Comparative Study of Resistance Spot Welding and Laser Spot Welding of Ultra High Strength Steel for Vehicles

Researchers from Annamarai University in India and South Ural State University in Russia reported a comparative study of resistance spot welding and laser spot welding of ultra-high strength steel for automobiles. The related research was published in The International Journal of Lightweight Materials and Manufacturing under the title "A comparative study on resistance spot and laser beam spot welding of ultra high strength steel for automotive applications".

This study investigated the effects of resistance spot welding (RSW) and laser spot welding (LBSW) processes on the microstructure evolution, load-bearing capacity, heat affected zone (HAZ) softening, and corrosion resistance of ultra-high strength steel (UHSS) joints welded using lap design. A dual phase 1000 grade (UHSDP1000) ultra-high strength steel plate with a thickness of 1.20 millimeters was welded using response surface methodology (RSM) optimized RSW and LBSW parameters. The microstructure characteristics of the welding area of RSW and LBSW joints were studied using an optical microscope (OM). The load bearing capacity of RSW and LBSW joints was evaluated using tensile shear failure load (TSFL) and cross tensile failure load (CTFL) tests. The fracture surfaces of TSFL and CTFL test samples were observed using scanning electron microscopy (SEM). The microhardness distribution of the RSW and LBSW joint area was evaluated and attributed to the TSFL and CTFL failures of the joint. The corrosion resistance of RSW and LBSW joints was analyzed using potential corrosion and immersion corrosion tests. The TSFL and CTFL durability of RSW joints are 183% and 62.79% higher than those of LBSW joints, respectively. Due to the smaller bearing area, the TSFL and CTFL durability of LBSW joints is not as good as that of RSW joints. This leads to stress concentration in the FZ and HAZ of the LBSW joint. RSW and LBSW joints exhibit TSFL and CTFL failures in fracture mode, accompanied by HAZ tearing. The reason for the failure of RSW and LBSW joints in the heat affected zone is softening caused by martensitic tempering and grain coarsening. The corrosion resistance of LBSW joints is worse than that of RSW joints, due to the higher content of martensite, which increases the proportion of pitting sites and reduces the corrosion resistance.

Figure 1. Potential applications of UHSS in automotive structural frameworks.

Figure 2. a) Images of RSW and b) LBSW machines used for manufacturing joints.

Figure 3. RSWed UHSDP1000 steel joint image: a) TSFL; b) CTFL test samples.

Figure 4. LBSWed UHSDP1000 steel joint image: a) TSFL; b) CTFL test samples.

Figure 5. a) Tensile testing machine setup; b) TSFL test setup; c) CTFL test setup.

Figures 6 and 7 respectively show a schematic diagram of the electrochemical corrosion test (top image) and a picture of the test sample (bottom image).

Figure 8. Typical macro images of spot welded joints: a) RSW and b) LBSW.

Figure 9. a) Images of TSFL samples with broken RSW joints and b) LBSW joints.

Figure 10. a) CTFL sample images of RSW joint and b) LBSW joint rupture.

Figure 11. a) Transverse BM section; b) Optical microstructure of longitudinal BM cross-section.

Figure 12. Microstructure of UHSDP1000 steel spot welded joint in different regions: a) FZ of RSW joint, b) UC-HAZ, c) IC-HAZ, d) LC-HAZ, and e) FZ of LBSW joint f) UC-HAZ、g) IC-HAZ、h) LC-HAZ。

summary
1. The FZ microstructure of RSW joint shows the evolution of acicular ferrite, Flat noodles martensite and polygonal ferrite. Since the cooling rate of LBSW is faster than that of RSW, FZ of LBSW joint shows a finer lath martensite structure and a certain proportion of acicular ferrite.

2. Compared with the LBSW joint, the TSFL and CTFL durability of the RSW joint have been improved by 183% and 62.79%, respectively. Due to the relatively small load-bearing area, the TSFL and CTFL durability of LBSW joints is not as good as that of RSW joints. This leads to stress concentration in the FZ and softened HAZ of the LBSW joint.

3. RSW joint and LBSW joint fracture failure, UHSDP1000 steel tearing in HAZ. The RSW joint exhibits a ductile failure mode, while the LBSW joint shows a combination of ductile and brittle failure modes, due to stress concentration in the softened HAZ leading to more tearing on the HAZ side.

4. The HAZ hardness of RSW and LBSW joints has significantly decreased, leading to HAZ softening problems, mainly due to the presence of a large amount of martensite in the microstructure of UHSDP1000 steel.

5. The TSFL failure of RSW and LBSW joints in the HAZ is attributed to softening caused by martensitic tempering and grain coarsening in the HAZ.

6. Compared with RSW joints, LBSW joints have higher FZ hardness, mainly due to the finer martensitic microstructure in FZ.

7. The corrosion resistance of LBSW joints is lower than that of RSW joints, because the martensite content is higher, which is conducive to an increase in the proportion of pitting sites and a decrease in corrosion resistance.

Source: Yangtze River Delta Laser Alliance