect of thread pitch and water cooling action on the mechanical and microstructural properties of FSP AZ31 Mg alloys. They found that the thread pin with one millimeter pitch improved mechanical properties and microstructure uniformity than the pin with a three millimeter pitch.
Some researchers investigated FSW of Mg with other materials. Simoncini, et al.
(2012) studied effect of tool configuration and process parameters on dissimilar weld of AA5754 and AZ31 alloys. They studied surface appearance, mechanical and microstructural properties. Chen et al. (2009) investigated the effect of tool geometry on microstructure and mechanical properties of lap welded steel and AZ31Mg and observed that short probe contributed to defect-free and high-strength joints. Malarvizhi et al.
(2012) performed welding of AA6061 and AZ31 Mg alloy to study the influence of shoulder diameters (12, 15, 18, 21 and 24 mm) and observed maximum tensile strength with 21 mm tool shoulder diameter i.e., 3.5 times of plate thickness. Firouzdor et al.
(2010) investigated on FSW joints of Al/Mg by varying travel speed, tool rotational speed and position of the plate to study formation of different IMCs and material flow in the stir zone and its effect on the welding conditions.
Grain size in the weld zone has a great influence on the microhardness and other mechanical and metallurgical properties. Each process parameters has more or less effect on the grain size and that contribute to micro-hardness in case of FSW process. The Effect of grain size on micro-hardness of FSWed AMX602 Mg alloy was investigated by
Chapter 2 Chen et al. (2012). They observed finer grain at the weld zone with lower micro- hardness value than the BM. Similarly, Sirong et al. (2010) studied the hardness profile of AZ31B Mg alloy added with cerium and Dobriyal et al. (2008) studied the hardness distribution of AE42 Mg alloy and found lower hardness values at the stir zone compared to the BM. However, Yu et al. (2009), and Rong-change et al. (2008) concluded that stir zone hardness is higher compare to BM during FSW of AE42, AM20 and AM50 Mg alloy, respectively due to finer grain formation at the stir zone by dynamic recrystallization. Hardness of welded specimens was correlated with the failure location of the welds by Padmanaban et al. (2010) during FSW of AZ31B Mg alloy.
Effect of thread pin tool was investigated by Nia et al. (2013) on FSP of AZ31B Mg alloy and concluded that lower pitch value leads to higher hardness values as well as improved mechanical and microstructure uniformity than the pin with higher pitch thread.