Study on Influence of Electrode Type on Weld ability of TL091 Al. Alloy in Medium Frequency RSW

The characteristics and difficulties of medium frequency resistance RSW (MFRSW) of high strength Al alloy TL091 were analyzed. In order to improve the service life of Al. alloy RSW electrode cap (EC), the shape of EC and welding parameters were studied. The weld ability of RSW of Al. alloy TL091 was studied by using a special pressing mold to make the surface of EC form a special shape. By studying the type and size of the surface-forming mold, pressing method and pressing parameters, the key factors affecting the weld ability of Al. alloy RSW were obtained. The specification and shape of compaction mold will affect the adhesion between alloy and EC, and pit depth, pit distance width and boss size of the mold have great influence on welding ability of Al RSW after single grinding. When indentation depth of the pressing mold is 0.5 mm, the width between the molds is 0.4~0.5 mm, and the width of bulge is 0.2 mm, it can obtain a RSW capability of at least 128 points after each grinding. The direction and ideas for improving the life of Al. alloy RSW EC were determined, which could provide reference for the optimization of automobile Al. alloy RSW.


Introduction
Lightweight promotes the wide application of Al alloy materials in automobile body.Al alloy RSW has characteristics of high flexibility, high efficiency and low cost [1].Service life of Al alloy RSW EC is related to EC material, shape, plate material and surface state, welding parameter and cooling state [2~4].These will have an important influence on weld ability of Al alloy.EC shape is one of the key factors affecting nugget formation of Al alloy RSW.
The research shows that using concentric rings with raised end faces of multi ring dome (MRD) EC can pierce the oxide film on the surface of Al alloy and reduce contact resistance between EC and plate.However, it is necessary to customize a special grinder, and with the wear of the EC, the state of the ring is inconsistent after each grinding, which cannot guarantee the consistency of welding quality, and once the ring is obviously flattened, alloying phenomenon on surface of EC will become more serious [5~10].
Welding ability of Al alloy RSW is improved by changing shape of EC.EC was compacted by using a compaction die with a specific specification.Through suitable specification parameters, the RSW method of Al alloy material was realized by repeated compacting, welding, re-compacting and welding, and RSW weld ability of TL091under different ECs was deeply studied.

Al alloy RSW characteristics
For the characteristics of Al alloy, with contact resistance between EC and work piece becoming high, thermal conductivity of Al alloy increases.In order to ensure formation of welding nugget, strong

Relationship between nugget shift and Peltier effect
When current flowed through the loop composed of different conductors, besides the generation of irreversible Joule heat, on the junction of different conductors, heat absorption release could occur respectively, due to the different directions of current.This phenomenon is the known as the Peltier effect.The charge carrier moves in the conductor to form an electric current.Due to different energy of levels of the charge carrier in in different materials, when it moves from a higher energy level to a lower energy level at the interface of two contacted conductors, extra energy would be released.On the other hand, as it moves from a lower energy level to a higher energy level, heat would be absorbed at the interface of the two contacted conductors since more energy is required.The effect is reversible between the endothermic process and the exothermic process with regard to the current flow direction (shown as in figure 1 [11] Based on the Peltier effect and the features of resistance RSW of Al alloy, the corresponding thermo-physical model was established as shown in figure 2.

EC size design
The specifications of the ECs used in this experiment were determined through experimental analysis and comparison of EC with different radius of the front-end.

Welding parameter
In order to compare and analyze welding effect of ECs with different radian at the front-end, the corresponding welding parameters (as shown in Table 1) were set and continuous welding was carried out on the premise that other welding conditions were unchanged.After welding is over, joint is chiselled, diameter of nugget is measured, and influence of EC shape on weld ability is analysed.

Design of ECs and molds
Compared with EC in the form of a boss with front end of 10~12mm, EC with radian is more conducive to the extrusion of alloy compounds along surface during formation of joints.However, with increase of welding points, alloy adhered to the EC will gradually increase, which leads to the need to grind EC after welding a certain number of points to ensure welding stability.Generally, EC needs to be grinded again after welding for 20 points in production.

Design of EC mold
By using a uniformly-distributed and four-prism shaped mold, surface of EC is compressed to form a layer of uniform concave-convex structure on EC surface.When EC of this structure contacts the work piece, contact between EC and work piece changes from large surface contact to small cross-sectional contact, which makes it easier to puncture oxide film on the Al alloy surface and realize welding.The honeycomb mold is shown in figure4.
The design of this structure can promote axial fretting and sliding between EC surface, thus further reducing interfacial tension and increasing the desorption of erosion layer.The corrosion layer is more easily adsorbed on surface of plate under welding pressure.When EC is in contact with work piece, the generated alloy is more likely to remain on the surface of plate.After welding a certain number of points, EC surface begins to have adherent alloys.Pressing die exerts pressure on the surface of EC, making EC surface form a concave-convex symmetrical structure, crushing the adsorbed alloy compound.When work piece is pressed, the broken alloy compound is adsorbed on work piece, reducing the alloy adhered on the surface of EC.EC surface is pressed and welded repeatedly in this way, which ensures the state of EC surface and greatly improves welding points of EC after a single grinding.

Design of EC die and compaction parameters
The pressure die should have high hardness and wear resistance.Die steel is chosen with hardness≥HRC63, and set different pressing specifications, in which the number of welding points after each EC pressing, the pressing pressure of EC, the pressing time and frequenct, the pressing angle between EC and the die are the key control indexes in the pressing process.As shown in Table 2, settings are as follows.2. 6 Welding equipment and parameters

Welding equipment
Water flow: 20 L/min, Gas pressure: 6Bar, Welding material: TL091, Welding material thickness:2mm +2mm, Welding fixture: a set of Al alloy non-magnetic welding fixture, Welding current reaches thousands of amperes, too high welding current will generate a magnetic field in welding process, which will magnetize the work piece, fixture and sensing facilities.Therefore, it is necessary to choose non-magnetic stainless steel chuck and non-magnetic wiring switch for fixture design.Welding equipment information is shown in Table 3.

Welding parameters
Test equipment has the functions of preheating welding and adaptive control.Through actual welding and metallographic examination, the recommended welding parameters are determined as shown in Table 4, and the welding curve is shown in figure 5.

Influence of EC size on welding.
The welding nugget size measured after continuous welding for 7 points with ECs with radians of R100mm and R150mm at the front end is shown in figure6.
After welding, the nugget diameter is between 7.4mm and 9mm, and the nugget size obtained by two ECs under different welding current input is the same basically, and it meets the requirements of welding standard of 5√t, that is, d p ≥7.07 mm.
Through comparison of experiments, it can be seen that the EC with radius size of 100mm can reduce welding current input and EC erosion compared with radius size of 150mm.

Influence of mold size
In order to determine the range of suitable mold specification, the welding state of joints with different mold sizes was studied under the same condition.Through test of a set of dies, it was found that there were more consecutive welding points on molds 1-3 and 1-4.
The experiment found that within a certain width and size range, the depth and width of the mold pit have a significant impact on welding.Therefore, continuous welding experiments were conducted after optimizing the mold.In the experiment above, it was found that RSW ability is different after being pressed by different mold, as shown in figure7.Under the same condition, after 48 points of continuous welding using mold 2-1, obvious adhesion phenomenon appeared on the joint surface.The depth of pressing dies 2-2 and 2-3 increased compared with that of die 2-1, with a depth of 0.4~0.5mm, a width spacing of 0.5mm and a boss size of 0.2 mm.After a single grinding, dies 2-2 and 2-3 are welded continuously for 128 points, and the surface state of the joints is good, shown in figure7.
It was found from the experiment that the depth of the die pit has a great influence on welding ability in a certain width and size range.If the depth is too small, the alloy oxide will adhere to EC, which will affect continuous welding ability of EC.Therefore, designing a die suitable for pecifications and sizes can reduce the adhesion of alloy on EC and improve welding ability.
After continuous welding, welding spot was sampled and metallographic state of welding spot was observed, and it was found that the nugget was in good condition.With the increase of welding points, the nugget shifted obviously to positive EC, as shown in figure8.EC compaction test of die 2-3was carries out, and observes the state of EC surface after first compaction.It can be seen that under the action of pressure, EC surface will form uniform and orderly surface pits after being compacted by the die, as shown in figure9.

Figure9. SEM EC surface
EC is pressed and welded again after every 16 welding points, and so on, the welding surface of a single EC for 128 points is shown in figure10 and figure11, and welding spot surface is good, so it can be seen that the EC can still be continuously welded EC can still be continuously welded.
After welding at 128 points, the surface compositions of the center and edge areas of the positive and negative ECs were measured, as shown in figure12 It can be seen that there are lots of Al in the central area of the positive EC, and at the same time, there is corresponding oxygen, and the existence of the alloy components intensifies the corrosion of EC.However, Al content in central area of negative EC is obviously lower than that of positive EC, and the alloying is relatively weak, because copper has a positive thermoelectro-motive force, which increases with the increase of temperature.The thermoelectric potential of Al is negative, which decreases with the increase of temperature.According to the definition of Paltta effect heat, when current flows into work piece from EC, the generated Paltta effect heat is positive, and heat should be released.When current flows into EC from work piece, the generated Paltta effect heat is negative, and it is necessary to absorb the heat [12], which shows that anode corrosion is relatively serious.
Subsequently, destructive test of joint samples was carried out.Maximum shear force measured is shown in Table 8, and the nugget size and shear strength are shown in figure13.

Figure 4 .
Figure 4. Principle of honeycomb mold A. principle B. mold shape C. design size D. profiled electrode cap

Table 1 .
Different ECs and welding parameters

Table 2 .
Mold Performance and Compaction Parameter Settings

Table 3
Welding equipment parameters

Table 4
Welding Parameters

Table 6 .
Performance Index of EC

Table 7
Continuous Welding results by molds with different size