Effect of Welding Time Variation in Resistance Spot Welding on Mechanical Properties of Dissimilar Joints on Mild Steel and AISI 304 Stainless Steel

Resistance spot welding plays an important role in the automotive industry for working on car bodies or frames. Welding with two different steel materials is used in automotive structures. However, the low strength of the joints of different steel materials is a major problem, limiting the scope of its application. The quality of the welding connection Resistance spot welding is influenced by several factors, namely, time, welding current, and electrode pressure applied during welding and post-welding treatment. In this research, mild steel plate with AISI 304 stainless steel is welded by Resistance spot welding with different variations of welding time, material hardness and tensile testing that will be investigated. The results showed that the highest Rockwell Hardness time was obtained at a welding time variation of 8 seconds with a current of 6000 Ampere was 112.2 so the higher the welding time, the higher the hardness obtained. welding 8 seconds with a welding current of 6000 Ampere obtained a maximum tensile strength of 20.4 kgf/mm2 so the greater the welding current, the tensile strength will increase.


Introduction
The automotive industry continues to grow in the face of competitive competition [1].Resistance spot welding (RSW) is an important manufacturing technology [2][3], its advantages include low cost, high production efficiency, high welding time and adaptability to automation so that it can be used for mass production [4][5].The characteristics of the resistance spot with dissimilar materials are very decisive to get a good weld joint, welding time, welding current, electrode force, preheat current, and electrode shape are one of the welding parameters, so that welding joints in automotive vehicles become safer as a solution to answer the needs passenger safety [6][7].Welding with two dissimilar steel materials is used in automotive structures However, the low strength of the joints of different steel materials is a major problem, limiting the scope of its application [8].
Lihu Cui et al, investigated the resistance spot welding of A6061 aluminum alloy and copper-coated steel.The results show that the joint strength and diameter of the nugget increase with increasing welding current and welding time and decrease with increasing electrode force [9].Nannan Wang et al investigated the resistance spot welding of mild steel Q235 and stainless steel SUS304 with a Ni interlayer.[10].Xinjian Yuan et al analyzed different joints of DP600 and DC54D steel, the results showed that the standard of welded joints increased with increasing Zn islands on the weld surface at that time [11].Dissimilar steel between carbon steel and stainless steel welded by resistance spot welding, shows that the increase in current, and welding time is directly proportional to the increase in the tensile strength of the welded joint [12][13][14] However, resistance spot welding between dissimilar materials accompanies some difficulties, due to the large difference in physical and thermal properties between the two dissimilar materials, and the formation of brittle reaction products at the welding interface so that the brittle intermetallic compound layer formed on the bonding surface after welding is the main cause of the limited joint strength [15][16][17].
In this paper, mild steel plate with AISI 304 stainless steel is welded by Resistance spot welding with different variations of welding time, material hardness and tensile testing that will be investigated.The goal is the application of dissimilar steel RSW for automobile manufacturing.

Methodology
The materials used in this study were mild steel plate and AISI 304 stainless steel with a specimen size of 1 x 25 x 100 mm as shown in Figure 1.The chemical composition of the two materials is presented in Table 1.

Figure 1. A Schematic diagram of test object used in the experiment
The material welding process is carried out using a pressurized air electrode hydraulic system which has a pressure regulating mechanism, for setting the welding current using an analog system and timing using a digital system as shown in Figure 2. Details of the welding settings are in Table 2.  Tensile testing is carried out to determine the mechanical properties of the welding quality in this study as shown in Figure 3.The hardness of a material can be determined by using a hardness tester using a hardness tester as shown in

Tensile test to determine the mechanical properties
The mild steel plate and stainless steel AISI 304 specimens were welded with resistance spot welding in accordance with the welding parameters in Table 1, then tensile testing was carried out which is the method used to test the strength of a material by applying an axial force.The principle of this test is that the test object is given an axial tensile force load that increases continuously at both ends of the tensile specimen until it breaks while the increase in length is measured.The data obtained are in the form of changes in length and changes in load which are then displayed in the form of graphs of yield stress, maximum stress and fracture stress [18].
Tensile test to determine the mechanical properties of Based on Figure 5. graph of the relationship between yield stress and welding time, at 2 seconds the lowest value obtained was 20.48 kgf/mm2, then at 4 seconds it decreased with a value of 19 .98 kgf/mm2, and at 6 seconds the highest maximum tensile strength is 20.52 kgf/mm 2 , and at 8 seconds it decreases by 20.4 kgf/mm 2 .

Figure 5. Yielding stress vs welding time
Based on Figure 6. the graph of the relationship between maximum stress and welding time, at 2 seconds the lowest value obtained was 20.48 kgf/mm2, then at 4 seconds it decreased with a value of 19.98 kgf/mm 2 , and at 6 seconds the highest maximum tensile strength is 20.52 kgf/mm 2 , and at 8 seconds it decreases by 20.4 kgf/mm 2 .

Figure 6. Maximum stress vs welding time
Based on Figure 7. the graph of the relationship between the maximum stress and the welding time, at 2 seconds it gets a value of 18.4 kgf/mm 2 , and in 4 seconds it has the lowest decrease of 17.6 kgf/mm 2 , then 6 seconds gets a value the highest was 19.6 kgf/mm, and the time of 8 seconds decreased by 18.4 kgf/mm 2 .Based on the results of the tensile test, it can be seen that the increase in time from 2 seconds to 4 seconds has an impact on increasing the tensile strength.While the increase in time from 6 seconds to 8 seconds shows a decrease in tensile strength.With increasing welding time, higher tensile strength is obtained, this is because the heat input that occurs is also greater, resulting in a strong tensile strength.But the connection is not always better, in the dissimilar metal point weld connection there is a condition where at 8 seconds the tensile strength decreases, this happens because the heat input exceeds the melting point of the material used so as to produce a wide weld metal area and deep penetration area, then the welding result will be damaged [19][20][21].

Hardness test
The Based on table 3, the hardness test data on specimens with spot welding results show that as the welding current increases in the welding process for the dissimilar connection point of 304 stainless steel and mild steel, the higher the hardness value is produced, the increase in the hardness value is caused by heat input.and rapid cooling that occurs during the welding process.However, in the HAZ area and the base metal area, the hardness value decreased significantly, this was due to the influence of heat received during welding [22][23][24].

Conclusion
The results obtained from the tensile test, and the hardness of the material, varied the welding time of the Mild steel connection and AISI 304 stainless steel to determine the mechanical properties.the maximum tensile strength is 20.4 kgf/mm 2 , so the greater the welding current, the higher the tensile strength.The highest hardness value obtained at 8 seconds of welding time with a current of 6000 Ampere is 112, so the higher the welding time, the higher the hardness obtained.

Figure 4 .
This test which is generally used is the HRC type.(Rockwell Hardness Test), HRB (Rockwell B Hardness Test), HRA (Rockwell Hardness Test), HRN (Rockwell N Hardness Test).Rockwell hardness test level is regulated according to standard DIN 50103.

Figure 7 .
Figure 7. Fracture stress vs welding time

Table 3 .
Results of data collection for hardness testing