Research on Tensile Test of Dissimilar Steel Body Solder Joints

With 22 mnb5 St16 and ordinary steel as the research object, under the conditions of medium frequency spot welding connection plate welding, solder joint for the electric logging under shear load test, determination of solder joints, solder joint and away from the solder joint around the base of strain, according to measurement data analysis of solder joints and the deformation regularity of parent metal. The test results show that the strain in the spot welding joint distribution is uneven, the center of the welding strain is small, large deformation mainly exist in the heat affected zone, near the solder heat affected zone is the main area prone to give in. Away from the solder joint area of the parent metal, the influence of solder joint is small, mainly tensile strain, and strain as the load increases, solder joint strain is negative, the center is linear, and basic antisymmetric location of strain values different test points.


Introduction
The manufacturing technology of the automobile has developed by leaps and bounds, and the technology in all aspects of the automobile body has improved rapidly compared with the past [1,2], especially the use of automobile body materials has begun to improve to the use of ultra-high strength materials, in order to effectively protect the driver during the daily driving emergency protection, it is necessary to develop the body material with good impact resistance and strong carrying capacity.
In recent years [3][4][5], many researchers at home and abroad have made great achievements in the research of various materials through various test methods, mathematical software, instrument test, etc. but the experimental research on the solder joint of ultra-high strength steel and low carbon steel is very few.Body frame cage as the main part of the ultra-high-strength steel and other low-carbon steel parts of the connection process, such as the base structure, door beam, AB column, central channel, etc.it is required at these locations.It is necessary to analyze the mechanical properties of solder joint by tensile test and study the mechanical behavior around the solder joint [6,7].
The mechanical properties and distribution of the solder joints of ultra-high strength steel and low carbon steel were measured by tensile test and strain gauge [8].The study of tensile test provides a reference for the establishment of numerical simulation model of spot welding.

Test Material
The test material is composed of ordinary low carbon steel st16 and 22MnB5 hot stamping steel.Among them, the former is a typical ordinary steel, while the latter shows no martensitic microstructure in the

Spot Welding Technology
The welding technology of medium frequency and servo resistance spot welding for dissimilar steel spot welding structure has a good effect.The spot welding lap joint of the test sample in this paper is processed by IF and servo spot welding equipment NIMAK machine X type spot welding pliers, SCM012 servo pressure controller and HWI2008 IF control cabinet.The main welding parameters are as follows: welding current 6.5kA, welding time 400ms; The cooling time is 400ms, the welding pressure is 4200N, and the forging pressure is 6500N after welding.

Tensile Testing
Tensile testing machine QBD-100, static strain gauge UCAM-60B.The plate of the tensile sample is uneven, there is initial curvature, processing and other reasons may make the load eccentric, in order to eliminate or reduce the influence, the multi-piece method is used.Two different materials welded structural members as figure 1.Schematic diagram of the placement position of the test sample and contains 1 to 33.The resistance strain gauge is affixed to both the positive and negative axes of the specimen.The four strain gauges are all working gauges and are measured by a quarter bridge.In order to eliminate the effect of temperature, temperature compensation plates are required.At least 4 to 5 stages of loading, each stage of loading should make a significant change in strain so.The experimental equipment is figure 2. Test equipment of Zwick/Roell-Z100 material testing machine.The tensile test sample is stuck on the tensile test machine, and the load is continuously increased in a certain increment, and the tensile speed is set at 5mm/min.At the same time, the strain value corresponding to each load is collected until the solder joint is pulled off, and the experiment ends.When the tensile load is applied to both ends of the metal sheet, the core center and the action line of the force are not in the same line in the initial state of the test.In accordance with the requirements of the standard BS EN ISO14273-2001 "Sample size and Procedure for solder joint shear test of resistance welding, seam welding and float welding", the modified tensile shear structure is used to conduct the test, that is, the tensile shear test is carried out by using a gasket as thick as the welding plate at both ends of the sample.At the beginning of the test, the center of the solder joint and the action line of the force are in the same straight line, as shown in figure 3.

Test Results
It is concluded from the test that the whole tensile process basically presents a nonlinear relationship.As can be seen from figure 5 and figure 6, Load-strain diagram of various parts on ultrahigh strength steel plate and Load-strain diagram of various parts on ordinary steel plate, indicating that the strain value and strain variation trend at the same section position.

Discussion and Analysis
As can be seen from figure 7, the load-strain diagram of 28, 29 and 30 pieces of ordinary steel plate far away from the welding core has a high coincidence degree, indicating that the strain value and strain variation trend at the same section position of ordinary steel plate far away from the base material of the welding core are basically the same, with little difference.Similarly, 15, 16, and 17 pieces of ultra-high strength steel plates also have the same situation, and their coincidence is also very high.The strain values of these test points are all positive, subject to tensile force, and increase with the increase of load.It can also be seen from figure 7 that the strain sizes on different steel plates with the same distance from the weld core are also different.Obviously, the strain values on ultra-high strength steel plates are smaller than those on ordinary steel plates.It shows that the deformation of ordinary steel plate is larger than that of ultra-rigid steel plate under the same load.Figure 8 shows the strain values of plates 1-3 and 19-20 far from the stress end.These positions of the strain gauge at the weld core near the free end, according to the theory of material mechanics, there is no tension.However, the experimental results show that strain is also generated at these locations, and deformation also occurs.
The strain value generated should be caused by the bending moment, and the strain value increases with the increase of the tension, which indicates that the bending moment also increases with the increase of the tension, and the two basically show a linear relationship.The generation of bending moment may be due to the fact that the spot welding structure is a lap form, resulting in the process of tension, the tension at the solder joint is not on the same line of action, so the effect of bending moment will be produced.
It can also be seen from figure 8 that the strain values of plates 1 and 3 located on both sides of plates 1-3 are positive, while the strain values of plates 2 located in the middle are negative.Similarly, in plates 18-19, the strain values of plates 18 and 20 located on both sides are positive, and the strain values of plates 19 located in the middle are negative.This indicates that at the free end near the weld core, the test points located on both sides are subject to tension, and the test points located in the middle are subject to pressure.It can be guessed that the deformation near the free end of the weld core is likely to be wavy, concave in the middle, and raised on both sides, resulting in middle pressure and tension on both sides.This deformation may be caused by the complex composition of the tissue around the weld core, uneven material properties, and the resulting bending moment makes the strain around the weld core complex.
Plates 1, 18,2, 19 and 3, 20 are in the antisymmetric position, but the load-strain curves of the two groups of test points are different, which may be due to two reasons.First, the strain gauge No. 1-3 is on the ultra-high strength steel plate, and the strain gauge No. 18-20 is on the ordinary steel plate.Under the same load, the deformation of the ordinary steel plate is different from that of the ultra-high strength steel plate.Second, the detection value of the electrical measurement method is determined by many factors, the quality of the patch or the interference between the wires, the slight change of the paste position will also lead to the change of the detection value, making the strain result complicated.Therefore, the deformation near the free end of the weld core is likely to be caused by the bending moment caused by the tension of the different action lines, the complex composition of the tissue around the weld core and the uneven material properties.However, the specific cause of this deformation remains to be further studied.As can be seen from figure 9, the strain values of No. 9-11 plates between the welded core and the stressed end on the ultra-high strength steel plate are all negative, and they are all under pressure.The strain values of the 25-27 plates between the welded core and the stressed end of the ordinary steel plate are positive and are all strained.The load of plates 9 and 11 on both sides coincides with the strain diagram, and the load of plates 25 and 27 coincides with the strain diagram.On the ultra-high strength steel plate, the strain of No. 10 in the middle is less than that of No. 9 and No. 11 on both sides.On the ordinary steel plate, the strain of the middle 26 plate is greater than that of the two sides 25 and 27 plates.(1) The strain values of the test points located at the antisymmetric position of the ultra-high strength steel and ordinary steel specimens are different, but the strains of both increase with the increase of the load, and are basically linear.

Conclusion
(2) The base material area away from the welding core is less affected by the welding core, mainly by tensile strain, and the strain size under the same load on the same section is the same.The tension and pressure around the weld core are different.At the center of the weld core, the strain value is negative and increases with the increase of the load on the specimen.

Figure 1 .
Figure 1.Schematic diagram of the placement position of the test sample.

Figure 4 .
Figure 4. Tensile curve of the test.

Figure 5 .
Figure 5. Load-strain diagram of each part of ultrahigh strength steel plate.

Figure 6 .
Figure 6.Load-strain diagram of each part of ordinary steel plate.

Figure 7 .
Figure 7. the load and strain between the solder joint and the clamped end, which are located in the antisymmetric position away from the weld nugget.

Figure 8 .
Figure 8. load and strain of plates 1-3 and 18-20 at the free end.

Figure 9 .
Figure 9.The load and strain of the piece in the antisymmetric position near the weld core between the solder joint and the gripper end.

Table 1 .
Chemical composition of ultrahigh strength steel 22MnB5.

Table 3 .
Mechanical properties of the two materials.