Investigation of induced magnetic force on liquid nugget during

Investigation of induced magnetic force on liquid nugget during resistance spot welding Y. Li1,2, Z. Luo*1,2, Y. Bai1,2 and S. S. Ao1,2 This paper investigates the source, magnitude and direction of magnetic force on the liquid nugget during resistance spot welding (RSW). High speed photography was used to observe the nugget formation process during half-sectioned RSW of steel (ferromagnetic substance) and aluminium alloy (paramagnetic substance). The induced magnetic force acting on half-sectioned and regular nugget was afterwards analysed and calculated. The results show that in the case of steel RSW, the magnetised workpieces generated a very strong magnetic field around the spot welding zone, while weak magnetic field appeared in the case of aluminium alloy RSW. This strong magnetic field causes a strong convection in the liquid nugget of steel even when the welding current is low. This strong convection will promote that the dimensions of nugget in the steel RSW become larger and closer to rectangle (observed on the cross-section) than in the aluminium alloy RSW. Keywords: Resistance spot welding, Induced magnetic force, Magnetisation, High speed photography

Introduction Resistance spot welding (RSW) has extensive application as a joining technique for the assembly of sheet metal components because of its high productivity, flexibility and suitability for automation and robotisation. The nugget formation process is very important for its influence on the strength and durability of the welded structure. The nugget formation process is affected by many factors, such as the welding current, electrode force, physical properties and surface condition of the workpiece. The electromagnetic stirring has been demonstrated to have significant effect on the improvement of weld quality in arc welding.1–3 At the same time, the magnetic force has gradually drawn attention of researchers in the RSW field. As early as 1965, Cunningham and Begeman found that the weld metal is apparently held in place by certain electromagnetic forces when they studied the cold rolled steel projection welding process by high speed photography.4 In 1990, Alcini measured the temperature field in the weld nugget of 1008 steel with microthermocouples and found that the temperature distribution in the weld nugget was relatively uniform. He considered that strong fluid convection caused by the induced magnetic force appeared in the weld nugget.5 Li et al. investigated the impact of external magnetic field (EMF) on the weld quality of RSW of advanced high strength steel DP 780 and found that the EMF could refine the crystal grains and increase the welding nugget diameter.6,7 He also analysed the magnetohydrodynamic 1 School of Materials Science and Engineering, Tianjin University, Tianjin, China 2 Tianjin Key Laboratory of Advanced Joining Technology, Tianjin University, Tianjin, China

*Corresponding author, email [email protected]

behaviours during RSW by numerical simulation and indicated that the molten metal in the nugget makes regular rotational flow in four symmetrical loops, which changes the temperature gradient in the weld nugget during heating and holding, and also yields significant effects on the shape and dimensions of the weld nugget.8 However, the above researches only generally point out that certain electromagnetic forces act on the nugget during the RSW process. They did not specify the source, magnitude and direction of the electromagnetic force. Hence, this paper investigates the action of magnetic force on the liquid nugget during the RSW process. To this end, this paper adopts high speed photography to observe the process of nugget formation during halfsectioned RSW. The reason for choosing high speed photography is that, first, high speed photography can visually observe the process of nugget formation and it has been proven to be an effective method to study the nugget formation.9–12 Second, some high speed photos showed that the weld metal was held in place by certain electromagnetic forces and was not squeezed out by electrode force when using the half-weld technique.4,12 Therefore, it is possible to use the movement pattern of the liquid nugget to determine the effect of the magnetic force. The magnetic force acting on half-sectioned and the regular nugget was analysed and calculated. In order to better study the effect of magnetic force on the liquid nugget, this paper compares the nugget formation process of steel (ferromagnetic substance) and aluminium alloy (paramagnetic substance).

Experimental In this research, Q235 steel and AA 5052 aluminium alloy were employed as the workpieces. The chemical composition is shown in Tables 1 and 2. The dimension

ß 2013 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 9 December 2012; accepted 22 January 2013 DOI 10.1179/1362171813Y.0000000110

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Induced magnetic force on liquid nugget during RSW

1 Sketch of welding process

of specimen was 4062062 mm. Truncated cone electrodes with a 6 mm diameter tip end made with copper alloy of RWMA class II chrome was used as the electrodes of the welding machine. The sketch of spot welding is shown in Fig. 1. Welding was performed on a 220 kW dc inverter RSW machine. The nugget formation of the welding zone was monitored with a high speed camera. The experimental set-up for the digital high speed camera is shown in Fig. 2. A FASTCAM SUPER 10KC high speed camera was used to monitor the nugget formation. The system was composed of a processor, power supply, camera, viewfinder and lighting sources. The camera system consisted of an adapter plate, camera head, tripod and lens. One 100 W halogen lamp was used to illuminate the welding zone. The welding process was recorded at 1000 frames/s. Figure 3 shows the experimental methods for high speed photography of half-sectioned RSW. Figure 3a and b is the half-sectioned RSW of aluminium alloy and steel, respectively. Figure 3c and d is specially designed to examine the induced magnetic force. Figure 3e shows the preparation method of Fig. 3c and d. First, a semicircular space with radius of 3 mm was cut from the aluminium alloy or steel by a wire cutting machine. Second, a semicircular steel or aluminium alloy was inlaid in the space. The radius of this semicircular steel or aluminium was slightly larger than 3 mm so that it could be fixed in the semicircular space. Figure 3c was used to examine the influence of the surrounding workpieces on the liquid nugget. Figure 3d was used to simulate the condition of steel RSW when the temperature of the welding zone was higher than its Curie temperature. Table 1 Chemical composition of Q235 steel/wt-% C

Mn

Si

S

P

0?12–0?20

0?30–0?70

(0?30

(0?045

(0?045

2 Placement of camera and lighting system

The welding parameters used in this study are shown in Table 3. The welding current of cases 2–4 was fixed at 6?5 kA to ensure the consistency of the induced magnetic force by the welding current. The welding current of case 1 was 16 kA because the aluminium alloy workpieces cannot be welded at a low welding current. After the half-sectioned RSW, the authors carried out regular RSW on Q235 steel and AA 5052 aluminium alloy to analyse the effect of induced magnetic force on the regular nugget. The welding parameters are shown in Table 4. An Olympus SZX12 stereomicroscope was used to observe the shape of nugget cross-section after welding.

Results and discussion Analysis of high speed photos The high speed photos of four cases are shown in Fig. 4. Eight high speed photos are arranged in time order for each case. The macroscopic morphology of the nugget for each case after the welding process is shown in Fig. 5. Figure 4a shows the RSW of the aluminium alloy. It can be seen that the nugget was first formed at the centre of the faying surface. Then a large amount of liquid metal was squeezed out after a welding time of 60 ms, and the total thickness of the two workpieces noticeably decreased. Figure 5a clearly shows macroscopic morphology of the nugget after the welding process. Finally, the molten metal formed a C shape. The RSW of steel is shown in Fig. 4b. From the welding time of 20 ms, the front face is darkened in colour due to the heat generated. A yellow-red heat zone, which related to the formation of the nugget,12 began to form from a welding time of 60 ms. However, the yellow-red zone was so bright that the nugget formation process cannot be observed clearly. The macroscopic morphology of steel after RSW is shown

Table 2 Chemical composition of 5052 aluminium alloy/wt-% Al

Mg

Cr

Si

Fe

Cu

Mn

Zn

Others

95?70

2?2–28

0?15–0?35

(0?25

(0?40

(0?10

(0?10

(0?10

(0?15


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a aluminium alloy (case 1); b steel RSW (case 2); c welding zone replaced by steel (case 3); d welding zone replaced by aluminium alloy (case 4); e sample preparation for cases 3 and case 3 Experimental methods

in Fig. 5b. It had a significant difference from the macroscopic morphology of aluminium alloy after RSW. That is, the molten metal at the centre zone was held in place by certain force, and the molten metal near the top and bottom electrode was ejected along the top and bottom electrode face. Finally, the molten metal formed an ‘anti-C’ shape. Figures 4c and 5c show that the case of welding zone was replaced by steel, while the surrounding zone was aluminium alloy. The authors found that the molten metal after welding formed a C shape, which was similar to the case of aluminium alloy RSW. This indicates that the surrounding metal has significant influence on the nugget formation process. In case 4, the block of aluminium alloy was overheated and melted completely (Fig. 5d). However, from the high speed photos of this case, shown in Fig. 4d, some concaves appeared on the molten metal. This

indicates that certain force was generated due to the surrounding steel. From the above discussion, the extrusion pattern of molten metal in the four cases can be summarised as shown in Fig. 6. The extrusion metal formed a C shape in the case of aluminium RSW, while it formed an anti-C shape in the case of steel RSW. The extrusion metal formed a C shape again in the case of welding zone replaced by steel, and the surrounding zone was aluminium alloy. The change of extrusion pattern of molten metal was due to the induced magnetic force from welding current. A detailed discussion is given as follows.

Analysis of induced magnetic force on halfsectioned RSW In order to be consistent with the above experiments, the induced magnetic force on the half-sectioned liquid

Table 3 Welding parameters for half-sectioned RSW

Case 1 Cases 2–4

Thickness/mm

Welding current/kA

Electrode force/N

Welding time/ms

2 2

16 6?5

1200 1200

200 200

Table 4 Welding parameters for regular RSW

Aluminium alloy Steel

Thickness/mm

Welding current/kA

Electrode force/N

Welding time/ms

2 2

14, 18 and 22 6, 9 and 12

3600 3600

200 200


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a aluminium alloy RSW (case 1); b steel RSW (case 2); c welding zone replaced by steel (case 3); d welding zone replaced by aluminium alloy (case 4) 4 High speed photos

a aluminium alloy RSW; b steel RSW; c welding zone replaced by steel; d welding zone replaced by aluminium alloy 5 Macroscopic morphology of nugget after RSW

6 Sketch of extrusion metal after RSW of a aluminium alloy, b steel, c welding zone replaced by steel and d welding zone replaced by aluminium alloy


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8 Dividing RSW zone for ferromagnetic substance

a induced magnetic field distribution during RSW process; b induced magnetic force acting on liquid nugget 7 Induced magnetic field and force by conduction current

nugget was analysed (see Fig. 7b). Based on magnetohydrodynamic theories, the magnetic force acting in the liquid nugget can be described as8 ! ! ! ! ! ! (1) F ~ j | B ~(jE z jv z jq )| B ! ! where jE is the conduction current density, jv is the ! current induced by fluid flow and jq is the convection current caused by the flow of charges. The induced current and convection current can be ignored because they are very small compared with the conduction current. Therefore, the magnetic force can be approximated by ! ! (2) F ~jE | B ! The magnetic flux density of B in equation (2) can be divided into two parts. In order to illuminate the ! component of B , the authors divided the welding zone according to its temperature (Fig. 8). TC is the Curie temperature of the workpiece, and Tl is the liquidus temperature. The area around the blue lines is the welding zone, where the current lines pass through. The surrounding zone S will not suffer induced magnetic force from the conduction current because no welding current passes through this zone. The surrounding zone

S will be magnetised by the induced magnetic field from the conduction current. After magnetisation of zone S, it ! will generate a magnetic field BM and the magnetic field line may pass through the liquid nugget, generating magnetic force that acts on the liquid nugget. Therefore, the magnetic force acting on the liquid nugget can be written as ! ! ! (3) F ~jE |(BW z BS ) ! where BW is the magnetic flux density generated by the ! welding zone and BS is the part, which passes through welding zone, of magnetic flux density generated by the surrounding zone. The authors used Biot–Savart law (equation (4)) and developed MATLAB programme code to calculate the ! ! magnetic flux density of BW and BM on the halfsectioned welding surface (Fig. 7b) when the temperature of this surface was beyond its Curie temperature. The r direction magnetic flux density induced by the welding current was calculated. ! ð m0 mr Id l |! r ! (4) B~ r3 l 4p ! In equation (4), B is magnetic flux density, m0 is permeability of vacuum (4p61027H m21), mr is relative permeability, I is current and r is the distance between a point on the calculated surface and a point in the electrodes and workpieces, where they pass through the welding current. The authors calculated the magnetic flux density on two surfaces shown in Fig. 9. The calculation parameters are shown in Table 5. The calculation results are shown in Fig. 10. Figure 10 shows that the magnetic flux density on the half-sectioned welding surface is nearly equal to zero in both aluminium alloy and steel RSW. That is, the ! induced magnetic force by BW was so small that it cannot hold the molten metal in place. However, in our experiments of half-sectioned RSW of steel and in Refs. 4 and 12, it indicated that a certain magnetic force acted on the molten metal, which held the molten metal in place. At the same time, in the case of the welding zone replaced by steel and surrounding zone replaced by aluminium alloy, the induced magnetic force that held the molten metal in place disappeared. Hence, the major magnetic flux density that passes through and magnetic force that acts on the liquid nugget was generated by ! surrounding zone BS (equation (3)). ! From the magnetic flux density BM on the faying surface shown in Figure 10, the authors found that in the case of aluminium alloy RSW, the induced magnetic field by the surrounding zone was small, while in the case of steel RSW, a very strong magnetic field (up to


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ferromagnetic substance that multiplies the EMF dramatically. ! The magnetic flux density BS , which acts on the ! liquid nugget, is part of BM . However, the proportion of ! ! BS in BM is difficult to determine because the space structure is very complicated near the welding zone and the real relative permeability of the steel is very difficult to determine during the RSW process. Although the ! exact value of BS is difficult to obtain, the authors can still conclude that the strong magnetic field that is generated by the surrounding workpieces after workpieces are magnetised is the most important source of magnetic force acting on the nugget.

Effect of induced magnetic force on regular RSW

9 Calculation position of magnetic flux density

40 T) appeared near the edge of welding zone. This is because aluminium alloy is a paramagnetic substance, which has little effect on the EMF, and steel is a

The most important influence of induced magnetic force on regular RSW is it causes strong convection in the liquid nugget. Li et al. showed that the strong rotational flow of liquid nugget brings high temperature metal to the nugget boundaries and causes more metal to melt.8 This paper indicates that the liquid nugget will suffer larger induced magnetic force during the RWS of steel than during the RSW of aluminium alloy. That is, weak convection may appear in the liquid nugget during aluminium alloy RSW. Accordingly, the authors propose that the nugget shape on the cross-section of

10 Distribution of magnetic flux density on half-sectioned welding surface and faying surface Table 5 Calculation parameters for magnetic flux density RSW of aluminium alloy

! BW on half-sectioned welding surface ! BM on faying surface

RSW of steel

Welding current/kA

Relative permeability

Welding current/kA

Relative permeability

16

1

6?5

1

16

1

6?5

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a weak convection; b strong convection 11 Effect of convection on shape of liquid nugget

a 14 kA; b 18 kA; c 22 kA 12 Shape of nugget in RSW of aluminium alloy

a 6 kA; b 9 kA; c 12 kA 13 Shape of nugget in RSW of steel

steel may be closer to rectangular than the aluminium alloy. The strong convection scours the boundary of the nugget and melts more metal. With the strengthening of the convection, the shape of the nugget cross-section will change from an elliptical shape (Fig. 11a) into a rectangular (Fig. 11b). To verify this point of view, the authors carried out regular RSW on Q235 steel and AA 5052 aluminium alloy. The experimental method has been mentioned in section on ‘Experimental’. The results are shown in Figs. 12 and 13. In order to give a quantitative description of the nugget shape, the authors defined a shape factor, which is equal to the ratio of the nugget area to its circum-rectangle area. The calculation method of this factor is shown in Fig. 14. The value of the shape factor is closer to 1, indicating that the shape of the nugget cross-section is closer to the rectangular. That is, stronger convection appears in the liquid nugget. MATLAB programme code was used to calculate the shape factor, and the results are shown in Tables 6 and 7. The shape factors of the nugget during steel RSW are larger than during aluminium alloy RSW in all cases. This indicates that stronger convection appeared in the liquid nugget of steel than that of aluminium alloy. In the case of steel RSW, the shape factors increased slightly with the welding current increasing. This shows

14 Definition of shape factor


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Table 6 Shape factors of nugget during aluminium alloy RSW


Table 7 Shape factors of nugget during steel RSW Welding current/kA

Welding current/kA 14

18

22

0?9081

0?9294

0?9598

Steel Aluminium alloy

that during the RSW of steel, a small welding current can cause a large convection. The magnetisation of surrounding workpieces can account for this phenomenon. This phenomenon also indicates that during the RSW process of steel it does not need to use a large current to obtain a preferable nugget dimension. However, in the RSW process of aluminium alloy, a large nugget may require a very high welding current. One possible solution is using EMF to promote the convection in liquid nugget of aluminium alloy. This requires further study and will not discuss in this paper.

Conclusions In this paper, the nugget formation process and magnetic force during steel and aluminium alloy RSW were studied using high speed photography and numerical calculation. The following conclusions have been drawn. 1. The high speed photography was carried out on four cases: the half-sectioned RSW of aluminium alloy, the half-sectioned RSW of steel, the welding zone was replaced by steel while the workpieces was aluminium alloy and the welding zone was replaced by aluminium alloy, while the workpiece was steel. The results showed that certain magnetic force changes the extrusion pattern of molten metal when the workpiece was steel (ferromagnetic substance). 2. The analysis of induced magnetic force indicated that the liquid nugget suffered an inward magnetic force. This induced magnetic force consisted of two parts: the induced magnetic force by the spot welding zone and the induced magnetic force by the surrounding workpieces after the workpieces were magnetised. 3. Numerical calculation was used to analyse the function of induced magnetic force. The results show that the induced magnetic force by spot welding zone was very small in both steel and aluminium alloy RSW. However, in the case of steel RSW, the magnetised workpieces generated a very strong magnetic field near the edge of the spot welding zone. The strong magnetic field is the main source of induced magnetic force on the liquid nugget. 4. The effect of the induced magnetic force on regular RSW was analysed. In the case of steel RSW, the strong magnetic field that appeared near the edge of the

6

9

12

0?9614

0?9629

0?9651

welding zone caused a strong convection in the liquid nugget even when the welding current was at a low level. This strong convection also promoted that the dimensions of the nugget become larger than in the aluminium alloy RSW. An external magnetic field may strengthen the convection in a liquid nugget of aluminium alloy, and this requires further research.

Acknowledgements This research is supported by the National Nature Science Foundation of China (grant nos. 50975197, 51275342 and 51275338) and the National Key Technology R&D Program of China (grant no. 2011 BAF11B00).

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