Yansong Zhang

Effect of welding energy on interface zone of Al–Cu ultrasonic welded joint

Abstract Ultrasonic welding is attracting increasing attentions in joining of dissimilar materials. The effect of welding energy on joint strength, failure behaviour and microstructure of Al–Cu ultrasonic welded joint has been experimentally investigated. The results showed that joint strength increased with welding energy initially and reached its maximum at 1000 J, then dropped significantly instead. Meanwhile, the failure mode changed from interfacial debonding to nugget pullout, and then back to cleavage failure. Various microstructures with different morphologies and properties were also observed at the interfacial region. At lower energy, the joint was only partly bonded by numbers of dispersed microbonds. A swirl-like structure appeared at the weld interface and led to a mechanical interlocking between the materials at higher energies (∼1000 J). However, cavity defects and intermetallic compound (IMC) were more likely to form under excessively high energies. A 0·5 μm thick IMC layer with dominant phase of Al4Cu9 was found in 2000 J joint.

Effect of welding speed on microstructure, hardness and tensile properties in laser welding of advanced high strength steel

Abstract The increasing demand of lightweight and durability makes advanced high strength steel attractive for future automotive applications. In this study, 0·8 mm thick bare 600, 800 and 1000 MPa grade dual phase steel and 1500 MPa grade martensitic steel were laser welded, and the effect of welding speed on weld bead geometry, microhardness, microstructure and tensile properties was investigated. The steels exhibited similar weldability, and a critical welding speed for acceptable joint was determined as 25 mm s−1. A linear relationship of the hardness at fusion zone with carbon equivalent was observed, while carbon content showed a poorer linear fit. Heat affected zone (HAZ) softening increased with the steel grades due to the higher martensite volume fraction of the base metal in stronger steels. In addition, decrease of welding speed led to longer tempering time and consequently higher degree of HAZ softening. Correlations between tensile strength and hardness were also investigated.

Effect of variable electrode force on weld quality in resistance spot welding

Abstract Resistance spot weldability is defined as the acceptable welding current ranges as determined by the weld lobe in resistance spot welding. Nowadays many studies have focused on the effect of welding current and welding time under constant electrode force on the weld quality and weldability. There is little research on the influence of variable electrode force on the weld quality and weldability because of the difficulty in controlling variable electrode force using pneumatic gun. In the present study, first, the influence of three stages of electrode force, including squeeze force, welding force and forging force, on the quality of welds is analysed. Then a design of experiment approach is applied to analyse the influence of the three stages of electrode force on welding quality and thus to obtain optimum parameter of variable electrode force by controlling the electrode force with servo gun. The comparisons of tensile shear strength, nugget size, weld lobe width and wear rate of electrode tip between variable force and constant force are carried out. The results show that the weld quality and weldability can be increased evidently using optimum parameter of variable electrode force without accelerating the electrode wear rate.

Monitoring and intelligent control of electrode wear based on a measured electrode displacement curve in resistance spot welding

Advanced high strength steels are being increasingly used in the automotive industry to reduce weight and improve fuel economy. However, due to increased physical properties and chemistry of high strength steels, it is difficult to directly substitute these materials into production processes currently designed for mild steels. New process parameters and process-related issues must be developed and understood for high strength steels. Among all issues, endurance of the electrode cap is the most important. In this paper, electrode wear characteristics of hot-dipped galvanized dual-phase (DP600) steels and the effect on weld quality are firstly analysed. An electrode displacement curve which can monitor electrode wear was measured by a developing experimental system using a servo gun. A neuro-fuzzy inference system based on the electrode displacement curve is developed for minimizing the effect of a worn electrode on weld quality by adaptively adjusting input variables based on the measured electrode displacement curve when electrode wear occurs. A modified current curve is implemented to reduce the effects of electrode wear on weld quality using a developed neuro-fuzzy system.

Numerical analysis of temperature distribution during ultrasonic welding process for dissimilar automotive alloys

Abstract Ultrasonic welding is attracting increasing attentions in lithium battery joining in the field of battery electric vehicle manufacturing. A three-dimensional finite element model was constructed to study the temperature distribution and heat generation in ultrasonic welding process. Numerical analysis showed that heat generation from plastic deformation accounts for nearly a quarter of the whole heat generation (material plastic deformation and interface friction). The fraction changes little with different sequence of specimens. The highest temperature locates at the contact interface of specimens and it is much lower than the melting point of the joining materials. Temperature distribution of the structure is not symmetric, and there are abnormal points under the effect of serrated ridges of sonotrode tip. Welding process can be divided in to three periods based on temperature evolution on the contact interface of lower specimen. The proposed model is validated by comparing simulated temperature evolution with experimental result.

Influence of initial gap on weld expulsion in resistance spot welding of dual phase steel

Abstract The weld expulsion is prone to occur and severely affects the nugget quality when the initial gap between dual phase (DP) steel sheets exist in resistance spot welding (RSW). To investigate the effect of initial gap on weld expulsion, a finite element model was developed to analyse the weld nugget formation process with different initial gaps for DP steels. An estimation method of expulsion occurrence based on the ratio of the nugget radius Rn and the contact radius Rc between sheets was proposed to get the critical initial gap without expulsion. The simulation and experimental results showed that the weld expulsion would not happen until the gap spacing reaches the critical value. The critical initial gap of DP steel is much smaller than that of low carbon steel. For both DP steel and low carbon steel, the critical initial gap would increase with the thickening of the steel sheet.

Online quality inspection of resistance spot welded joint based on electrode indentation using servo gun

Abstract Resistance spot welding is one of the major joining techniques widely used for car body assembly. Weld quality may significantly influence the durability and reliability of the automobile body. Automotive manufacturers often rely on destructive testing and monitoring variables which indirectly reflected weld quality to assess the weld quality and control the welding process. However, these approaches have inherent limitations and are difficult to be implemented in plant environments. Therefore, it is imperative to develop an online inspection method to evaluate weld quality. In the present study, a method of producing a series of substantially uniform spot welds between two metal parts using a servomotor driven movable electrode and an axially opposing fixed electrode is proposed. The indentation in the workpiece surface is suitably measured by the displacement of the movable electrode as it applies an electrode force and welding current is passed through the weld site of the workpiece. The optimal indentation range is determined by peel test and metallographic examinations with respect to various sheet gages and grades. Consequently, online weld quality inspection results are achieved based on developed optimal indentation range.

Experimental study of single sided sheet to tube resistance spot welding

Abstract To reduce weight and improve performance, hydroformed tubes are being widely used in automotive structure fabrication and the single sided sheet to tube resistance spot welding (SSRSW) is considered as a feasible method for joining a tube to other parts. However, in the sheet to tube SSRSW process, it is difficult to assure welding quality because of large welding deformation due to a lack of support inside the tube. The present paper investigates the influences of welding parameters, such as electrode force and welding current, on the welding deformation and quality of the sheet to tube SSRSW using electrode displacement and tensile shear tests. The effects of different electrode force patterns on the welding quality are investigated utilising the force characteristics of a servo gun. It is found that the welding deformation is influenced by both the electrode force and the welding current, and the tensile shear strength declines with larger electrode force and higher welding current. However, the tensile strength could be enhanced significantly and the welding deformation decreased greatly by reducing the electrode force in the welding stage or holding stage. In order to decrease manufacturing cost and improve weld quality, the reduced electrode force is recommended for the sheet to tube SSRSW process.

Effect of acoustic softening on the thermal-mechanical process of ultrasonic welding

HighlightsConstitutive model considering acoustic softening effect (ASE) was proposed.ASE can promote joint formation by increasing plastic deformation.Structure deformation under various welding time can be predicted numerically.Heat generated from plastic deformation in ultrasonic welding cannot be ignored. ABSTRACT Application of ultrasonic energy can reduce the static stress necessary for plastic deformation of metallic materials to reduce forming load and energy, namely acoustic softening effect (ASE). Ultrasonic welding (USW) is a rapid joining process utilizing ultrasonic energy to form a solid state joint between two or more pieces of metals. Quantitative characterization of ASE and its influence on specimen deformation and heat generation is essential to clarify the thermal‐mechanical process of ultrasonic welding. In the present work, experiments were set up to found out mechanical behavior of copper and aluminum under combined effect of compression force and ultrasonic energy. Constitutive model was proposed and numerical implemented in finite element model of ultrasonic welding. Thermal‐mechanical analysis was put forward to explore the effect of ultrasonic energy on the welding process quantitatively. Conclusions can be drawn that ASE increases structural deformation significantly, which is beneficial for joint formation. Meanwhile, heat generation from both frictional work and plastic deformation is slightly influenced by ASE. Based on the proposed model, relationship between ultrasonic energy and thermal‐mechanical behavior of structure during ultrasonic welding was constructed.

Numerical simulation of spot welding for galvanised sheet steels

Abstract Galvanised sheet steels are now widely used to be the substrate for body in white (BIW) construction in the automotive industry. Weldability of galvanised sheet steels much worsened compared to spot welding of low carbon steels. The present paper develops a 2D axisymmetric model and employs an incremental coupled thermal–electrical–mechanical analysis to predict the nugget development during resistance spot welding (RSW) of galvanised sheet steels. Temperature dependent contact resistance for faying surfaces was determined to take into account of the influence of zinc coat for spot welding galvanised sheet steels. The effect of dynamic contact radii on temperature distribution was studied and compared with results under constant contact area assumption. The predicted nugget shape and size agreed well with the experimental data. Higher current and longer welding time should be applied for galvanised sheet steels compared to low carbon steel spot welding. The proposed model can be applied to predict weld quality and choose optimal welding conditions for spot welding galvanised sheet steels.