*2.3. Friction Stir Processing of Similar*/*Dissimilar Alloys*/*Metals*

Friction stir processing is a fairly new material processing technique. This then suggests that there are many works that are still in progress focusing in different aspects of this new technique. This involves the processing of plates and welded joints. The evaluation of the mechanical properties of the friction stir processed dissimilar AA2024 and AA6061 welded joint was performed by Hameed et al. [46]. The friction stir processed joints used were formed through the use of FSW technique. The authors did the mechanical analysis of the processed joint in comparison with the unprocessed joints. The parameters used in performing the processing were similar to those used to perform FSW. The tensile properties of the processed joint were higher than the unprocessed joint. The hardness value for the processed joint was higher than the unprocessed joint. The microstructural analysis for processed joint reveal finer grain sizes compared to the unprocessed one.

Karthikeyan and Kumar, [47] studied the relationship between process parameters and mechanical properties of a single pass friction stir processed AA6063-T6 plate. Processing was performed at different axial forces, traverse speeds and tool rotational speeds. The tensile results revealed a linear relationship with the axial force than any other parameter used. The improvement in ductility was found to be linearly depending on the axial force than on the other parameters. The impact of applying the FSP on 6 mm AA6056-T4 plates was investigated by Hannard et al. [48]. It is well known that the ductility of each material plays a very crucial role towards the formability of the material and the chosen material is mostly used in forming different components and structures. Hannard et al. discovered that the ductility of the plate increased with the increase in number of processing pulses. The existence of pores from the base metal was suppressed completely by the increase in multi-pass FSP. The multi-pass FSP was found to be the method in breaking the intermetallic particles and to redistribute them homogenously. Hannard et al. work have proven that the proper processing of the material occurs when the multi-pass FSP is used.

Mazaheri et al. [49] have shown the capabilities of FSP in producing surface composites. This capability was tested when they used the FSP technique to produce the A356/A2O3 surface composites. The microstructural analysis results of the A356/A2O3 indicated that A2O3 particles were well distributed in the aluminium matrix, and good bonding was also observed. The nanoindentation technique revealed that the microhardness for A356/A2O3 and A356–*n*A2O3 surface composites was higher than the samples processed without A2O3 particles and the as–received A356 material. A similar study was performed by Kalashnikova and Chumaevskii [50] where they used the FSP technique to develop surface composite between titanium carbide (TiC) and AA6082. The microhardness of the composite was found to be higher compared to the AA6082 metal. The tensile properties of the composite were found to be matching those of AA6082.

The effect of FSP on AA2024-T3 was studied by Hashim et al. [51]. The performance of the FSP was based on the pin-less cylindrical shoulder. The hardness results revealed that the application of FSP increased the hardness of the processed sample compared to the base material. There was also a notable increase in tensile properties of the processed sample compared to the base metal. The microstructural grain size was also refined compared to the base metal and this was found to be in correlation with tensile results.

The impact of FSP technique on the mechanical properties of cast Al-Si base alloy was analyzed by Tsai and Kao [52]. The tensile properties of cast AC8A alloy were improved after FSP, particularly the tensile elongation, which increased from <1% to 15.4%. FSP resulted in improvement of the tensile strength as the result of a combination of dissolution, coarsening and strengthening precipitates, which were attained by the FSP parameters. Jana et al. [53] investigated the FSP effect on fatigue behaviour of the cast Al–7Si–0.6 Mg alloy. The results showed five times improvement in fatigue life for a hypoeutectic Al–Si–Mg cast alloy. FSP eliminated the porosities and refined the silicon (Si) particles resulting in a decrease of the crack growth rate. In addition, FSP resulted in both break-ups of the dendritic microstructure and complex material mixing.

Kurt et al. [54] performed FSP on the aluminium alloy AA1050 to improve respective mechanical properties. Samples were subjected to the various tool rotating and traverse rates with and without silicon carbide (SiC) powders. The optimum processing parameters that were found to give better results were the rotational speed of 1000 rpm and traverse speed of 20 mm/min. The results revealed that FSP reduced the AA1050 grain size, which subsequently increased its hardness. A good dispersion of SiC was obtained and a good formation of composite layer. The hardness of the formed composite surfaces was improved significantly compared to that of base metal. Bending strength of the produced metal matrix composite was significantly higher than that of the processed specimen and untreated base metal.

The impact of using various chromium molybdenum (Cr-Mo) steel tool profiles in performing the FSP on AA2014 was studied by John et al. [55]. The hexagonal profile was found to be the best profile in producing good mechanical properties of the processed sample. The highest value for the hardness was also achieved through the hexagonal pin profile. The hexagonal pin profile was found to be suitable in producing highly refined grains compared to other profiles tested in the study. The influence of FSP on the microstructure and mechanical properties in terms of hardness for AA6061 sheet was investigated by Prakash and Sasikumar [56]. The cylindrical shaped high steel tool was employed in performing the multi-pass FSP. The microstructural evaluation reveal that the grain size of the processed specimens was 70% smaller than the base metal. There was also a linear relationship between the hardness value and the number of pulses used. The tensile properties were found to be linearly depending on the number of pulses used during FSP.

Sinhmar et al. [57] have analyzed comparatively the mechanical properties of the processed and unprocessed AA7039. The modified surfaces were characterized in respect to macrostructure, microstructure, hardness and tensile properties. The results showed an increase in ductility from about 13.5% to 23.6% while the ultimate and yield strength were adversely affected. The results showed higher ductility on the longitudinal direction than in traverse direction. The multi-pass friction stir processing produced higher hardness than the single pass one. Santella et al. [58] showed that FSP created a uniform distribution of broken second-phase particles of A319 and A356 and eliminated the coarse and heterogeneous structure of the alloys. The study was performed to assess the mechanical properties and reported that the tensile and fatigue behaviour of the material were improved with friction stir technique. The transmission electron microscopy (TEM) observations revealed the generation of a fine-grained structure of 5–8 μm for FSP A356. Furthermore, TEM examinations revealed that the coarse Mg2Si precipitates in the as-cast A356 sample disappeared after FSP, indicating the dissolution of most of the Mg2Si precipitates during FSP. FSP was found to be generally beneficial for dissolution of precipitates and structure homogenization [59].

Wrought AA5059 was friction stir processed by Izadi et al. [60] with the purpose of finding the best tool profile suitable for such class of aluminium alloy. Amongst the profile tested, the 3-flat threaded pin profile outshine the profile in all aspects. The microstructural analysis revealed that the average grain sizes were about 1.24 μm and this size was far less than the grains of the base metal. This grain size contributed towards the improvement of the microhardness. The yield strength and the ultimate tensile stress were also found to be higher than the base metal. The percentage elongation was also found to be higher than 25%. Ni et al. [61] have used FSP to modify the surface of cast Mg-9Al-1Zn alloy. The processed specimens were found to be defect free with fine-grain microstructure dominated by fine β-Mg17Al12 particles. The fatigue properties of the processed specimens were found to be higher than the base metal. The employment of FSP resulted to the transformation of quasi-cleavage fracture to dimple fracture. It was also found that the employment of FSP brought about the suppression of porosities and coarse β particles.

Sakurada et al. [62] were the first to perform a study on underwater FSW on AA6061. Their results showed that it was possible to generate enough friction for processing even though the workpieces were underwater. The stirred region of the underwater weld joint showed a finer microstructure in comparison to the one exposed to room temperature air conditions. The hardness of the underwater specimens was found to be relatively higher than those of the room temperature based specimens. Hofmann & Vecchio [63] studied the effect of submerging FSP on the grain size of AA6061-T6 compared to in air FSP. Their results showed that more grain refinement was attained under submerged conditions due to a faster cooling rate. They also demonstrated the feasibility of predicting the grain size of the processed specimens through the use of boundary migration model.

Zhang et al. [64] performed the joint analysis produced through the processing that was performed under water. They used variation in rotational speed in assessing the joint quality. They discovered that the fracture of the underwater joints was mostly dependent on the tool rotational speed. Their tensile analysis results showed a linear relationship with the rotational speed. Darras & Kishta [65] investigated the friction stir processing of AZ31 magnesium alloy in normal and submerged conditions. There were three condition used in performing the analysis of the joints i.e., air, hot underwater and cold underwater. The grain size for the cold underwater specimens was relatively smaller than the specimens produced at other different conditions. The thermal results revealed that the highest peak temperature for weld was in air-based processing compared to the other conditions. The longest processing duration was found when the processing was performed on air. Sabari et al. [66] have performed similar study on different material and processing parameters. They reported that the higher temperature gradient (along transverse and longitudinal weld axis) and higher cooling rate in underwater friction stir welds were a result of uniform heat absorption capacity of water when compared to the air-cooled welds.

El-Danaf et al. [67] have used commercial AA5083 rolled plates in analyzing the impact of FSP towards the ductility and the grain size of the processed specimens. The microstructure analysis showed a fine grain and an average disorientation angle of about 24◦. Ductility was enhanced with a factor ranging between 2.6 and 5 when compared to the base metal. The strain rate sensitivity of the processed material was 0.33 while for the base metal was 0.018. Akinlabi et al. [68] investigated the effect of the tool rotational and traverse speeds as well as the number of passes on tribological characteristics of the modified surfaces. The FSPed samples exhibited lower wear rates than the as-cast A390 hypereutectic Al–Si alloy. The wear rates were found to decrease by reducing the tool rotational speed while increasing the tool traverse speed. There was a notable inverse correlation between the wear rate and the number of FSP passes.

Toma et al. [69] investigated the effect of FSP tool cutting depth on the mechanical properties of AA6061-T6. The cylindrical tool without the pin was employed in performing this analysis. The hardness was found to be increasing with the increase in cutting depth. The engineering flaws granules became smaller and the size of these granules increased with cutting depth. The tensile properties of the processed specimens were found to be improving with the increase in cutting depth. Abrahams et al. [70] investigated the properties and microstructure of friction stir processed 7075-T651 using various tool designs. Trials were conducted on AA5005-H34 with the aim of determining the most suitable FSP tool design out of all the considered pin profiles. Fully recrystallized fine microstructure and a defect free processed zone were achieved through the use of some of the FSP pin profiles. The grain sizes were reduced from the initial 192 μm pancake-like microstructure for AA5005-H34 base material to the range between 10 and 20 μm in the processed regions. The similar behaviour was also observed on the case of AA7075-T651. The traverse speed had a greater influence on the microhardness and mechanical properties compared to the tool rotational speed. It was also discovered the traverse speed suppressed the precipitates free zones which have negative impact towards the mechanical properties of the material.

The effect of the processing parameters of friction stir processing on the microstructure and mechanical properties of AA6063 was performed by Zhao et al. [71]. Post FSP produced fine equiaxed α-Al grains formed in the weld nugget of AA6063. The size of those α-Al grains was increasing with the increase in rotational speed. Tunnel defects were observed in the TMAZ region for a low tool rotation speed. When the rotational speed exceeded 700 rpm, a good combination interface was formed between the weld nugget (WN) and the TMAZ. Electron backscatter diffraction results showed that the fraction of the high-angle grain boundary was increased after FSP in the WN. The TEM analysis results showed that the densities of needle-shaped precipitates were reduced in the WN. There was an observed linear relationship between the ultimate tensile strength (UTS) and the tool rotational speed.

Rouzebehani et al. [72] have used AA7075 plate to perform FSP underwater and room temperature with the purpose of analyzing the metallurgical and mechanical properties. The variable process parameters were used. The temperature during FSP was monitored and recorded using the K-type thermocouple placed underneath the plate close to the abutting line of the workpiece. The average grain and precipitate sizes of the weld nugget zones were significantly reduced by the submerged conditions. The best metallurgical and mechanical properties were achieved when the processing was performed under water. There are numerous attempts that are being reported where the FSP technique is being utilized to produce surface composite. These attempts look into different alloys of aluminium and different dopants. Singh et al. [73] have produced surface composite through the use of FSP technique. The approach used by Singh et al. was to deposit SiC particles inside the holes drilled on the surface of AA6063 plate. The microhardness of the fabricated composite was relatively high compared to the one for the base metal. It was discovered that the increase in microhardness was due to the pining effect of hard SiC particles. The good bonding between the SiC particles and AA6063 results to the improvement of tensile strength of the composite when compared with base metal.

The microstructural modification of AA206 through the use of FSP was also reported by Sun et al. [74]. This modification was performed so as to comparatively evaluate the mechanical properties of processed and unprocessed AA206 material. A 6.26 mm and 16 mm thick plates were used for tensile and fatigue test respectively. The two key processing parameters were tool rotation speed and tool traverse speed. The results showed an increase in both yield strength and UTS after FSP when compared to those of the base metal. There was a notable improvement in yield strength and UTS on the processed plates compared to the base material. The percentage of elongation and fatigue strength also increased compared to the unprocessed ones.

Thakral et al. [75] used FSP to enhance the tensile properties and hardness of the TIG welded AA6061-T6 joint. Tensile results showed that the average UTS value for the base metal was 299 MPa, 85 MPa for the TIG welded joint and 125 MPa for the friction stir processed (FSPed) TIG welded joint. An increase of 48% was reported on the UTS on the TIG welded joint. The hardness values of FSP TIG specimen ranged from 72–74 HV which was almost similar to that of base metal which was 74 HV whereas in TIG specimen hardness value ranged from 66–68 HV. Microstructural analysis was performed on the weld zone to evaluate the effect of welding parameters on welding quality and grain structure. The microstructure of FSPed TIG joint showed very fine equiaxed recrystallized grains compared to the microstructure of TIG joint.

The effect of a single pass FSP on the mechanical properties and microstructure of the commercially pure aluminium was investigated by Yadav and Bauri [76]. The grain size of the FSPed specimens were way smaller than those of the base metal. The TEM results showed fine grains with well-defined boundaries. The tensile results showed UTS increase of about 25% while the ductility decreased by about 10%. The impurity particles observed in TEM resulted in the yield strength decrease. The hardness also improved substantially compared to the base metal. Feng et al. [77] investigated the effect of SFSP on the microstructure of the AA2219 sheet. The grain size on the stir zone was less than that of the base metal (BM). The area fraction of the ultra-fine grains in the stir zone increased as heat input decreased. The results showed a decrease in microhardness of the SFSP stir zone compared to that of the unprocessed BM. The processed zone exhibited microhardness that was higher than that of the base metal.

The 6-mm thick aluminium alloy AA6082 was subjected to underwater FSP to test the changes in the UTS [78]. The high carbon high chromium steel rod of diameter 20 mm material was used as the processing tool for this investigation. The result revealed that the maximum tensile strength of the underwater joints was higher than that of the normal air. The effect of SFSP on the mechanical and microstructural properties of 10 mm thick AA7075 was investigated by Nourbakhsk and Atrian [79]. A thermocouple was used to record the temperature of water during the processing. The single pass FSP was used in carrying out the analysis. The results obtained from the submerged processing were similar to those obtained by other researchers [64,65,72]. Mabuwa and Msomi [80] used a single pass FSP to improve the mechanical properties of the TIG and friction stir welded AA5083-H111 joints. The processing was performed under normal room conditions. The FSPed joints showed better mechanical properties compared to the unprocessed ones. The DRX that happened during FSP resulted in ultra-fine grain refinement of the FSPed joints. Table 3 presents typical summary of the friction stir processed literature with the purpose of showing the mostly affected material property post friction stir processing. Table 4 shows different types of tool that are used on FSP extracted from cited literature. Figure 2 shows typical stress strain curves for the unprocessed and friction stir processed joints.


**Table 3.** Typical results of friction stir processed plate and joints. (TS—traverse speed, RTS—tool rotational speed, UTS—ultimate tensile strength, SZ—stir zone, NS—not specified.).

**Figure 2.** Tensile stress and strain curves for processed and unprocessed surface.


**Table 4.** Typical FSP tools. (PL—Pin length, PD—Pin diameter, SD—Shoulder diameter, SL—Shoulder length, PH—Pin height, NS—Not specified, SBPL—Square base pin length, CHL—Conical head length).

*2.4. Advantages of Friction Stir Welding, TIG Welding and Friction Stir Processing*

Table 5 presents the advantages of the friction stir welding, TIG welding and friction stir processing techniques.

**Table 5.** Advantages of Friction stir welding, TIG welding and friction stir processing (IMC—intermetallic compounds, BM—base materials, UFG—ultra-fine grains, DR—dynamic recrystallization, El—elongation, YS—yield strength, UTS—ultimate tensile strength).

