Advanced materials joining methods

M. Amirthalingam, E.M. van der Aa, N. den Uijl, M.J.M. Hermans & I.M. Richardson: Phosphorous and boron segregation during resistance spot welding of advanced high strength steels; 9th International Conference on Trends in Welding Research; Chicago, Illinois, USA; June 4-8 2012.

The mechanical properties of advanced high strength steels (AHSS) are sensitive to their multi-phase microstructures.
Phosphorous is added in AHSS mainly as a solid solution strengthener and to suppress the formation of cementite, thereby enhancing the room temperature stabilisation of austenite in transformation induced plasticity (TRIP) and quench and partitioning (Q&P) steels. However, the weldability of phosphorous containing steels is poorer than conventional low carbon steels due to embrittlement resulting from the segregation of phosphorus to grain boundaries during solidification of the weld pool. In order to reduce the grain boundary embrittlement, the addition of boron has been suggested; however, this also increases hardenability and the ratio of boron to other alloying elements present in the steels also affect the mechanical properties of the welds.
In this work, the partitioning behaviour of phosphorous and boron during the solidification of weld pools under various weld thermal cycles has been studied using experimental simulations and a DictraTM based diffusional model. Steels with varying carbon, phosphorous and boron contents were designed and subjected to various welding thermal cycles.
Mechanical properties were evaluated by hardness, cross tension tests and correlated with the weld microstructure. Diffusion calculations show that the phosphorus concentration in the last area in the weld pool to solidify can reach about 0.38 wt. % for a steel with a bulk concentration of 0.08 wt. %. Elemental analysis indicates that in the absence of boron, the grain boundaries of columnar grains in the weld pool are decorated with phosphorous. As a result, a complete interface failure occurs during cross tension testing. With the addition of boron, apart from an increase in weld strength, the failure mode switches to a complete plug mode.

The present investigation analyzes the force and torque developing during Friction Stir Spot Welding (FSSW) of thermoplastic sheets varying the main process parameters. In addition, measurements of the tool temperature and that of the material close to the welding region were carried out to better understand the variation of the forces during FSSW and quality of the joints. Experimental tests involving an instrumented drilling machine were performed on polycarbonate sheets.The study involved the variation of dwell time, tool plunge rate and rotational speed. Mechanical characterization and dimensional analysis of the joints were performed in order to assess the influence of the process parameters on the joints quality under considered processing conditions. According to the achieved results, using low values of the plunging speed has beneficial effects on both the process (reduction in the force and torque) and the mechanical behaviour of the joints. Increasing the tool rotational speed results in reduced processing forces, higher material mixing and temperature. The dwell time has negligible effect on developing forces while it highly influences the material temperature, dimension of the welded region and consequently the mechanical behaviour of the joint.

This study investigates the influence of the tool dimensions on the mechanical behavior of Friction Stir Spot Welds performed on polycarbonate sheets. Different tools were used by varying the pin geometry (cylindrical and tapered) and the diameters of the tool pin and that of the tool shoulder. Morphological analysis was conducted to determine the extension of the welded zone and main defects of the welds. Single lap shear tests were carried out to determine the influence of the tool dimension on the mechanical behavior of the welds. The onset and evolution of failure during the shear tests was analyzed. Five different failure modes were identified which showed both a brittle and ductile behavior. According to the achieved results, the strength of the welds reduced with increasing the pin diameter and taper angle. On the other hand, the strength increased with increasing the tool shoulder diameter. However, the highest specific strength, calculated as the ratio of the shear strength by the tool area, reduced with increasing the shoulder diameter. Under the optimal processing conditions, the strength of the joints reached up to 88% of the shear strength of the base material.

An experimental investigation has been conducted on mechanically clinched joints, produced with fixed and extensible dies with different forming forces. Mechanical testing involving single lap shear tests, both with one and two joining points, and peeling tests were conducted under quasi-static conditions to assess the different mechanical behaviours of these joints. The effect of the processing conditions on the main mechanical response of the joints, namely the maximum strength, stiffness and absorbed energy were investigated.
The results showed that the joints produced with the extensible die exhibited a similar strength as compared to those produced with the fixed die in single lap shear tests, while they are characterized by a higher strength (up to 40%) when loaded during the peeling test because of a larger interlock. In addition, the employment of extensible dies allows a drastic reduction of the forming loads as compared to those required by adopting the fixed dies.

The present investigation is aimed at analysing the influence of the processing speeds and processing times on mechanical behaviour of Friction Stir Spot Welding (FSSW) joints produced on polycarbonate sheets. The analysis involved the variation of rotational speed, tool plunge rate, pre-heating time, dwell time and waiting time. Mechanical characterization of joints was carried out by means of single lap shear test. Experimental tests were conducted according to two full factorial designs. First, an exploratory 25 full factorial plan was carried out to determine the most influencing factors determining the mechanical behaviour of FSSW joints. Then, a 33 optimization plan was performed by varying the most relevant process parameters among three levels. Therefore, analytical models were developed to predict the mechanical behaviour of welds (maximum shear strength, stiffness and absorbed energy) produced under different processing conditions. In addition, an Artificial Neural Network (ANN) model was developed to improve the matching between experimental measurements and model predictions. On the basis of the achieved results, a framework for improving the mechanical performances of thermoplastic joints was established. According to the achieved results, tool plunge rate, dwell time and waiting time are the most influencing parameters for the joint strength and weld extension. On the other hand, pre-heating time and tool rotational speed have lower influence on the mechanical behaviour of FSSW joints.

The present investigation is carried out to assess the suitability of the clinching process for production of plastic-metal hybrid joints. To this end, a prototypal apparatus was developed to preheat the sheets before joining. A campaign of experimental tests was carried out on aluminium alloy AA5053 and polystyrene. Experimental tests were conducted by varying the main process parameters i.e. pre-heating conditions (heating time and temperature of heating air) and forming pressure. The joints produced under different operating conditions were observed using optical and stereoscope microscope. In addition, the mechanical behaviours of joints were assessed by conducting single lap shear tests on single joints. Based on the achieved results, the main failure modes were identified and the effect of process parameters on mechanical behaviours of the joints were clarified.

Friction Stir Spot Welding (FSSW) experiments were carried out on polycarbonate sheets to assess the influence of the tool geometry on the joining loads and material flow. An instrumented drilling machine was used to measure the plunging load and the torque developing during the process. Different tool geometries were tested involving the variation of the tool pin shape (cylindrical or tapered), diameter of the tool pin and diameter of the tool shoulder. The analysis of material flow enabled understanding the behavior of the load and torque trends measured during the process, such as the presence of load peaks during the plunging phase and reduced values of the axial pressure during the process. It was discovered that the average axial pressure was higher during pin plunging (such value was mainly determined by the pin diameter) while it dramatically decreased (almost to 1.2 MPa) during shoulder plunging owing to the higher heating resulting from higher tangential speed. The pin taper angle had negligible influence on the plunging load and torque. The tool shoulder influenced the plunging load and torque while it produced a negligible variation in the average axial pressure. On the other hand, the pin diameter influenced both the process loads and the axial pressure.

The present investigation is aimed at analysing the influence of the processing speeds and processing times on mechanical behaviour of Friction Stir Spot Welding (FSSW) joints produced on polycarbonate sheets. The analysis involved the variation of rotational speed, tool plunge rate, pre-heating time, dwell time and waiting time. Mechanical characterization of joints was carried out by means of single lap shear test. Experimental tests were conducted according to two full factorial designs. First, an exploratory 2 5 full factorial plan was carried out to determine the most influencing factors determining the mechanical behaviour of FSSW joints. Then, a 3 3 optimization plan was performed by varying the most relevant process parameters among three levels. Therefore, analytical models were developed to predict the mechanical behaviour of welds (maximum shear strength, stiffness and absorbed energy) produced under different processing conditions. In addition, an Artificial Neural Network (ANN) model was developed to improve the matching between experimental measurements and model predictions. On the basis of the achieved results, a framework for improving the mechanical performances of thermoplastic joints was established. According to the achieved results, tool plunge rate, dwell time and waiting time are the most influencing parameters for the joint strength and weld extension. On the other hand, pre-heating time and tool rotational speed have lower influence on the mechanical behaviour of FSSW joints.

The present investigation analyzes the force and torque developing during Friction Stir Spot Welding (FSSW) of thermoplastic sheets varying the main process parameters. In addition, measurements of the tool temperature and that of the material close to the welding region were carried out to better understand the variation of the forces during FSSW and quality of the joints. Experimental tests involving an instrumented drilling machine were performed on polycarbonate sheets.The study involved the variation of dwell time, tool plunge rate and rotational speed. Mechanical characterization and dimensional analysis of the joints were performed in order to assess the influence of the process parameters on the joints quality under considered processing conditions. According to the achieved results, using low values of the plunging speed has beneficial effects on both the process (reduction in the force and torque) and the mechanical behaviour of the joints. Increasing the tool rotational speed results in reduced processing forces, higher material mixing and temperature. The dwell time has negligible effect on developing forces while it highly influences the material temperature, dimension of the welded region and consequently the mechanical behaviour of the joint.

The joinability of aluminium alloy sheets with reduced ductility produced by mechanical clinching is analysed. A modified tool set geometry was developed to reduce the localization and magnitude of plastic strain. Sheets pre-heating was adopted to increase the material formability. Optical microscopy and scanning electron microscopy were utilized to observe possible presence of cracks in clinched connections and to perform dimensional analysis. Mechanical characterization tests comprising micro-hardness test and single lap shear tests were conducted to evaluate the influence of the processing conditions on joints strength.

This work investigates the thermal performance of four novel CFC-Cu joining techniques. Two involve direct casting and brazing of Cu onto a chromium modified CFC surface, the other two pre-coat a brazing alloy with chromium using galvanisation and sputtering processes. The chromium carbide layer at the interface has been shown to improve adhesion. Thermal conductivity across the join interface was measured by laser flash analysis. X-ray tomography was performed to investigate micro-structures that might influence the thermal behaviour. It was found that thermal conductivity varied by up to 72%. Quantification of the X-ray tomography data showed that the dominant feature in reducing thermal conductivity was the lateral spread of voids at the interface. Correlations were made to estimate the extent of this effect.

The joinability of aluminium alloy sheets with reduced ductility produced by mechanical clinching is analysed. A modified tool set geometry was developed to reduce the localization and magnitude of plastic strain. Sheets pre-heating was adopted to increase the material formability. Optical microscopy and scanning electron microscopy were utilized to observe possible presence of cracks in clinched connections and to perform dimensional analysis. Mechanical characterization tests comprising micro-hardness test and single lap shear tests were conducted to evaluate the influence of the processing conditions on joints strength.

This work investigates the thermal performance of four novel CFC–Cu joining techniques. Two involve direct casting and brazing of Cu onto a chromium modified CFC surface, the other two pre-coat a brazing alloy with chromium using galvanisation and sputtering processes. The chromium carbide layer at the interface has been shown to improve adhesion. Thermal conductivity across the join interface was measured by laser flash analysis. X-ray tomography was performed to investigate micro-structures that might influence the thermal behaviour. It was found that thermal conductivity varied by up to 72%. Quantification of the X-ray tomography data showed that the dominant feature in reducing thermal conductivity was the lateral spread of voids at the interface. Correlations were made to estimate the extent of this effect.

The joinability of aluminium alloy sheets with reduced ductility produced by mechanical clinching is analysed. A modified tool set geometry was developed to reduce the localization and magnitude of plastic strain. Sheets pre-heating was adopted to increase the material formability. Optical microscopy and scanning electron microscopy were utilized to observe possible presence of cracks in clinched connections and to perform dimensional analysis. Mechanical characterization tests comprising micro-hardness test and single lap shear tests were conducted to evaluate the influence of the processing conditions on joints strength.