Effect of friction stir processing on microstructure and mechanical properties of weld made by metal inert gas welding

Friction stir processing (FSP) is a surface processing technique to improve the microstructure and different characteristics of metal surfaces. In the current research, the effect of FSP on microstructure and mechanical properties of weld made by MIG between St37 steel parts was studied. Materials and methods Butt-welding procedures were carried out in flat and vertical positions. Some of welded specimens were processed using FSP process. Shoulder was in contact with weld area surface while the pin was plunged into the weld. For all FSP processes, the transverse speed was 31.5 mm/s and rotating speed was 1200 rpm. Hardness was evaluated while force was 0.05kgf and dwell time was 10 s. For wear test based on pin and disk method, normal load was 20 N, distance was 100 m. Results and discussion It comes from fig.1 that microstructures for FSP processed specimens are coaxial and ultrafine ( 1 µm) while for those which have not been processed by FSP, the microstructure is dendritic. Additionally, presence of voids and porosities was not found in samples which have been FSP processed. During FSP, due to heat form friction, the material is softened and stirred. As a result, dislocations are generated and dislocations density enhances. Due to occurrence of dynamic recrystallization, sub-grains are formed in microstructure and a fine and equiaxed microstructure develops [1]. Additionally, material stir during FSP, leads to removal of porosities. Fig.1 Microstructures of different specimens a) flat, b) vertical, c) flat + FSP, d) vertical + FSP. According to Fig. 2, strength and displacement to failure of FSP processed specimens (for both welding conditions) are larger than those relating to specimens without FSP. This can be related to ultrafine andequiaxed microstructure of FSP processed specimens. As grain size decreases, volume fraction of grain boundaries increases. Grain boundaries impede the movement of dislocations and increase strength [2]. Additionally, Mikhaylovskaya [3] found that intergranular fracture transforms to transgranular fracture as grain size decreases and as a result, displacement to fracture enhances. Fig.3 Stress-displacement diagrams of the studied specimens. Hardness and wear test results in Table 1 indicate that for both welding conditions, by application of FSP, hardness increases and worn weight decreases. In fact, development of ultrafine grains ( 1 µm) with high volume fraction of grain boundaries increases hardness. According to Archard relation [4], wear debris weight decreases as hardness increases. Table 1 Hardness and wear test results for the studied specimens. Hardness(VHN) weight decrease(mg) Flat-weld 190 37.4 Flat+FSP 920 0.3 Vertical-weld 245 10.5 Vertical+FSP 820 0.6 Conclusions In the current research, the weld region made by MIG welding method between ST37 steel specimens was processed by FSP. Two welding conditions were considered: flat and vertical. The results indicated that an ultrafine grain microstructure developed in the weld region. Additionally, it was found that strength and ductility values of FSP processed specimens were higher than those relating to non FSP processed specimens. The results also indicated that wear resistance increased as FSP was applied