Experimental analysis of microstructure and mechanical properties of welded joint of dissimilar alloy AA6082 and AA7075 by TIG and FSW

Abstract

Tungsten inert gas welding is the most commonly used for joining of dissimilar alloy, which are highly recommended in aircraft and automobile engineering. The quality of the weld and strength of the welded joints is higher than the other fusion welding, but there are some unavoidable microstructure defects formation such as porosity and micro cracks is found in the fusion zone. The formation of these defects will result in the reduction of weld strength. On the other hand friction stir welding removes these types of defect and improve the weld quality of dissimilar material. 
The present work will focus on the improvement of welded joint of dissimilar material. The friction stir welding destroyed the coarse grains structure in the weld zone and help to dissolves the precipitates of secondary particles, which exist along the grain boundaries. In addition the formation of very fine grain structure was observed in the stir zone as compare to the fusion zone in the TIG welded joint. The ultimate tensile strength of dissimilar alloy (AA6082 and AA7075) increases by increasing the tool rotation. On the welded joints the friction stir welded joint fabricated using tool rotation 1300 rpm have higher tensile stress of 173 MPa with higher 33.5% elongation. The joint efficiency of welded joint with 1300 rpm tool rotation is much higher than the TIG welded joint. Due to grain refinement in friction stir welding the hardness value was found maximum as compare to tungsten inert gas welding. At the high tool rotation speed with same feed rate, welding quality is improved and solve the welding defect like porosity which affect the welded joint. It is found that the residual at the center of the weldment decreases with increases the tool rotational speed. The maximum compressive residual stress 75 MPa was found at TIG weldment, whereas the minimum compressive residual stress 36 MPa was found at the center of the weldment having tool rotation speed 1300 rpm.                                                                             ©2019 ijrei.com. All rights reserved 



1. Introduction 

It is a solid-state joining technology which has been used to successfully weld aluminum and its alloys. FSW is performed with a non-consumable rotating tool consisting of a smaller diameter pin and larger diameter shoulder. The forces generated during FSW are significant; and a proper fixture design is critical to the success of the weld. The working principle of FSW process is schematically represented in the Fig. 3. Friction stir weld can be accomplished in any position. [1-3]. The ultimate tensile strength and hardness of bimetallic weld joint increases by increasing the pre-stress, and ductility was decreases when thermal loading increases. The tool contacts and penetrates into the abutting edges of the sheets being joined and traverses along the faying interface of the joint. While the tool rotates, it generates a large amount of frictional heat on the work piece. 
This heat softens the material surrounding the pin and facilitates movement of material flow around the pin to displace material from the front of pin to the backside of the rotating pin. Since no melting occurs in this process, the process was patented as a solid-state joining technology. The center of the joint, the weld nugget, namely, stir zone (SZ), exhibits a size and morphology which depends on the size and geometry of the tool involved. In terms 11 of the weld nugget microstructure, it is grouped into three features of the adjacent space, consisting of the stir zone, thermo-mechanically affected zone (TMAZ), and heat affected zone (HAZ). The stir zone (also known as the dynamically recrystallized zone) is a region of heavily deformed material that roughly corresponds to the location of the pin during welding. The grains within the stir zone are roughly equiaxed and often an order of magnitude smaller than the grains in the parent material. The tensile strength of the joint is lower than that of the parent metal and it is directly proportional to the travel/ welding speed. Welding parameter such as tool rotation, transverse speed and axial force have a significant effect on the amount of heat generated and strength of FSW joints [4-8]. The following conclusion has been made from the literature review which are as below. 

2. Experimental method and material 

2.1 Tungsten Inert gas welding 

Manual tungsten argon arc welding is generally considered to be the most difficult of all welding processes commonly used in the industry. Because the welder must maintain a short arc, the length of the electrode, and requires great care and skill to prevent contact between the workpieces. The torch is similar to welding, GTAW, which usually requires two hands, because for most applications, the welder is manual, and on the other hand increases the torch to the filler metal into the weld zone. To strike the welding arc, similar to a high frequency generator (Tesla coil) is to provide an electrical spark; this spark is used to conduct a conductive path through the shielding gas and allows rotation of the electrode and the working split piece The arc, except for the usual 1.53 mm (0.06-0.12 in).

2.2 Friction stir welding 

The experiments have been carried out on the friction stir welding machine with necessary equipment details such as tool, process parameter and safety precautions. Process parameter involved here is the tool rotation speed, welding speed, tilt angle and tool geometry. the FSP tool geometry, aluminum alloy plates, friction stir welding machine, processed zone and various tool manufactured to perform the desired experiments. The process of FSP begins with the tool design and fabrication. The main and the crucial thing of this work were the tool design for friction stir processing process, which would fix in the available friction stir welding machine shank. Initially FSP tool designed in such a way that the tool geometry was very simple with cylindrical tool, shank dia-25 mm, shoulder dia-20 mm, pin dia-8 mm, pin lenth-5.5 mm. 

2.3 Chemical composition of Al- alloy 

Aluminium alloy of AA6082 and AA7075 are selected to fabricate dissimilar joints using TIG and friction stir welding (FSW). The length, width and thickness of both the alloy plates are chosen as 120, 40 and 6.3 mm respectively. The chemical composition of AA6082 and AA7075 are given in table 1. 

2.4 Specimen Dimensions 

Tensile testing was performed on ASTM E8 standard samples to evaluate the mechanical properties of different welds. In all cases, the failure occurred in the original metal of AA 6082. Before the fracture, Welds produced a large amount of plastic deformation in the ductile failure mode. 

2.5 Processing Parameter 

The Processing parameter for Tungsten inert gas welding and friction stir welding were chosen by trial and error attempts until no visually detected defect could be identified. The penetration depth was adapted to fully penetrated butt joint in a material of 5.5 mm thickness. 

3. Results and Discussions 

3.1 Tensile strength 

Friction stir welding may be used to join a different member of material. Defect free welds with excellent mechanical properties can be achieved by FSW. The stress strain curves for TIG and FSW joints is shown in figure 18. The tensile properties like ultimate tensile strength and % elongation of the weldments are presented in table 3. The ultimate tensile strength and hardness of dissimilar alloy (AA6082 and AA7075) increases by increasing the tool rotation as shown in figure. On the welded joints the friction stir welded joint fabricated using tool rotation 1300 rpm have higher tensile stress of 173 MPa with higher 33.5% elongation. The joint efficiency of welded joint with 1300 rpm tool rotation is much higher than the TIG welded joint. 


3.2 Tool rotation speed and welding speed 

Processing parameters of friction stir welding are the main factor affecting the welded joint. If the rotating speed of FSW tool is too low then the frictional heat will not generated enough to induce plasticized flow which lead to defect in the weldment. The other important factor is welding speed. When welding speed is too low then the frictional heat makes the temperature too high then there is the possibility of excess heat flow in the welded joint, whereas when the weld speed increases the material just below the tool softens to such a degree that it act as a lubricant, lowering the friction and reduce the temperature. 

3.3 Residual Stress analysis 


It is found that in the region where the equivalent plastic strain is increases, the residual stress is decreases. When away from the stir zone point of the welded joint, the residual stress is slightly increases but after stir zone the distribution of residual stress remains almost steady. Because of unsymmetrical deformation at the welding zone, the residual stresses are not symmetric to the welding line. When the fixture are released and the temperature is going to reduce to room temperature then the material in the nugget zone tends to recover. But the weldment in the HAZ has smaller deformation and will prevent the recovery process in the nugget zone. So the maximum residual stress (RS) occur in the boundaries of the heat affected zone (HAZ) and minimum in the nugget zone (NZ). 
There are two types of residual stress distribution found in the weldment, usually tensile residual stress located in the weld area, whereas compressive residual stress can be found at heat affected zone. The results are obtained by the computational method as shown in fig. 21 for the five specimen with different processing parameters at the center of the weldment. It is found that the residual at the center of the weldment decreases with increases the tool rotational speed. The maximum compressive residual stress 75 MPa was found at TIG weldment, whereas the minimum compressive residual stress 36 MPa was found at the center of the weldment having tool rotation speed 1300 rpm as shown in fig. 22. 


3.4 Microstructure Analysis 

The pin influenced region in the friction stir welding is defined as the bottom portion of stir region, which experiences the effects like heat generation and material flow, which are solely created by the rotation and rubbing of the tool pin during friction stir welding. The strength of dissimilar alloy mainly concern on the mechanical interlocking of the material, thus the material should be flowed and mixed properly, so the dissimilar material flow decide the formation of defect free stir zone and strength of the dissimilar joint. Fig.22 (b-e) shows the microstructure of welded joint of AA6082 and AA7075 of the nugget zone of the joint interface of the weld produced tool rotation speed of 1000-1300 rpm with 44 mm/min transvers speed. The microstructure shows good stirring and more consolidate between AA6082 and AA7075 which improve the weld quality of the weldment. TIG welded joint influenced region shows larger grain size than the friction stir welded joint. 

Additionally, most grain in heat affected zone contained a high dislocation density with a network structure as shown in fig. 22(a), suggested that recovery has not been completed or was continuous in nature. Likewise, dislocations of particles were also observed in stir zone as shown in fig.22 (b-e). 

3.5 Microhardness 

The graphical representation of microhardness of welded joint with different processing parameter as shown in fig. 23. The microhardness values are less momentous in affecting the mechanical properties of the welded joint, because processing parameter (tool rotation speed, current, feed rate etc.) have more influencing factor over the hardness value [64]. 
The microhardness values at the middle and bottom of the welded joint detected the major effect, because the grain size and microhardness number were changed due to solidification sequence ad cooling rate of the weldment. The microhardness number also play a very important role to recognizing the metallurgical phase. The highest micro-hardness was found at the center of the welded joint in friction stir welding at 1300 rpm with feed rate 44 mm/min and the lowest micro-hardness was found at the center of TIG welded joint as shown in fig. 23. 

4. Conclusions

Experimental analysis of microstructure and mechanical properties of welded joint (TIG and FSW) of dissimilar alloy AA6082 and AA7075 with different processing parameter has been done, and the following conclusion can be made.
  • The ultimate tensile strength of dissimilar alloy (AA6082 and AA7075) increases by increasing the tool rotation. On the welded joints the friction stir welded joint fabricated using tool rotation 1300 rpm have higher tensile stress of 173 MPa with higher 33.5% elongation. The joint efficiency of welded joint with 1300 rpm tool rotation is much higher than the TIG welded joint.
  • Due to grain refinement in friction stir welding the hardness value was found maximum as compare to tungsten inert gas welding.
  • At the high tool rotation speed with same feed rate, welding quality is improved and solve the welding defect like porosity which affect the welded joint.
  • It is found that the residual at the center of the weldment decreases with increases the tool rotational speed. The maximum compressive residual stress 75 MPa was found at TIG weldment, whereas the minimum compressive residual stress 36 MPa was found at the center of the weldment having tool rotation speed 1300 rpm.

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