Friction stir welding (FSW) is an innovative solid-state joining technique benefiting both industries and research laboratories because of its numerous returns e.g., it’s fumeless, external medium-less or filler-less, melt-less due to its solid-state nature, no requirements of personal protective equipment, etc. FSW was first used for aluminum and its alloys at the Welding Institute of the United Kingdom and patented by Wayne Thomas in 1991 [1]. Now, it has become a state-of-the-art assembling technique that can be employed to weld a variety of materials other than aluminum e.g., steel, magnesium, copper, plastics, composites, and several combinations of dissimilar materials taking two dissimilar materials at a time of welding [2].
Brass is generally an alloy of copper and zinc. Brass properties mainly depend on heedful arrangements of percent copper in percent zinc. Brass has already become a good candidate for engineering and industrial applications owing to its striking properties such as high strength, high corrosion resistance, high electrical and thermal conductivities. Brass may easily be processed with exceptions to fusion welding processes, and it looks apparently nice before and after processing. However, brass provides a lot of difficulties when it’s subjected to fusion welding which involves melting of brass, since fusing brass usually evaporates zinc leaving brass with porosity issues. Therefore, weldability studies on brass material are scarce. C. Meran et al. [3] reported welding problems associated with the brasses when they are fused by tungsten inert gas (TIG) pulse welding. Zinc evaporates at 907 oC (boiling temperature of zinc) and porous appearance is achieved at the weld bead. The yellowish color of brass turns into reddish color due to the zinc evaporation. Although authors addressed severe difficulties in welding brasses, it was an excellent effort of that time when it was hard too to find considerable studies on joining brass materials.
Hence a need arises for such a novel joining process which doesn’t give rise to the melting of brass keeping it in its solid-state during and after welding to prevent zinc from evaporating. To fulfill this need, novel FSW may be used to join brass considering solid-state joining of brass. For instance, F. Hugger et al. [4] performed the laser beam welding of brass. They realized a huge evaporation of zinc, since fusion of brass happens during laser welding. Although the joining of brass via laser welding is novel, the joint properties of brass are expected to be greatly compromised due to the evaporation of zinc as weld interface temperature goes beyond 1200 oC justifying the viability of FSW for brass. On the other hand, C. Meran [5] welded the brass (CW508L) using FSW. Author has validated that melting of brass was never observed leading to preserving excellent joint properties in the presence of zinc. Moreover, a welding efficiency of joint was achieved to be 94.44%.
G. Cam et al. [6] joined two brass alloys separately using FSW. Brass alloys used are 70/30 and 90/10. Authors have found microscopically that no porosity exists when joining these brass alloys with FSW. This validates that there is no evidence of evaporation of zinc during FSW of brass alloys. Therefore, it has been reconfirmed that FSW is a solid-state joining process. Although researchers have welded brass specimens with welding efficiency of 117% for 70/30 brass alloy and 98.88% for 90/10 brass alloy, other brass alloys could also be welded to explore further the FSW’s potentials with excellent welding efficiencies. M. B. Durdanovic et al. [7] established a better comprehension of FSW by segregating it into five stages with a supposition that tool keeps rotating until all the FSW stages are covered. These phases are namely plunging the FSW tool into weld specimens, primary dwell, translation in a straight line, secondary dwell, and pulling the FSW tool out of welded specimens. In addition, authors have also settled a mathematical model to calculate the heat generation during FSW. Although authors have made an excellent effort to present comprehensive understanding of FSW, they did not take any welding material into account, this FSW understanding should be applied on novel materials like brass 405-20.
T. Murakami et al. [8] worked on the microstructural variations when joining brass using FSW. At friction stir weld zone (FSWZ), two phases recognized were named as alpha and beta phases. Alpha phase was the bright phase whereas the beta phase was dark phase. With a proper weld setting, beta phase was neglected owing to its presence up to 17 – 20% as compared to the base brass where degree of this phase was present to be 16%. Since evaporation of zinc was not observed even in a single welding situation, FSW was again verified as a solid-state joining technique. It was also found that alpha grain size reduces up to a certain degree with reducing the heat input. Maximum tensile strength of 398 MPa was reported by the authors, therefore, the welding efficiency was obtained to be 101% for 60/40 brass. This weld strength can also be reduced by mitigating defect’s formation at the FSWZ owing to non-recrystallization of alpha phase of brass. A. Heidarzadeh et. al. [9] explored the microstructure of 63/37 brass to reveal the presence of any defects. They utilized optical microscope, scanning electron microscope (SEM), and scanning transmission electron microscope (STEM) to reveal the microstructure of brass under investigation. It was disclosed that the alpha grains turn into finer grains with FSW owing to dynamic recrystallization (DR) after FSW. However, beta phase divides it between the alpha grains without DR. In this work, welding efficiency was acquired to be 88.85%.
Z. Y. Ma et al. [10] studied the effect of rotational speed on the FSW joint quality when joining brass. The welding efficiency was found to be 80% of parent material (PM). Although authors have achieved a good quality of joint, the specimen’s dimensions were not following any ASTM standards and effect of transverse speed was also neglected. P. V. C. S. Rao et al. [11] tested the FSW joint of brass alloy using hardness test and many other tests. The hardness of joint was found to be higher than that of parent material. Although authors have conducted a good study on the FSW of brass, the hardness of joint should be lower than that of parent material to minimize the weld brittleness whose high value can be detrimental to joint quality.
Therefore, few studies were found on FSW of brass alloys. These studies may establish a strong foundation for trial experiments along with the results from numerical studies when novel brass materials/alloys are intended to be welded by FSW.
Few trial experiments are usually deemed to be essential with novel combinations of FSW factors in the light of relevant literature to determine the effect of FSW factors on response parameters. Sometimes, trial experiments seem essential in the absence of background knowledge as the most of the novel research studies are never conducted before. In this context, simulation studies can help researchers minimize the cost and time required for trial experiments. Therefore, validated numerical studies, based on thermo-mechanical settings, may be executed towards evaluating the suitability of weld factors which may lead to shrink the efforts required for trial experiments. The numerical studies have also economical advantage [12].
K. M. Rao et al. [13] performed a thermal simulation study for FSW of Al6061-T6 alloy. Ansys was used to perform the simulation studies. The numerical study was found to be in good agreement with the empirical study. The peak temperature at the FSWZ obtained was 69.2% of melting temperature of the material under investigation. Although authors provided the researchers with better understanding of heat calculations at the FSWZ, the peak temperature is approaching the melting point of Al6061-T6 and the samples’ geometry was non-standard too. Z. Zhang et al. [14] also reported a full thermo-mechanical model to study the effect of shoulder size on the thermal distributions and material deformations in the FSWZ. In this work, the workpiece used was Al6061-T6 alloy. Authors found that temperature rise at the FSWZ directly depends on the shoulder size except the need for the boundaries of FSWZ where recrystallization is dominated by the material deformation. Since FSW is a solid-state joining technique, the maximum temperature rise noted at the FSWZ, was 63.8% of the melting temperature of workpiece material. Although authors provided the researchers with a better simulation methodology to address the temperature distributions at the FSWZ, the peak temperature is still reaching the melting point of Al6061-T6 and the samples were again non-standard too.
B. G. Kiral and H. T. Serindag [15] performed a numerical and experimental studies on FSW of Mg alloy (AZ31). Ansys APDL was used to conduct the numerical studies for temperature distributions. Hardness and joint strengths were also reported. The maximum temperature achieved was 450 oC which was 75% of the melting temperature of base material. Hardness achieved was 80% of the base material’s hardness. Although authors have executed an excellent study on FSW numerically and empirically, the peak temperature is quite high and reaching the melting point of base material with hardness higher too. As higher hardness may result in increasing brittleness of joint.
M Song and R Kovacevic [16] presented the thermal studies on FSW addressing three dimensional (3D) transient thermal model using finite difference method. Numerical results were validated by the empirical FSW data with good agreement between these results. Although authors have delivered a unique numerical work, this thermal and numerical study should further be used for determining the joint strength. Moreover, authors used aluminum and tool steel in this study. Therefore, a research space for comprehensive study on brass material, still, exists which should be properly filled.
P. Biswas and N. R. Mandal [17] have established another thermal study finding mainly the tool geometry’s effect on the thermal history using aluminum alloy. Numerical results were deduced to be agreed nicely with those of empirical confirming that the thermal study assumptions were appropriate. Although authors have delivered another good simulation idea, several vital aspects of FSW were not addressed e.g., standard weld sample design, effect of other possible FSW factors on thermal results, aluminum material replacement with other novel materials, etc.
H. Zhang et al. [18] executed a thermal study on magnesium alloy AZ31 that is a good candidate for FSW. This study is an effort towards investigation of thermal distribution for the preheating period of FSW looking for suitable preheating weld parameters. Although authors have finally studied numerically FSW of AZ31, similar kind of problems, still, exists including specimen design was not standardized and temperature measurement involves utilizing conventional k-type thermocouples.
S. Bag et al. [19] conducted a thermal analysis numerically and experimentally using aluminum alloys. In the study, heat input was given as a symmetric heat flux at the union line of flat tool shoulder surface, tool pin side, and bottom surfaces. Although numerical and empirical results were agreeing with each other, aluminum was considered again which should be replaced with any other novel material like brass. Moreover, effect of transverse tool speed was neglected. Similarly, A. R. S. Essa et al. [20] determined numerically the effect of eccentric cylindrical pin on the heat generation while FSW of aluminum alloy. Although the numerical study was again in good agreement with that of experimental, the same numerical and empirical studies may be applied to brass to validate whether a very good agreement, still, exists between numerical and empirical results or not.
H. A. Derazkola et al. [21] used a new numerical method using computational fluid dynamics (CFD) to understand how materials flow and mix together during FSW. They also worked on finding a link between the materials mixing and materials bonding before and after FSW respectively. Although this CFD approach was never used to understand the intermixing of materials during FSW of Al-Mg-Si alloy T-Joints, this numerical study was not validated with any empirical methods. Moreover, the material flow during FSW like a fluid is unlikely, since materials to be welded remain in solid-state during FSW.
From the literature on FSW, it is obvious that limited researchers have welded brass as welding problems may arise from fusion welding of brass. Researchers have welded various materials other than brass employing FSW process factors. In fact, researchers have tried their best to improve the welding efficiency in term of various output parameters/responses which include joint strength, joint hardness, and joint temperature. A study gap inevitably exists to improve these output parameters for brass material. Statistical analysis is also found to be scarce on the way to FSW of brass. Therefore, a thorough statistical analysis involving main effects, interaction plots, PCRs of each friction stir welding factor (FSWF), was a need of the time. Numerical and empirical studies on FSW of brass were also limited too; with good agreement in them. Although H13 HSS is frequently used for FSW, effect of new tool material e.g. M2 tool steel was never investigated. As far as research methods are concerned, many researchers used k-type thermocouples to measure weld temperature which are economically full of wastage in terms of time, cost, precision, and accuracy. Hence precise temperature measuring devices might be used which could save time and money. In this study, a noncontact thermal camera was used instead of contact thermocouples. In fact, improvements of weld strength, weld hardness, and weld temperatures pertinent to friction stir welding of brass 405-20, were never studied before and their interrelationships. Present study is an effort to cover these research gaps highlighted towards FSW of brass 405-20.