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Spot weld growth on medium carbon steel.
Part 2: Servo based electrode actuation system

Nachimani Charde

Department of Mechanical Engineering,
Faculty of Engineering,
University Malaya, Malaysia
E-mail: nachicharde@yahoo.com

          In the part 1 of this research, the medium carbon steel was welded using pneumatic based electrode actuation system and subsequently it was investigated for the fatigue strength under tensile shear load, hardness and micro structural changes. Similar type of welding conditions and strength tests were conducted in this experiment but the electrode actuation system was replaced by servo based electrode actuation system; instead. A 1.5 kW powered-servo motor and its driving mechanical assembly were electro-fitted as to improve the force profiles before, during and after the welding process takes place. In doing so, the force exertions are systematically distributed and the corresponding changes are analyzed for the welds improvements. As such the specimen sizes and corresponding alignments were kept constant that of the previous experiments had but the welding lobe parameters and force presets were slightly calibrated for. The servo based electrode actuation system improves the electrically generated forging forces during welding process and consequently minimizes the porosity occurrences at the welded region. Specifically the diameters of fused regions were increased for the same welding conditions as compared to part...

Medium carbon steel, Carbon steel welding, Spot welding of steel, Electrode actuation.

           Force changes during the entire welding process are important parameter to understand as it diminishes the quality of welded regions in terms of fatigue strength. When the pneumatic based and servo based electrode actuation system are separately analyzed, it noticeably varies from the force profiles distribution, particularly at the increment of forging forces during weld cycles [1]. These forging forces are irremovable properties as it was induced due to the high AC current flow. As the welding current movement goes on both direction (positive to negative on half cycle and vice versa) during welding process in AC spot welder; the forging forces do exist significantly in electrical means [2]. This effects, of course, can be reduced by using servo based electrode actuation system with supportive braking system. Literally, the servo based system squeezes the base metals very smoothly and locks the mechanical lever using brakes once it reaches the exact pressing force levels [3]. So the strong hold of base metals during weld cycles have resulted low generation of forging force effect and hence the heat diffusion is significantly improved

The entire research was carried out using 75 kVA spot welder, powered by AC waveform and capable of handling up to 99 weld cycles in whole. Each cycle consumes 0.2 mS in time scale. Pre and post welding mechanism was available but only single welding current and single welding force (SISF) method was taken into consideration for this paper. The base metals were prepared in rectangular shape metal sheets (200 mm x 25 mm x 2 mm) as used in part 1 before and it has been shown in Fig. 1. The chemical properties are: C = 0.40; Cu = 0.016; Mn = 0.90; P = 0.040; S = 0.050 and Si = 0.006 for medium carbon steel. Hardness was 65 HRB when measured with Rockwell hardness tester on scale ‘B’ and the truncated-electrode tip was 5 mm diameter which was selected from RWMA’s class two (copper and chromium) category.

3.1. Tensile shear test results

The tensile-shear test (Figure 5) was carried out using hundred kilo Newton (100 kN) capacity machine to determine the strength of spot welded samples of both (current versus weld time; current versus force) sets [5]. Average strength values from the five samples were taken as the equivalent

This experimental investigation looks into the spot weld nugget growth of medium carbon (0.4%) steel using servo based electrode actuation system and it finalizes the facts that: the tensile shear force was slightly higher in the servo based experiment as compared to the pneumatic based experiment (part 1) because of the weld beads diameters increment. The strength increments in terms of percentage were noticed from 11 to 25% upon the parametric variations. A combination of 8 kA, 3 kN and 20 cycles (weld schedule 9) was the highest limit of parameter set up for 1 mm thickness, from which the widest diameter was recorded at 5.899 mm. A combination of 6 kA, 3 kN and 10 cycles was the lowest limit of parameter set up for 1 mm thickness, from which the least acceptable diameter was recorded at 3.676 mm. The common crack initiation and post crack propagation modes that noticed during tensile test was only the PF and TF modes. There was no IF mode appeared at all due to the suppression of forging force fluctuations. The hardness distribution was almost same as the pneumatic based system because the solidification process was same for both systems. Thus, the values were increased from 55 HRB (unwelded areas) to 105 HRB (welded areas) in average as how noticed in the pneumatic system previously. Carbon content was slightly increased at the welded zones but it does not produce profound changes on either hardness or micro structural orientation. Macro structures of BM, FZ and HAZ were remained almost similar to the pneumatic based...

I would like to thank Ministry of Science, Technology and Innovation, Malaysia (MOSTI) for their financial support during the experimental work. This research outcome is part of Nachimani Charde’s doctoral research.

[1]. Nachimani, C. (2012). Spot weld growth on medium carbon steel (Part 1). International Journal of Mechanical and Materials Engineering, No. 1, Vol. 7, 36–40.
[2]. Aslanlar, S. (2006). The effect of nucleus size on mechanical properties in electrical resistance spot welding of sheets used in automotive industry. Materials and Design, 27, 125–131.
[3]. Niu, B. (2009). Improving productivity and quality in automotive spot welding-servo gun based. Automation and Control, Vol 2, 120-131.
[4]. Shih, F.L.; Li, X.W; Yoke, R.W and Rung, W. (2010). Input electrical impedance as quality monitoring signature for characterizing resistance spot welding. NDT&E International, 43, 200–205.
[5]. Darwish, S.M. (2004). Peel and shear strength of spot-welded and weld-bonded dissimilar thickness joints. Journal of Materials Processing Technology, Vol. 147, 366-378.

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