Critical Nugget Diameter of Resistance Spot Welded Stiffened Thin Plate Structure

Stiffened thin plate structure which is mostly applied to the car body structures consists of frame and sheet. It is a low cost method to reach a high-performance vehicle structure because of requirement low volume materials. Frame is thicker than sheet, so it acts as the reinforcement of this structure. The resistance spot welding (RSW) is generally used to join it. In this study failure modes of resistance spot welded thin plate structure, pullout and interfacial failure modes, were investigated based on thickness of joined plates and load secondary voltage to determine the critical nugget diameter. Three pairs of joined plates thickness of 1-1, 2-1 and 3-1 mm were spot welded using varied voltage in a range of 1.6 V to 2.3 V. The spot weld nugget of the stiffened thin plate structure joint were asymmetric form. Increasing welding voltage made the shear load bearing capacity of RSW joint increase. In the pullout failure, the bearing capacity increased if the thickness of joined materials increased. The experimental critical nugget diameter to guarantee pullout failure mode in the unequal sheet thickness joint of the stiffened thin plate structure decreased if the sheet thickness of joined materials increased. It was higher than nugget diameter obtained by AWS formula. AWS formula should be used carefully in the dissimilar sheet thickness joint of the stiffened thin plate structure.


Introduction
Stiffened thin plate structure which is mostly applied to the car body structures consists of frame and sheet.It is a low cost method to reach a high-performance vehicle structure because of requirement low volume materials.Frame is thicker than sheet, so it acts as the reinforcement of this structure (Gean et al., 1999).The resistance spot welding (RSW) is generally used to join it.RSW is a joining process of two or more metal sheets by fusion and pressure at discrete points at the interface of joined sheets.The heat required for the weld pool is created by means of resistance when a high welding current is directed through the welded workpieces.Welding heat and pressure will make the joined metal melt and fuse to form nuggets.After a certain period of time called the welding time, electrical current is then turned off and the nuggets cooled and hardened under pressur (Pouranvari & Marashi, 2010).The advantages of using RSW are that it is a quick and economical joining, no filler material is required and dimensional accuracy is better preserved during welding with local heating.
Generally, shear and tensile load combination will be occurred when spot weld lap joint was exposed load.In this condition, two distinct failure modes were observed during static tensile-shear testing: interfacial failure (IF) and nugget pullout failure (PF).When failure occures in the form of crack propagation through the interface of joined sheet and fusion zone (weld nugget), it will be interfacial failure mode, whereas pullout failure mode is indicated by complete (or partial) nugget withdrawal of one sheet.The load bearing capacity of RSW joint can be significantly affected by failure mode.Pullout failure mode specimens have higher load bearing capacity than interfacial failure mode specimens.For this reason, welding process parameters should be controlled so that the pullout failure mode is guaranteed to ensure the reliability of spot welds during vehicle lifetime (Pouranvari & Marashi, 2010).
There are many standars and recomendations from professional organizations such as the American Welding Society (AWS) and Society of Automotive Engineering (SAE) that express the requirements to guarante the joint fail in pullout failure mode (Triyono et al., 2012).They are generally a formula with respect to the critical nugget diameter as a function of plate thickness of joined material.For example, AWS's formula is where t is the thickness of the plate in millimeters.It is very helpful formula to find suitable spot welding parameters for joint of equal thickness of joined sheet.However, there are a lot of spot welds applications in general engineering which involve dissimilar sheet thicknesses joint.In this case, a big question appears is which thickness plate is used in the formula, the thicker one, the thinner one or the average of them.Generally, schedules for this case are developed by and practiced within individual manufacturers (Pouranvari & Marashi, 2010;Agashe & Zhang, 2003).
Unfortunately, the most of spot welding research investigations, as well as most of practice recommendations, have been conducted on welding of equal thickness sheet.Despite of various application of unequal thickness RSWs, there are limited articels which expose physical-mechanical behaviors of them.Therefore, the objective of the present study is to determine the critical nugget diameter of the unequal sheet thickness resistance spot welded carbon steel by using failure mode investigation and compare to nugget diameter of AWS formula.

Materials and Welding Procedure
Three pairs of mild steel SS400 plates were joined in a lap joint by resistance spot welding (RSW) using welding schedule as shown in Table 1.Tables 2 and 3 respectively show the chemical composition and mechanical properties of research materials.

Metallography and Microhardness Measurements
Standard metallographic procedure according to the ASTM E3-01 was used to prepare macrostructure specimen.It cut cross section passing through the center of weld nugget.To reveal the macrostructure, HNO 3 2.5% solution was used.Macrostructure observations were carried out using a stereo zoom microscopy.
The hardness of the weld nugget, HAZ (heat affected zone), and the base metal were measured using microvickers methode on the metallographic specimens with a load of 500 g.It is difficult to characterize the spot weld joint in its strength due to the limitations of nugget area measurement methods.The bearing capacity is usually used to express the characteristics of the welded joints.It is the quantity that states the ability of spot weld joint to continue load between joined parts.The experimental results indicated that welding voltage had a significant effect on the shear load bearing capacity of the spot welds under tensile-shear static test.As can be seen in Figure 4, for all pairs of sheet thicknesses, increasing welding voltage made the shear load bearing capacity of RSW joint increase.It was primary due to increasing nugget area and shown by Figure 5.It is well known that increasing RSW's parameters such as welding current, welding voltage and holding time make nugget area increase.Figure 5 shows that increasing welding voltage led to increasing nugget diameter.Special case was occurred in 1-1 mm specimens, due to sheet damage under over voltage, the peak load decreased in the voltage of 2.3 V although its nugget area increased.In this condition, thin plate underwent excessive deformation and became thinner than initial thickness at the end of the nugget.Failure would lead from this area and the strength of the RSW joint was very low.On the contrary, if the welding current was lowered so thin plate was not damaged, fusion of joined plate would be imperfect, and the joint strength would be also very low.Shear and tensile load combination will be occurred when lap joint was exposed load.In this condition, two modes of failure were occurred during static tensile-shear testing: interfacial failure (IF) and nugget pullout failure (PF) as shown in Figure 6.Failure mode has a significant influence on the bearing capacity of spot welds.Figure 4 shows that bearing capacity of pullout specimen was higher than that of interfacial specimen.In the pullout failure, the bearing capacity increased if the thickness of joined materials increased.The small load component in the through thickness direction due to the increased stiffness and decreased rotation is the main reason (Nordberg, 2006).There was transition condition in which interfacial failure (IF) turned into pullout failure (PF).It was called critical condition and commonly determined by the critical nugget diameter.Spot weld joints which had smaller nugget diameter than the critical nugget diameter would fail in IF mode whereas the other would fail in PF mode.
Based on the experiment result, it was found that critical nugget diameter of the spot weld joint would decrease due to the increasing plate thickness.It did not agree with AWS formula stated that 4√ where t is the thickness of the plate in millimeters.The experimental critical nugget diameter was higher than the AWS nugget diameter.Figure 7 shows the comparison of the experimental critical nugget diameter and the nugget diameter determined by using AWS formula.If it used the thinnest plate in AWS formula, constant nugget diameter of 4 mm would be obtained for all pairs of sheet thickness.The AWS nugget diameter would increase if it used the thickest or the average of sheet thickness.As can be seen in Figure 7, if AWS formula was used in 1-1 mm and 2-1 mm joints, it would fail in interfacial failure (IF) mode and have low bearing capacity.AWS formula with the thickest plate or average plate thickness guaranteed pullout failure (PF) in 3-1 mm joint.It indicated that AWS formula should be used carefully in the unequal sheet thickness joint of the stiffened thin plate structure.

Conclusions
The spot weld nugget of the stiffened thin plate structure joint were asymmetric form due to the difference in electrical resistance of joined materials.Increasing welding voltage made the shear load bearing capacity of RSW joint increase.In the pullout failure, the bearing capacity increased if the thickness of joined materials increased.Due to the interfacial failure, in the lower voltage range, increasing thickness decreased both nugget diameter and bearing capacity.The experimental critical nugget diameter to ensure pullout failure mode in the unequal sheet thickness joint of the stiffened thin plate structure decreased if the sheet thickness of joined materials increased.It was higher than nugget diameter obtained by AWS formula.AWS formula should be used carefully in the dissimilar sheet thickness joint of the stiffened thin plate structure.

Figure
Figure 2. M Figure 4. Effe Figure 7.Comparison of the AWS's critical nugget diameter and experiment results

Table 1 .
Thickness of joined plate and welding schedule