| Automotive lightweight is an effective way to realize the vehicle development for safety, energy conservation and environmental protection. Advanced high strength steels, such as dual phase steel, are steadily applied more and more widely in light-weighting auto-body manufacturing in the advantages of good forming and crash performance. For example, in side rail and B pillar assemblies, there are many joints made with three steel sheets including low carbon steels and dual phase steels. The application of multiple steels joint reaches about 33% of a joint on a common vehicle. However, when resistance spot welding, as the main joining method in production, applied to multiple sheet stackups, issues such as weld expulsion, narrow weld lobe and interfacial failure have developed. To deal with these issues, some vehicle manufacturers try to partly replace resistance spot welding with adhesive bonding technology. But the bond quality is significantly affected by the production environmental, like temperature, humidity, etc., and consequently the adhesive bonding is hard to meet the requirements of the vehicle body. As a result, the combination of adhesive bonding and resistance spot welding, called as weld-bonding, is recognized as the preferred method to join three steel sheets. However, weld-bonding of three steel sheets is affected by both the adhesive layer and steel properties. The viscous adhesive would make the steel sheets hard to contact enough. Dissimilar steel grades and thicknesses would lead to the unbalance distribution of the electric and temperature field in weld-bonding. The combination of these effects resulted in the weld nugget shift, and consequently it is hard to control the weld quality in weld-bonding of three sheet stackup. Thereforce, it is imperative to study the mechanism of weld-bonding of three sheet stackup to improve the weld quality.In this dissertation, the weld-bonding of three sheet stackup is focused on by starting from the adhesive layer. Theoretical analysis and experimental methods have been employed to study the contact resistance between the steel sheets with the adhesive during the squeezing stage, and the heat transfer and dynamic resistance during the welding stage. Based on these two aspects and considering the different properties of dissimilar steel sheets, a finite element model was developed to model the welding process and reveal the mechanism of weld-bonding of three steel sheets with various strengths and thicknesses. Furthermore, a new method using asymmetric electric field input was brought up to deal with the weld nugget shifting phenomenon and improve the weld quality in weld-bonding of with three steel sheets. Process specifications for weld-bonding of three steel sheets were established to widen the application in the auto-body. The main content in this dissertation contains four parts:1) Contact resistance between the steel sheets with the adhesive during the squeeze cycleSince the viscous adhesive is used, the steel sheets could have a poor contact under a given electrode force, and consequently result in the weld expulsion. To deal with this issue, a theoretical analysis model containing the influence of the adhesive has been brought up to study the contact regulations between the steel sheets with adhesive layer. The results showed that the remaining adhesive on the surface of the steel sheet increases the film resistance, and consequently the total contact resistance between steel sheets rises. The larger the adhesive viscosity is, the more the contact resistance would increase. However, the thickness of the adhesive layer takes little influence. It has also been found that a suitable electrode force is required to obtain the proper contact between the steel sheets in weld-bonding. The minimum electrode force to ensure the contact state in weld-bonding of 0.8 mm thick DC04, 1.4 mm thick DP600 and 1.8 mm thick DP780 is about 1.0 kN, 3.0 kN and 4.5 kN, respectively.2) Heat transfer and dynamic resistance between the steel sheets with the adhesive during the welding cycleDuring the welding cycle of weld-bonding, the adhesive change in heat is complicatedly coupled with the joule heat generation, which makes the process of weld-bonding obviously different from the traditional spot welding. To solve this problem, an equation for the balance between the heat generation and cooling in weld-bonding was established. On this basis, it was found that the adhesive not only increased the heat generation during welding due to the great contact resistance, but also minimized the heat loss on the steel surface. As a result, the dynamic resistance during welding for weld-boding increased significantly comparing to that for spot welding. Furthermore, test results showed that the weld-bonding with a high viscous adhesive had greater dynamic resistance than that with a low viscous adhesive. The adhesive thickness affects the dynamic resistance little. Extensive tests were peformed to asses the effect of the adhesive placement on the weld-bonding of three sheet stackup and test results showed that two layers of the adhesive palced between the three sheet stackup produced the maximum dynamic resistance.3) Weld nugget formation in weld-bonding with three dissimilar steel sheetsThe weld nugget shifting phenomenon is common in weld-bonding of three steel sheets and it would degrade the weld quality. A finite element model considering the effect of the adhesive on the contact state between the steel sheets has been established to study on this issue. Model results showed the weld nugget formation process of weld-bonding with various types and thicknesses of steel sheets and different adhesive layers by structure, electric and temperature field to reveal the mechanism of weld-bonding with three steel sheets. It was found that the different time of nugget initiation on the two interfaces between steel sheets is the key reason for the weld nugget shifting. However, the nugget initial time is closely related to the properties of steel sheets and adhesive. In the weld-bonding with 0.8 mm thick DC04, 1.4 mm thick DP600 and 1.8 mm thick DP780, the initial time of weld nugget on the interface between 1.4 mm thick DP600 and 1.8 mm thick DP780 was about 80 ms earlier than that between 0.8 mm thick DC04 and 1.4 mm thick DP600. Additionally, the weld nugget in weld-bonding initiated earlier 40 ms than that of spot welding with the same welding parameters. Weld-bonding with a high viscous adhesive gets a big weld nugget; however, the adhesive thickness affects little.4) Weld nugget shifting and process optimization of weld-bonding with three dissimilar steel sheetsBased on the research result about the mechanism of weld-bonding of three dissimilar steel sheets, process specification for detail production condition was established including different placement of adhesive layer, sheet combinations with different thicknesses and orders. It was found that the weld-bonding with one adhesive layer on the thin steel side could get a better weld nugget only on that interface, so it could improve the weld nugget shifting obviously. In a weld-bonding of one low carbon steel and two dual phase steels, the maximum thickness ratio between the top and bottom steel sheets is about 1:3. To further improve the weld quality, a symmetric electric field input method was developed by using different electrode caps to adjust the weld nugget shifting. The results showed that a couple of electrode caps with 4 mm tip diameter as the top one on the thin sheet side and 6 mm tip diameter as the bottom one on the thick sheet side, simply called 4+6 electrodes, had the best efficiency to improve the weld quality.In summary, modeling and experimental methods have been used in this dissertation to study the mechanism of weld-bonding of three steel sheets. Process specifications and optimization are developed to improve the implementation of weld-bonding of three steel sheets in manufacturing.
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