Dynamic Compression Properties of the Resistance Spot Welding of High Strength Steels under Varying Temperature Conditions

In this paper, we have studied the dynamic compression performance of the RSWof QP980 steel and TRIP800 steel by using a split Hopkinson pressure bar (SHPB), and we have also examined the fracturemode of the two research objects. It is found that the spot welding zone is primarily composed of the martensite structure, and there is a sparse defect of crystal structure adjacent to the center of nugget. In addition, there are evident gaps between the plates on both sides of the spot welding zone. (rough the measurement of the microhardness of the two grade steel, it is found that the average hardness of the RSW of QP980 steel is higher than that of TRIP800 steel. (ere is a softening region in the interface of the heat affected zone and the substrate zone. (e dynamic compression experiments are carried out on the RSW of QP980 steel and TRIP800 steel under 200°C and 300°C conditions, and it is found that the strain rate would increase with the rising temperature, but the compressive strength would experience declines. Furthermore, the sparse defects of crystal structure adjacent to the center of nugget would lead to stress rebound when the specimen is compressed. Moreover, through the observation of the fracture surface of the recovered specimens, it is found that the fracture of the nugget is brittle, whereas the fracture mode of the sample is more complicated. In addition, the fracture surface features a number of “river pattern” cleavage facets, and there are very few dimples of ductile tearing.(is study is expected to have huge implications to the safety of vehicle body under high-speed impact.


Introduction
Over the recent years, there has been constantly increasing use of high-strength steel plates in the automobile manufacturing industry. e QP980 steel and the TRIP800 steel manufactured by the Bao Steel Group have shown their advantages in terms of ultrahigh strength and ductility [1]. e QP980 steel is manufactured based on a novel process of quenching and distribution [2], whereas the TRIP800 steel features a TRIP effect. Some scholars have conducted studies on the deformation and fracture of QP980 steel [3][4][5]. Hu [6] determined the critical resolved shear stresses and hardening parameters of the constituent phases in QP980. Wang [7] provided the effects of strain rate and temperature on the dynamic tensile fracture strain of QP980 steel, whereas other scholars have studied the mechanical properties of TRIP800 steel [8][9][10][11]. Kussa [12] studied the austenite transformation kinetics and tensile properties of constructional 0.2 wt%C-Si2Mn2CrMoVNb TRIP-assisted steel. Fu [13] studied a low-alloyed C-Mn-Al-Si cold-rolled TRIP steel was tensile tested at room temperature and low temperature. In general, the sheet steel on the car body is connected by resistance spot welding. ere are roughly 4,000 spot welds in a car, whose strength and stability under load are critical, as the car will bear all sorts of loads during manufacturing and use [14][15][16]. Noh [17] analyzed the failure of welded structures considering dissimilar combinations of advanced high-strength steels (AHSS) and a conventional mild steel. Soomro [18] carried out in situ postweld heat treatment by employing a double pulse welding scheme with the aim of improving the mechanical performance of DP590 steel resistance spot weld joint. Over the past few years, numerous scholars have conducted studies on solder joints. Sezgin [19] examined the electron beam welding (EBW) method of TRIP 800 steel and DP 1000 steel and found through observation that the TRIP 800 joining absorbed more energy than the DP 1000 joining at impact notch experiment for each temperature. Emre [20] studied the impact imposed by the welding current and welding time on nugget shape, while conducting the tensile shear experiment on welding specimens in the meantime. Wu [21] studied the fatigue properties of TRIP steel subsequent to resistance spot welding. e research findings indicate that the fatigue strength of the solder joint is only 8.9% of the original trip 800 steel. rough the observation with the scanning electron microscope (SEM), it is verified that the fatigue crack between HAZ and matrix is the cleavage fracture. Zsara [22] studied the resistance spot welding performance of AHSS steel TRIP800 (transformation induced plasticity) and HSLA steel. Analysis is conducted on the impact imposed by the welding time parameters on tensile and shear properties. Moreover, the study is conducted on the optimal parameters of tensile shear strength and the separation mode encountered. e maximum tensile shear load can be obtained with the use of 15 cycles. Most of them focus on the shear failure and fatigue properties of solder joints, but few center around their dynamic compression properties.
In this study, we have observed the microstructure of the RSW of QP980 steel and TRIP800 steel, and we have studied the metallographic composition and distribution of the two research objects. Moreover, we have conducted experiments on the Vickers hardness of the spot welding zone of both types of steel, studied the distribution of their hardness, and compared the measurement results of their Vickers hardness. We have conducted multiple dynamic compression experiments on the cylindrical specimens of the RSW of QP980 steel and TRIP800 steel at room temperature and under heating conditions, thereby obtaining the stress-strain curves to study the impact imposed by the metallurgical structure and temperature of the spot welding zone on dynamic compression properties. By using the scanning electron microscope (SEM), we have observed the fracture surface of the specimens so as to analyze the fracture patterns. When a car body suffers high-speed impact, strength and stability of the RSW joints will influence the security of the car body. Failure mechanism under impact loading is extremely vital in the safe design of the car body. Our research is expected to provide a certain basis and reference for the strength and safety design of RSW joint in automobile manufacturing.

Methods
e base materials used in the experiment include QP980 steel and TRIP800 steel, whose thickness amounts to 1.2 mm and 1.4 mm, respectively. rough the overlap welding of the two types of steel, the overlap structure for RSW is obtained with one central spot weld, whose diameter amounts to 5 mm, as shown in Figure 1. e spot welding electrode is composite copper electrode, whereas the welding parameters are shown in Table 1. Subsequently, the overlap welding structure of both types of steel is cut around the central spot weld, so as to obtain the cylindrical compression specimens of the RSW of QP980 steel and TRIP800 steel, respectively.
During the observation of the microstructure of the spot welding zone of the two grade steel, the spot welding zone is cut starting from the center. Subsequent to plastic packaging, grinding, and polishing, the metallographic specimens are prepared accordingly. Furthermore, the metallographic specimens are etched with the use of the 4% nitric acid alcohol solution as an etching agent before observation of their microstructure with a metallographic microscope.
To test the Vickers hardness of the spot welding zone of the two grade steel, the microhardness meter is adopted. During the experiment, the first step is to locate the points horizontally on the substrate. Next, subsequent to the entrance into the heat affected zone, we go through the nugget center along the diagonal line to reach the boundary of the heat-affected zone before going horizontally into the substrate. e spacing of adjacent points amounts to 0.2 mm, whereas 44 points have been subject to the Vickers hardness experiment in total. e quasi-static compression experiments are conducted on the cylindrical specimens of the RSW of QP980 steel and TRIP800 steel with a material testing machine. Furthermore, the dynamic compression experiments are carried out on the RSW of both grade steel with a split Hopkinson pressure bar, which is illustrated in Figure 2. e bullets, incident bars, transmission bars, and absorption bars used in the experiments are all made of the same grade high-strength steel with a diameter of 14.5 mm and a length of 30 cm, 100 cm, 100 cm, and 60 cm, respectively. e specimens are dynamically compressed at 25 degrees centigrade under heating conditions. e heating equipment is illustrated in Figure 3. e original structure of RSW is cut (with the range of cutting as illustrated in Figure 4) to obtain the compression specimens of the RSW of the two grade steel. e diameter of the specimen of QP980 steel's spot welds amounts to 4.0 mm, whereas the height amounts to about 2.3 mm. e diameter of the specimen of TRIP800 steel's spot welds amounts to 4.0 mm, whereas the height amounts to about 2.6 mm.
According to two basic assumptions of Hopkinson test [23], the bar is in one-dimensional stress state and the stress and strain are uniformly distributed along the length of the specimen. e stress σ W (t) and strain ε W (t) rate can be calculated in the spot welding specimen with the following equations: where t is time, ε t (t) is the transmitted wave strain signal, ε r (t) is the reflected wave strain signal, A B is the crosssectional area of bar, E B is elastic modulus of bar, C B is elastic wave velocity of bar, A W is the cross-sectional area of specimen, and L W is length of the specimen.

Results and Discussion
e microstructure of the spot welding zone of the twograde steel is illustrated in Figure 5. e spot welding zone is divided into three zones, namely, melting zone, heat-affected zone, and substrate zone. e microstructure of the spot welding zone is primarily subjected to the control by temperature during welding. It can be evidently notified that transitional changes took place in the microstructure between the melting zone and the heat-affected zone as well as between the heat-affected zone and the substrate zone.
Given that, during the cooling procedure, the cooling rate is higher than the transformation rate of martensite, whereas the melting zone located in the center is subjected to the significant impact of the heat cycle. Consequently, the coarse lath martensite is generated, which features the coarse columnar grains ( Figure 6). By contrast, the heat-affected zone is subjected to slighter impact of the heat cycle. As a result, the martensite is subjected to uniform transformation during cooling, leading to the generation of refine grains (Figure 7).
During the heat cycle of welding, crystallization took place in the inward direction, and the density of the martensite derived from crystallization exceeds the original density of the plate, leading to the formation of pores in between the coarse columnar crystals. In other words, porosity defects arise (Figure 8), and the closer to the center it is, the more severe the porosity would become. Welding cracks are observed on both the left and right sides of the spot welding zone of QP980 steel and TRIP800 steel (Figure 9). In addition, the microstructures of the substrate zone of QP980 steel's spot welds consist of martensite, ferrite, block M/A island, and retained austenite. e microstructures of the substrate zone of TRIP800 steel's spot welds consist of ferrite and block M/A island.
e Vickers hardness distribution of the spot welding zone of QP980 steel and TRIP800 steel is illustrated in Figure 10. It could be notified that the Vickers hardness of the spot welding zone of QP980 steel surpasses that of TRIP800 steel due to the higher level of carbon content (Table 2) of the former type of steel than that of the latter one, and this phenomenon consists of the relationship between the strength of the two grade steel. In addition, judging from the figure on the distribution of hardness in the spot welding zone of both grade steel, we may conclude that the average hardness of the melting zone ranks as the highest, followed by the heat-affected zone, and the substrate zone is the lowest. However, the hardness value of the heataffected zone is subject to the greatest range of changes. is is caused by the co-existence of two transitional zones in the heat-affected zone. One is the transitional zone between the melting zone and the heat-affected zone, where the martensite grain structure has experienced a transition from the coarse-grained lath structure into the uniform fine-grained   structure. e other transitional zone is located between the heat-affected zone and the substrate zone, where the martensite grain structure has undergone a transition from the fine-grained structure into the substrate structure. In the heat-affected zone adjacent to the substrate zone, there is a region where the hardness value is lower than that of the base metal, which is referred to as the softening region (S). Such softening is caused by the tempering taken place in the martensite of this region, leading to its decomposition and the further decrease of hardness [24][25][26][27].  Figure 11. Judging from the two stress-strain curves, it can be found that the strength of QP980 steel's spot welds is evidently higher than that of TRIP800 steel's spot welds. e conditions for the dynamic compression experiment on the cylindrical specimens of the spot welds of QP980 steel and TRIP800 steel at room temperature are specified in Tables 3 and 4. In addition, the dynamic compressive stress-  Advances in Materials Science and Engineering strain curves of the spot welds of QP980 steel and TRIP800 steel at room temperature are illustrated in Figure 12. Judging from Figure 12, it can be found that, as the strain rate climbs up, the compressive stress in the RSW of the twograde steel would experience increases accordingly, but such increases would remain at a small range. Our research findings indicate that the material of the spot welding zone could impose an effect on the strain rate to a certain extent, though it is not quite evident. e recovered specimens for the dynamic compression experiments on the spot welds of QP980 steel and TRIP800 steel at room temperature have been flattened, and there has been huge plastic deformation without any fracture taken place. e conditions for the dynamic compression experiment on the cylindrical specimens of the spot welds of QP980 steel and TRIP800 steel under heating conditions are specified in Tables 5 and 6. e dynamic compressive stress-strain curves of the RSW of both grade steel under heating conditions are illustrated in Figure 13. Judging from Figure 13, we may conclude that, under heating conditions, the strain rate is high, whereas the stress peak value is rather low. e research findings indicate that, with the rising temperature, the strain would rise accordingly, but the strength would decrease correspondingly. Moreover, it should be noted that, with higher strain rate, the stress would enter into a stage of decreasing before experiencing a rebound. From our perspective, this is because porosity and crystal defects exist at the nugget center, and when compressed smaller under heating conditions, such porosity would result in a diminished compressive strength of nugget and stress. By contrast, with the disappearance of the pores, the compressive strength of nugget would increase, leading to a larger stress. e recovered specimens for the dynamic compression experiments on the spot welds of QP980 steel and TRIP800 steel under heating conditions are illustrated in Figure 14. Judging from Figure 14, the specimens have been flattened and even broken, whereas the fracture surface has expanded from the nugget center to the circumference.
At present, the majority of the existing studies focus on the base material of the steel. However, few empirical studies have been conducted on the dynamic compression of cylindrical specimens of spot welding materials. In addition, the dynamic compression under heating condition is less discussed, so it is the fracture mode of spot welding materials under dynamic compression. In this study, we have observed the fracture surface of the recovered specimens of the spot welds of QP980 steel and TRIP800 steel using a scanning electron microscope. It is found that the fracture mode of the sample is more complicated, whereas the fracture surface features a number of "river pattern" cleavage facets. Moreover, there are very few dimples of ductile tearing [28][29][30], whereas the fracture of nugget is brittle (Figure 15),   Advances in Materials Science and Engineering indicating its fragility in nature. At the microscopic scale, it is found that the damage evolution of original microcracks exists on the fracture surface of the recovered specimens [31]. Subsequent to the nucleation, expansion, and connection of the original microcracks with the porosity defects at the nugget center, a continuous surface would be formed, leading to the formation of the brittle fracture in the recovered specimens.

Conclusions
In this paper, we have studied the dynamic compression performance of the RSW of QP980 steel and TRIP800 steel, and we have also examined the fracture mode of the two research objects. Judging from our research, we may draw the following conclusions: (1) rough the observation of the microstructure of the RSW of the two-grade steel, it is found that the nugget center is located in the crystal structure of the coarse martensite featuring porosity defects. In addition, there are evident gaps between the plates on both sides of the spot welding zone. (2) rough the experiments on the microhardness of the two-grade steel, it is found that the average hardness of the RSW of QP980 steel surpasses that of TRIP800 steel. ere is a softening region in the interface of the heat-affected zone and the substrate zone.
(3) e dynamic compression experiments are carried out on the RSW of QP980 steel and TRIP800 steel under heating conditions (20°C and 300°C). It is found that the strain rate would increase with the rising temperature, but the compressive strength would experience declines. Furthermore, the porosity defects of the crystal structure at the weld nugget center would lead to stress rebound when the specimen is compressed. (4) rough the observation of the fracture surface of the recovered specimens, it is found that the fracture of the nugget is brittle. Moreover, the fracture mode of the sample is more complicated, whereas the fracture surface features a number of "river pattern" cleavage facets, and there are very few dimples of ductile tearing.

Data Availability
e data used to support the findings of this study are included within the article.