Investigation of preheating method on joint strength of aluminium-stainless steel dissimilar welding using metal inert gas (MIG) process

This study investigates AA6O61-O and SUS304 dissimilar welding with preheating of stainless steel SUS304 prior to welding process. The welding method used was metal inert gas (MIG) with butt joint type weld. The mechanical strength was investigated using tensile test. Meanwhile, the macrostructure and microstructure of the specimens were analyzed using optical microscope, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The tensile tests indicate that the preheated specimen with 90 °C have the maximum ultimate tensile strength of 111 MPa. In addition to that, the intermetallic compound (IMC) of the all the specimen was observed to be in the range of 1.59 μm to 10.8 μm. Fracture failures occur at the IMC interfaces on all specimen, where a thicker IMC layer consequently yields a lower tensile value. It can be concluded that the optimum parameters for AA6061-0 to SUS304 welding can be achieved at 90 °C with 17.5 V welding voltage and 110 A welding current.


Introduction
Metal inert gas (MIG) welding is a prominent fusion welding method and is widely used in various industrial applications such as in shipbuilding, construction and automotive sector. Due to its long history, many researches have been done on its feasibility in various environments, and this is evident through the many international standards that have been established on MIG welding. To meet new industrial demands, a new welding technique known as dissimilar welding was introduced. Such technique opens the possibilities to join metals of dissimilar alloy or material together to form a unique part. Dissimilar welding research have since seen many successful joining of different materials to aluminium, such as aluminium to steel [1], aluminium to copper [2], aluminium to magnesium [3] and aluminium to titanium [4].
Out of the many combinations, dissimilar welding between aluminium alloys and various steel alloys has been the most widely studied upon. This is primarily due to the high strength and toughness characteristics of steel alloys that compliments well with the low cost, light weight and corrosion resistant characteristic of aluminium alloys [5,6]. However, such combination is difficult due to the near zero solubility and the stark contrast of the melting temperature between aluminium and steel. Consequently, this facilitates the formation of a notorious Fe x Al y brittle intermetallic (IMC) compound between the two metals during the joining process [7,8]. Since these IMC layer formation is inevitable but adversely affects the weld strength, Kreimeyer and Sepold [9] have suggested that a weld joint with an IMC layer thickness of less than 10 µm is acceptable and can be considered mechanically sound. To mitigate the formation of such brittle structure, several researches have proposed the use of preheating prior to the welding process. Recent researches by Kreimeyer and Vollertsen [10] and Bang et al. [11] have suggested a secondary heat source such as a tungsten inert gas (TIG) torch in order to preheat the steel counterpart prior to welding to enhance the joint strength. Other works also suggested the use of laser as a preheating source to enhance fusion and solid state welding between aluminium and steel [12][13][14]. These researches result in significant improvements to the joint strength due to the suppression of the deleterious IMC layer [15].
However, such additional preheating source set up is costly and complex. Shah et al. [16] conducted the preheating method using a mobile burner torch on tungsten inert gas (TIG) dissimilar welding of AA1100-SUS304 and AA6061-SUS304, respectively. The method proved to be simple, effective and cheap. Both results have shown an increase in the tensile strength of the joint when steel is preheated up to 150 ˚C.

Materials
The base materials used in this study was a 2 mm thick aluminium alloy AA6061-O and stainless steel SUS304. Both metals were cut into plates with dimensions of 150×55×2 mm. The filler metals used for this study is ER4043. Table 1 shows the chemical composition (in wt%) for base metals AA6061 and SUS304 as well as filler material ER4043, respectively.

Welding process
The joining process was carried out by using an automatic Dr Well DM-500EF MIG machine. The oxide layers on specimen surface were removed using a metal brush to prevent inclusion of impurities in the weld joint. They were then carefully cleaned using acetone and left to dry to remove dirt and grease. Welding with the parameters mentioned in table 2 without preheating also yielded unsuccessful joints. After the welding set up and just prior to the welding process, the stainless-steel counterpart was preheated using a commercially available gas burner torch up to the desired temperature. The temperature was monitored closely using a mobile Center 300 Series digital thermometer with surface probe. Once the desired temperature was reached, the welding process commenced.

Mechanical and microstructural analysis
In order to investigate the specimen joint strength, tensile tests were conducted using the Shimadzu Universal AG-X tensile test machine. The dimension for the tensile specimen was cut using an electrical discharge machining (EDM) wire cut according to the ASTM-E8 standard. Figure 1 shows the dimension (in millimeters) for the tensile specimen. The weld joint is to be located at the middle of the dog bone shape. For the microstructure analysis, the specimens were ground using 240, 320, 400 and 600 grit papers in that order and polished using 6 µm and 0.05 µm polishing plates to remove scratches on the surface to be observed. To reveal the grain boundaries, they are finally etched by dabbing a cotton bud immersed in an HF aqueous solution for the stainless-steel region, and a separate cotton bud immersed in Keller's reagent for the aluminium counterpart, respectively, in ambient temperature. Microstructure and elemental analysis of the specimen were observed using optical microscope, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Figure 2 shows the best surface appearance of the welded specimens for each preheating temperature group. From the figure below, it can be seen that sound weld joints with minor defects can be obtained from the process. A better weld bead consistency can be observed from figure 2(a) using the 90 ˚C preheating temperature (sample 2), but better wetting can be observed for sample 4 and sample 9.  Figure 3 shows the side and top view of the fractured tensile test specimen. A more detail view of the weld bead before and after the fracture can be observed in figure 4. It can be seen in figure 4(a) that the welding process yielded a welding-brazing joint; the aluminium side combined with the aluminium filler to create a welding joint, whereas the ER4043 filler only brazed at the contact surface of the steel side. After fracture, figure 4 Figure 6 shows the microstructure morphology of the welded joint using an optical microscope at a low magnification (figure 6(a)-(c)) and at a high magnification (figure 6(d)-(f)), respectively. A mixed region can be observed between the weld zone and the aluminium base metal ( figure 6(a)). On the other hand, a clear thin boundary between the weld zone and stainless steel base metal can be seen ( figure 6(b)-(c)).

Microstructure analysis
With a higher magnification, the transition between weld zone and heat affected zone (HAZ) of aluminium can be seen at the aluminium side ( figure 6(d)). Coarse grains can be seen on the left side of figure, indicating that HAZ experienced thermal cycle without deformation. On the contrary, figure 6(e)-(f) show the existence of a thin inter metallic compound (IMC) layer on the inner and top faying surface of the steel sheet (inner and top IMC layer), respectively.
To better understand the IMC layer and the region surrounding it, the region is further examined using a scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Figure  7 shows the result of the SEM analysis at the inner side and top side of the bonded aluminium-steel surface. Due to the inconsistency of the thickness, several points were taken to determine the thickness range. The IMC thickness range for sample 2 was measured to be between 1.59 and 10.8 µm in size, indicating that a mechanically sound joint was achieved.
A line elemental analysis of the IMC layer region was conducted using EDX and its count-persecond versus position graphs of the main elements are shown in figure 8. From the individual elemental spectrums, it is evident that the left side of the analysis region ( fig.8(a)) is dominated by aluminium and magnesium ( fig.8(b)-(c)), whereas iron is mostly present on the right side ( fig.8(d)). As for the IMC layer, it consists of aluminium, magnesium, silicon and carbon. Apart from the main elements of base metals, magnesium and silicon ( fig.8(c) and (e)) are present primarily from the aluminium base metal and filler metal. However, since there is low carbon content in the initial set up, it is still unclear as to how carbon emerged in the IMC content. It can be assumed that the abrupt increase of carbon at the IMC layer ( fig.8(f)) is primarily due to hydro carbon contaminants present at the faying surface of each material and at the filler metal surface. Further study is needed to clarify this issue.  The premise of the preheating method is to enhance diffusion between the base metals at a shorter duration and lower heat input. The IMC thickness under diffusion control process is governed by:

= √t
(2) and = exp (− ) where X is the IMC thickness (mm), t is the time (s) for diffusion, K o is a constant, Q is the activation energy (J) for the growth of the layer, R is gas constant and T is the absolute temperature (K) [18,19]. It is postulated that preheating on the stainless-steel counterpart provides additional energy to the system to surpass the threshold of Q in order to initiate IMC formation with lower heat input. With lower excessive heat input introduced to the system during the welding process, a shorter duration can be achieved, thus, from the equation (2), a thinner IMC can be formed. The correlation between tensile strength, IMC thickness and the welding parameters can be explained as follows. As shown in figure 2, the samples with the best weld appearance have shown moderate heat input in the mid-range between the lowest and highest heat input, 1785c kJ/mm and 2070c kJ/mm, respectively. However, contrary to sample 2, tensile strength for both sample 4 and sample 9 show low tensile strength compared to other samples. This implies that despite the moderate heat input from the MIG torch which facilitates the wetting and consolidating process, the higher preheating temperature (180 °C and 270 °C respectively) of both samples may have caused the brittle IMC layers to grow thicker and cause detrimental effects on the samples' mechanical property. A general trend of decreasing tensile strength as preheating temperature increases can also be seen in figure 5, except for sample 3. Having the highest heat input (2070c kJ/mm), the excess heat input in sample 3 may have also caused further IMC growth and therefore a low tensile strength is produced.

Conclusion
Dissimilar metals joints between aluminum alloy AA6061-O and stainless steel SUS304 were successfully joined by MIG welding with preheating on the stainless-steel specimen before welding process. Major conclusions of this study could be summarized as follows: i. All specimen yielded quality joints with minor defects. ii. Fracture failure occurs at the IMC interface on all specimens. The preheated specimen with 90 °C preheating obtained the maximum ultimate tensile strength of 111 MPa. iii. The IMC of the specimen with highest tensile strength was observed to be in the range of 1.59 µm to 10.8 µm. iv. EDX analysis show that the IMC mainly consists of the base metal elements as well as contaminants. v. The optimum parameters for dissimilar welding of AA6061-O and SUS304 can be achieved at 90 °C with 17.5 V welding voltage and 110 A welding current.