Effects of Welding Procedures on Resistance Projection Welding of Nuts to Sheets

Transcription

1 ISIJ International, Advance Publication by J-STAGE ISIJ International, ISIJ International, J-Stage Advanced Advance ISIJ Publication International, Publication, ISIJ International, by DOI: J-STAGE, Advance Vol. Publication 57 DOI: (2017), /isijinternational.ISIJINT No. by International, J-Stage 12 Vol. 57 (2017), No. 12, pp. 1 7 Effects of Welding Procedures on Resistance Projection Welding of Nuts to Sheets Xiaopei WANG 1,2) * and Yongqiang ZHANG 1,2) 1) Shougang Research Institute of Technology, Beijing, China. 2) Beijing Key Laboratory of Green Recyclable Process for Iron & Steel Production Technology, Beijing, China. (Received on April 26, 2017; accepted on July 25, 2017; J-STAGE Advance published date: October 21, 2017) In this study, experiments were conducted to investigate the effects of welding current, welding time, electrode force and types of nuts on resistance projection welding of nuts to sheets. The microstructures and welded joint size of welded joints in different welding procedures have been analyzed. The failure mode of welded joints was also discussed. It is found the microhardness distribution of the fusion zone gradually becomes homogeneous with the increase of the welding current or the welding time. Lower electrode force causes the occurrence of splash and large fluctuations of the microhardness in fusion zone. Different types of nuts lead to different heat distribution in the projection welding process. The welded joint in nut is deeper than that in base metal for welding nut with higher electrical resistivity. Button pull fracture, partial thickness fracture and interfacial fracture are the three failure modes of nut projection welded joints. The interfacial fracture is brittle rupture and the partial thickness fracture is ductile rupture. KEY WORDS: projection welding; nuts; weld quality; welding procedures; fracture mode. 1. Introduction Projection welding is the variations of the spot welding, which is a kind of resistance welding. The projection points are arranged on the workpiece and the nuggets are formed at the projection points. The projection welding technology of nut to steel is a key process in auto manufactories. The welding position of the nut can be accurately positioned and the welding quality can be improved by the projection welding process in the manufacturing process of the white body. In the spot welding, the current flow is determined by the size of the electrode tip, whereas in projection welding the current flow is constricted to the contact surfaces by an embossed or machined projection. Metal sheets, wires and fasteners such as nuts, bolts, screws and pins can be joined with projection welding. 1) In the study of Tolf and Hedegard, 1) different resistance welding equipments were compared and both alternating current and direct current were evaluated in resistance projection welding of weld nut to thin sheet steels. They found that alternating current (AC) power sources, which outputted sinusoidal current, increased the current range and the tensile strength of weld nut joints compared to medium frequency direct current (MFDC) machines. However, significant differences in pull-out strength were recorded in joints welded with different AC power sources due to the different initial heating capacity. Wang 2) investigated the effects of surface * Corresponding author: wangxp198710@163.com DOI: treatments of galvanized steels on their weldabilities in the projection welding and pointed out that surface treatments had great effects on the weldabilities of galvanized steels, especially the surface greasing, with which the steel sheets had the worst weldability. Jonas 3) studied the sheet metal fatigue near nuts welded to thin sheet structures. He found that the continuous welded nut and 4-point welded square nut configuration showed the similar sheet metal fatigue characteristics and they were less damaging to the sheet than the 3-point welded nut configuration. For the same magnitude of reversed fatigue loading, the 2.0 mm sheet showed less fatigue resistance in comparison with the 1.2 mm sheet. Chris 4) accomplished the numerical simulation of projection welding of nuts to sheets by taking into consideration complex three-dimensional interactions involving plasticity, electricity and an example is provided to show the potential of finite element numerical predictions in setting up the welding parameters for square nut welding. Zhu 5) developed a new incrementally coupled finite element method using PC-based hardware to investigate more than one projection being welded simultaneously on a workpiece and the coupled FE method in his study was proved to be capable of simulating a projection process. Although the resistance projection welding of nuts to sheets is widely used in industry, few research articles have been published in the literature and the projection weldability of nuts to sheets is not as clear as the spot weldability of sheet to sheet. Comprehensive studies are still need for a better application of nut projection welding in automotive industry. In the current work, the effects of welding cur- 1

2 rent, welding time and electrode force on microstructure and microhardness were investigated experimentally, as well as the chemical composition of weld nuts. In addition, the failure modes of the welded joints were also discussed. 2. Experimental Procedures Two different types of weld nuts were used in this study. They are M10 square nut and M10 round nut, which are marked as SN-M10 and RN-M10, as shown in Table 1. There are four projections in the underside of each weld nut, which were used to generate projection welds. The projection welding electrodes are also displayed in Table 1, which are applied to M10 weld nuts. Table 2 shows the chemical composition of RN-M10 and SN-M10, the contents of carbon and manganese in SN-M10 is much higher than that in RN-M10. Table 3 shows the chemical composition of the base metals used in this study. They are DP590 steel and H340LAD hot dip galvanized steel, which are widely used in automobile manufactures. Resistance projection welding was performed using a PLC controlled 220 kva DC pedestal type resistance projection welder operating at Hz. The projection welding electrodes have been displayed in Table 1, which were made from RMAW Class Ⅱ chromium- zirconium-copper material. After punched a hole of 10 mm in base metals, the projection welding of nuts to sheets tests were conducted. The specimens for optical metallography, which were prepared from test welding coupons, were mechanically polished and then etched by a 4% nital solution at room temperature. The microstructure of the welded joints were examined by optical microscope (OM, Leica DMI5000M). Vickers microhardness testing was conducted along the diagonal direction of joints by microhardness testing machine (Leica HXD-1000TM). A indenter with 200 g load and dwell time of 10 s was adopted. The distance between two indentations was set to 0.4 mm. 3. Results and Discussion 3.1. Microstructures The welding tests of DP590 steel and RN-M10 have been carried out and the welding procedures are shown in Table 4. There are different combinations of electrode force, welding time and welding current, thus all of them can be investigated in the resistance projection welding of nuts to sheets. Figure 1 shows the microstructure of welding nugget of RN-M10 welded to DP590 steel (4 kn, 50 ms, 16 ka). The matrix microstructure of DP590 steel is composed of ferrite and martensite islands, as displayed in Fig. 1. Figure 1(f) shows the matrix microstructure of RN-M10, which consists of ferrite and pearlite. Figures 1 and 1(e) show the heat affected zone (HAZ) near DP590 base metal and the HAZ near RN-M10 base metal. The fusion zone (FZ) is displayed in Fig. 1(d), which is composed of bainite and martensite Microhardness As there are four projections in the underside of each weld nut, four welded joints will be generated in the projection welding of nut to sheet. Under each welding condition, a typical one was selected from the four welded joints for comparison and analysis. The microhardness of the fusion zone across the welded joints in different welding conditions are displayed in Fig. 2. There are three curves in each graph, the maximum hardness, the minimum hardness and the average hardness. Figure 2 shows the relationships between the welding current and the microhardness. The Table 2. Chemical composition of weld nuts. Weld nut type C Si Mn P S Al RN-M SN-M Table 3. Chemical composition of base metals. Steel Thickness C Si Mn P S Al DP mm H340LAD +Z 2.5 mm Steel DP590 Weld nut type RN-M10 Table 4. Welding procedures. Electrode Tip Force F/kN Welding Time t/ms Welding Current I/kA Holding Time t/ms 200 Table 1. Weld nuts and electrodes. Weld nut type ID Top side Under side Electrodes Round nut M10 RN-M10 Square nut M10 SN-M10 2

3 ISIJ International, Advance Publication by J-STAGE ISIJ International, ISIJ International, Advance Vol. Publication 57 (2017), No. by J-Stage 12 (d) (e) (f) Fig. 1. Fig.1 Microstructure of welding nugget of RN-M10 welded towelded DP590 steel (4 kn,steel 50 ms, 16 ka) welding Microstructure of welding nugget of RN-M10 to DP590 (4kN, 50ms, 16kA) nugget, DP590 base metal, the HAZ near DP590 base metal, (d) the center of FZ, (e) the HAZ near RN-M10 base metal, (f) RN-M10 base metal. a welding nugget, b DP590 base metal, c the HAZ near DP590 base metal, d the center of FZ, e the HAZ near RN-M10 base metal, f RN-M10 base metal measured average hardness drops gradually as welding current gets higher, which is caused by a sustained increase in grain size.6) The microhardness distribution of the fusion zone gradually becomes homogeneous with the increase of the welding current. When the welding current is 24 ka, the difference between the maximum hardness and the minimum hardness is small. As the current is high, the heat generation is large and the cooling rate is slow. There is plenty of time to accomplish the phase transition and a homogeneous microstructure of the fusion zone can be gotten, leading to homogeneous microhardness. Figure 2 shows the microhardness distribution of welded nugget in different welding time. With the increase of welding time, the difference between the maximum hardness and the minimum hardness is small, which is similar to that of welding current. However, the average microhardness changes little in different welding time. Figure 2 shows the microhardness distribution of welded nugget in different electrode force. As the electrode force enhances, the average microhardness increases, which is displayed in the Fig. 3. Figure 3 shows the microhardness distribution of welded nugget in 2 kn and 5 kn. When the electrode force is 2 kn, the microhardness distribution of the fusion zone (FZ) is inhomogeneous and the fluctuations is very large. The interface contact of the projections of the nut and the base metal is not good in small electrode force, leading to the occurrence of splash and non-uniform distribution of temperature, both of which cause the large fluctuations of the microhardness in fusion zone. As the electrode force 3

4 Fig. 3. Microhardness distribution in different electrode force. Fig. 4. Effects of welding parameters on welded joint size. Fig. 2. Microhardness variation under different welding conditions welding current, welding time, electrode force. enhances, the interface contact of the projections of the nut and the base metal becomes good, and the splash decreases or even disappears. Thus, the microhardness distribution of the fusion zone is homogeneous, as can be seen in the force of 5 kn. The width of the heat affected zone (HAZ) in the DP590 side is narrower than that of the nut side. The softening phenomenon of HAZ in the DP590 side is obvious in the force of 2 kn, which is caused by the occurrence of tempered martensite in the HAZ. 7) 3.3. Welded Joint Size Figure 4 shows the welded joint size in different welding procedures. The size of welded joint, which includes fusion zone and the affected zone, becomes larger with the increase of welding current. When the welding current increases to 20 ka, the size of the welded joint reaches maximum. Continue to enhance the welding current, the size of welded joint no longer increases, and the splash becomes more serious in the process of projection welding. The influence of welding time on welded joint size is similar to that of welding current. When the welding time is 60 ms, the spatter becomes serious, which restricts further increase in the size of the welded joint. Welding current seems to be more significant than the welding time, due to heat generation is proportional to the square of welding current (Q = I 2 Rt). So the effects of welding time on the size of welded joint are smaller, relatively. As can be seen from the Fig. 4, except for the small electrode force of 2 kn, the size of welded joint has no obvious change. The electrode 4

5 force has little effect on the joint size, which is very different from the welding current and welding time. When the electrode force is small, the splash is relatively intense, such Table 5. Welded joint size in the nut and the steel. Welding current I/kA Nut type Joint in the nut a/μm Joint in the steel b/μm Ratio λ 12 SN-M RN-M SN-M RN-M as 2 kn. The splash decreases significantly with the increase of electrode force Effects of the Types of Nuts Two different types of nuts, the RN-M10 and the SN-M10, have been welded to DP590 steel. The microstructures of welded joint are shown in Fig. 5. The welding currents are 12 ka and 16 ka, respectively. The welded joint is divided into two parts, and they are the joint in the nut and the joint in the steel. Figure 5 shows the welded joint size Fig. 6. Electrical resistivity of RN-M10 and SN-M10. Fig. 7. Morpology of different failure modes in base metal sheet (4 kn, 50 ms, 20 ka). (d) Fig. 5. Microstructure of welded joint in different types of nuts 12 ka, SN-M10, 12 ka, RN-M10, 16 ka, SN-M10, (d) 16 ka, RN-M10. 5

6 in the two parts in different welding procedures. The welded joint size in the nut and the steel are a and b, and the ratio of them is λ = a/b. Then, the values of them can be gotten, as displayed in the Table 5. The ratio λ of SN-M10 is bigger than that of RN-M10 in both 12 ka and 16 ka, which means the welded joint in nut is deeper than that in base metal for SN-M10, compared with RN-M10. The electrical resistivity of RN-M10 and SN-M10 has been Fig. 8. Cross section morpology of button pull fracture, partial thickness fracture and interfacial fracture in weld nut. (d) Fig. 9. Fracture micromorphology of (a, b) interfacial fracture and (c, d) partial thickness fracture in base metal sheet. 6

7 calculated by JMatPro and the results are shown in Fig. 6. As can be seen from Fig. 6, the electrical resistivity of SN-M10 is larger than that of RN-M10 under C, the resistance of SN-M10 is bigger than that of RN-M10, which leads to the different heat distribution Failure Modes The destructive separation has been conducted to the welded joints of SN-M10 welded to H340LAD+Z steel, and Fig. 7 shows the morphology of fracture surface. There are three failure modes of welded joints. They are button pull fracture, partial thickness fracture and interfacial fracture. As can be seen, the nugget is completely pulled out from the base metal sheet in button pull failure mode, leaving a hole in the metal sheet, while the nugget is broken out from the faying surface in interfacial failure. In partial thickness fracture, the nugget is broken out from the center of the base metal sheet. The cross section morpology of three failure modes in weld nut are displayed in Fig. 8. In button pull fracture, the base metal has been damaged down, and the whole base metal in welded joint was pulled out. The base metal in welded joint was partial pulled out in partial thickness fracture. The failure location is in faying face for interfacial fracture. Figure 9 shows the fracture micromorphology of interfacial fracture and partial thickness fracture in base metal sheet. The fracture surface of interfacial fracture is quasicleavage or cleavage pattern, which is brittle rupture, as shown in Figs. 9 and 9. While the fracture surface of partial thickness fracture is dimple pattern, which is ductile rupture, as shown in Figs. 9 and 9(d). 4. Conclusions (1) The microhardness distribution of the fusion zone gradually becomes homogeneous with the increase of the welding current, and the welding time has the similar effects on the microhardness distribution. (2) Lower electrode force causes the occurrence of splash and large fluctuations of the microhardness in fusion zone. (3) With the increase of welding current or welding time, the welded joint becomes larger. However, extremely big welding current or welding time leads to serious splash, restricting further increase in the size of the welded joint. (4) The electrode force has little effect on the welded joint size. (5) Different types of nuts leads to the different heat distribution in the projection welding process. With higher electrical resistivity, the welded joint in nut is deeper than that in base metal for SN-M10, compared with RN-M10. (6) Button pull fracture, partial thickness fracture and interfacial fracture are the three failure modes of nut projection welded joints. The interfacial fracture is brittle rupture and the partial thickness fracture is ductile rupture. REFERENCES 1) E. Tolf and J. Hedegard: Weld. World, 51 (2007), 28. 2) C. Wang and M. Wang: Weld. World, 49 (2005), 39. 3) J. W. Ringsberg, P. Orvegren, H. F. Henrysson and G. Akerstrom: Int. J. Fatigue, 30 (2008), ) C. V. Nielsen, W. Zhang, P. A. F. Martins and N. Bay: J. Mater. Process. Technol., 215 (2015), ) W. F. Zhu, Z. Q. Lin, X. M. Lai and A. H. Luo: Int. J. Adv. Manuf. Technol., 28 (2006), 45. 6) X. Wan, Y. Wang and P. Zhang: ISIJ Int., 54 (2014), ) J. Liang, H. Zhang, X. Qiu and Y. Shi: ISIJ Int., 55 (2015),