Joining Zone Research Articles

A New Design for Friction Stir Spot Joining of Al Alloys and Carbon Fiber-Reinforced Composites

Friction stir spot welding (FSSW) has been recently developed to join dissimilar materials. However, the traditional requirement for a rotating tool consists of a pin and shoulder in FSSW leads to a complex joining process and unpredictable defects. In this study, a new static-shoulder design in FSSW was proposed and developed to join Al alloys to carbon fiber-reinforced polymer (CFRP) composites. The main joining parameters, including pin rotational speed, pin feed rate and pin plunge depth, were varied to investigate their effects on the joining temperature, materials interaction and the strength of joints. The pin rotational speed had the largest influence on the joining temperature. Lap shear tensile testing was conducted to evaluate the performance of the joints. The joints exhibited the ultimate lap shear force from 230 to 260 N. A brittle fracture occurred with the displacement-at-fracture load of 0.35-0.41 mm. Cross-sectional images revealed the creation of undulations on the surface of Al alloys in the joining zone. The undulations created a macro-mechanical interlocking bonding between the materials, which determined the performance of the joints. For a flat pin, by increasing the plunge depth from 1.25 to 1.30 mm, the undulation size increased from 0.21 to 0.26 mm, which can enhance the macro-mechanical interlocking bonding between Al alloys and CFRP and accordingly increased the ultimate shear force of the joints from 230 to 241 N. Use of a fluted pin significantly influenced the flow of the plasticized Al alloy which created pronounced undulations and large Al alloy spikes of 0.46 mm. These features seemed to establish an efficient macro-mechanical interlocking bonding, which resulted in a noticeable improvement in the performance of the joint. For a plunge depth of 1.30 mm, the ultimate shear force increased to 261 N using the fluted pin.

Weld seam morphology and bond strength of infrared and vibration welded SLS parts of polyamide 12 as a function of the layer build-up direction and the welding process

Selective laser sintering (SLS) offers the possibility of manufacturing complex parts without tools and thus enables the cost-effective production of individualized products. In contrast to this advantage, the producible component dimension is restricted due to the dimensions of the building chamber of commercial SLS systems. Furthermore, a great amount of cost-intensive powder is necessary for the production of large SLS parts. In order to overcome these limitations, reliable joining processes, which enable high mechanical bond properties in SLS assemblies, are indispensable. Especially welding processes are suitable in this case. However, there is only limited knowledge available about the welding of SLS parts. In particular, the influence of the characteristics of the SLS process on the resulting bond quality has not been analyzed so far. Within this paper, the influence of the SLS layer build-up direction on the resulting weld seam morphology and bond strength as a function of the welding process is investigated. Infrared and vibration welding, which differ in the method of energy input into the joining zone, are used.The investigations carried out show that high weld seam strengths can be achieved in SLS assemblies for both welding processes. The resulting bond strength is thereby largely independent of the SLS build-up direction. Vibration welded SLS parts exhibit high bond strengths in the range of the base material strength. SLS joints produced via infrared welding have lower bond strengths, which is expected to be caused by an already beginning recrystallization in the generated melt layer during the changeover phase. Furthermore, the investigations show that the weld seam morphology for SLS parts deviates from the characteristic structure of polymeric weld seams, as welded SLS samples do not show the typical separated multi-layer structure. The weld seam shows a very fine spherulitic, partly optically amorphous structure. This is probably the case because of additives contained in the SLS powder (e.g. pyrogenic silicon dioxide), which have an accelerating effect on crystallization. Deformed spherulites in the transition area to the base material are only visible for vibration welded SLS specimens. Due to larger occurring spherulites within the SLS samples compared to injection molded parts, their deformation takes place over a much wider area. The analysis of the weld seam morphology further substantiates the assumption that recrystallization already begins during the changeover phase in the infrared welding process. A highly oriented area with growing spherulites can be detected in the area of the joining plane. In summary, however, it can be noted that both welding processes are well suited for joining SLS parts.

Tailoring Soft Local Zones in Quenched Blanks of the Steel 22MnB5 by Partial Pre-cooling with Compressed Air

The high-strength boron steel 22MnB5 steel is widely used for automotive lightweight constructions. A novel approach is promising to tailor strength and ductility in the hardened condition using locally pre-tempered sheets for the hot stamping process. It results in the formation of locally soft spots where mechanical joining is subsequently intended. A slow pre-cooling of the later joining zones with cooling rates below a certain critical cooling rate for obtaining a decreased strength in these regions is required. A tubular air cooling system suited for this task is presented and tested in a process where the subsequent quenching of the overall sheet is realized by rapid cooling in a water bath. Varying the air pressure and cooling duration allows controlling the size of the softened local spot in a wide range and still obtaining a bainitic microstructure. Using two-stage cooling with point jet nozzles and a subsequent hot stamping process with water-cooled dies resulted in a main sheet hardness of about 470 HV5 and of 260 HV5 in the pre-cooled spots, respectively.

Feasibility study of using integrated fiber optical sensors to monitor laser-assisted metal\u2013polymer joining

The possibilities and challenges of using fiber optical sensors to monitor the laser-assisted joining of metal–polymer joints have been described in this article. Fundamental investigation proves the basic suitability of the measuring method for this application and studies the effect of essential influencing variables of the joining process—e.g., the clamping force—on the resulting sensor signals. In addition, the strain state (because of the process temperature and shrinkage of the polymer) of the parts to be joined can be traced as a function of the joining partners, the process parameters, and the material thicknesses. It is shown that the fiber optical method is suitable for process monitoring directly in the joining zone of metal–polymer hybrids and providing a tool for detailed strain measurements in the joint zone during subsequent component testing.

Effect of the water depth on the hydrogen content in SMAW wet welded joints

Hydrogen-induced cold cracking is a huge challenge in underwater wet welding. In the present study, the influence of water depth on the diffusible and residually stored hydrogen content is investigated for the case of underwater wet shielded metal arc welding. The welding is carried out in a simulated water depth of 5, 20, 40, and 60 m with four stick electrodes specifically developed for underwater wet welding. The influence of the welding current, the arc voltage and the electrode’s composition on the diffusible hydrogen content are considered. To obtain reproducible welding conditions, a fully automated multi-axis welding system is used inside a pressure chamber. The water depth is simulated by setting the internal pressure up to 6 bar, equivalent to 60 m water depth. A large amount of samples are analysed and statistical method are used to evaluate the results. The results show a significant reduction of the diffusible hydrogen and an increase of residual hydrogen in the joining zone with increasing water depth.

Influence of process parameters on the formation and quality of joining zones during induction brazing

Contact materials for electrical switch gears are widely used for connecting and disconnecting electric circuits. These contacts are two component systems, where the contact tips are brazed to the carriers prior to installation. The contacts used in the present work is a 3 layered sandwich structure consisting of a silver-tin oxide (Ag-SnO2) layer, a silver (Ag) inter-layer and a thin layer of the braze. The carrier itself is copper (Cu) - plated steel. The brazing process is accomplished by a high frequency inductive heating process. By varying the brazing parameters, namely, temperature and time, their influence on joint formation is investigated. The brazing joint of the contact tips were characterized by microscopy (light and electron microscopy) and physical analytical techniques (energy dispersive X-ray analysis. The mechanical characteristics were determined with Vickers micro-hardness. Investigations indicated that, comparatively, the variations in temperature had a greater influence on joint zone formation and their mechanical properties. It was further demonstrated that the extended hold times lead to a higher degree of joint zone filling due to a large reduction in pore volume fraction.

Local shear strength distribution of ultrasonically welded hybrid Aluminium to CFRP joints

This paper presents an approach to evaluate the local distribution of lap shear strength in the joining zone of ultrasonically welded single lap joints of aluminum and fiber-reinforced thermoplastics, which also show a sufficient resistance against ageing or galvanic corrosion. Both the characteristics of the process and the microscopic evaluation of the fracture surfaces suggest that the strength is distributed inhomogeneously over the joining area and is composed of adhesive and cohesive fractions. For this evaluation, two procedures were developed to subdivide the joining zone into sections and to investigate them separately. Subsequently, a local strength distribution in the joining area can be determined using mathematical operations. With the help of this interpretation, different bonding mechanisms can be identified and assigned. Additionally to mechanical investigation, Finite Element simulations were used to calculate the stress distribution of the joints depending on the carrying cross section to validate the experimental data. Despite the change of the geometry e.g. due to drilling, the calculated stress distribution pattern did not change.

Influence of the properties of the joining partners on the load-bearing capacity of shear-clinched joints

Abstract Shear-clinching requires a minimum ductility for the punch-sided sheet material. A promising approach to overcome this limitation is a local short-term heat treatment of the upper joining partner in the joining zone. However, the design of heat treatment layouts for high-strength aluminum alloys necessitates fundamental knowledge on the mechanisms in shear-clinching processes in dependence of the properties of both joining partners. Therefore, within the paper, the mechanisms in shear-clinching were analyzed for a common aluminum magnesium alloy of the 5000 series, an alloy of the 6000 series with higher strength and a high-strength alloy of the 7000 series. On the die-side, hot stamped 22MnB5 and the dual phase steel HCT780X was utilized. For upper sheet materials with higher strength, the radial material extrusion of the upper joining partner decreases, which leads to the increase of the material volume in the joining zone and therefore to a larger bottom thickness and interlock. In addition, the decreased penetration of the outer punch results in a larger upper neck thickness. Due to the larger load-bearing cross section and the higher tensile strength, higher forces can be transmitted for high-strength materials. However, the low ductility of high-strength materials results in a failure of the joint at lower displacements and therefore in a decreased energy absorption. The found mechanisms provide the theoretical basis for the design of heat treatment layouts for high-strength aluminum alloys.

Visualization of the molten pool of the laser beam submerged arc hybrid welding process

Submerged arc welding is currently the predominant industrial welding process for joining various steel grades in the plate thickness range of about 40 mm. A high energy input and the associated component distortion are unavoidable due to the high number of welding layers. By combining the conventional submerged arc welding process with the laser beam welding process to form a common hybrid welding process, this welding task can be mastered in only two welding layers by means of layer-counter-layer technology. This considerably reduces the welding time and thermally induced component distortion to a minimum due to the symmetrical energy input. A characteristic feature of a hybrid welding process is the formation of a common weld pool. In previous studies, the mixing of the molten pool has already been proven using chemical analysis methods. In the context of this paper, the shaping of the hybrid weld pool is also discussed. The molten material is expelled out of the joining zone by means of a gas pressure surge. Subsequently, a light section sensor was used to scan this area and produce a three-dimensional image from the formerly molten weld seam area. This shows that a single contiguous melt pool is produced. Finally, the first one-sided welds with full penetration and backing will be presented and discussed.

Innovative joining technology for the production of hybrid components from FRP and metals

Abstract The joining of fiber-reinforced plastics (FRP) currently poses particular process engineering challenges. Adhesive bonding or mechanical joining is normally used for such joints. However, bonding processes require complex measures for surface pre-treatment, fixing concepts and usually requires long curing times. On the other hand, mechanical joining processes such as screws or semi-hollow punch rivets lead to significant damage to the reinforcing fibers. Although different approaches for the production of plastic/metal hybrids are being pursued, there is still a need for a joining process that adequately meets the technical and economic challenges of joining metal and FRP. An innovative joining concept bypasses the problems of conventional joining techniques by using small-scale form-fitting elements in combination with established resistance welding processes. The investigations presented here include temperature measurements of the welds by means of contact thermocouples and analysis of the matrix material after welding by IR spectroscopy. The mechanical properties of the joint were verified in static shear tensile tests on coupon samples and in 3-point bending tests on geometries close to the component. The investigations carried out so far show high static bond strengths, which can be increased by coordinated welding parameters and component geometries. The joining process offers many possibilities with regard to the design freedom of the joining zone, since the pin structures can be flexibly adapted with regard to their arrangement, number and geometry, so that different requirements can be met. The Multi-Spot-Joint generates a quasi-ductile post-break characteristic. Thermal damage to the plastic matrix can be avoided by the targeted use of particularly short resistance welding processes. No damage to the plastic matrix could be detected during the IR spectroscopy.

A novel laser welding method for the reliable joining of NiTi/301SS

In this paper, a novel laser welding method with Nb/Cu multi-interlayer was developed for improving the mechanical properties of NiTi/301SS joint. A hybrid joint consisting three metallurgical joining zones was obtained based on different welding mechanisms through a single pass welding. The self-fusion welding was formed on the Nb interlayer, and the fusion zone was composed of Nb-based solution. NiTi and Nb were well bonded by eutectic reaction, and the NiTi/Nb interface contained Nb-based solution, NiTi phase and Nb-NiTi eutectic structure. Nb and 301SS were joined by brazing because of the melting of Cu, and the Nb/Cu/301SS interface consisted of Nb-, Cu-, and Fe-based solutions and a little Fe7Nb6 intermetallic. The strength and ductility of the joints with interlayer were 126% and 188% higher than the joints without interlayer, respectively. The use of Nb/Cu multi-interlayer completely prevented the formation of Ti-Fe and TixNiy IMCs, which can improve significantly mechanical properties of joints.

The bonding temperature effect of the diffusion brazing Inconel 625 superalloy on the microstructure changes, corrosion resistance, and mechanical properties

The aim of this survey was an evaluation of the diffusion brazing (DB) bonding properties of the Inconel 625 superalloy using a BNi-3 interlayer. During the inter-diffusion and the dissolution of the alloy elements at the bonding temperature, and also cooling-cycle process, the joining zone (JZ) divided into three section namely isothermal solidification zone (ISZ), diffusion affected zone (DAZ), and athermal solidification zone (ASZ) which included a Ni-rich solid solution phase, Mo and Cr-rich boride precipitation, and Mo-rich boride precipitation and also Ni-Si binary phase, respectively. The cyclic potential-dynamic polarization tests indicated an inverse relationship between the corrosion behavior and the width of the ASZ. Also, the bonding process leads to the loss of resistance to pitting corrosion. The completion of the ISZ, together with the highest corrosion resistance attained at the bonding temperature at 1180 ᵒC. Likewise, with increasing bonding temperature, the eutectic compounds decreased and ISZ completed. The highest hardness belonged to the JZ/DAZ boundary at 1060 ᵒC and ASZ at 1120 ᵒC measured as 422 HV and 390 HV, respectively; which decreased by increasing bonding temperature, but the ISZ hardness increased.

Absorber-free laser transmission welding of transparent polymers using fixed focus optics and 3D laser scanner

Laser transmission welding is a well-known joining technology for thermoplastics, providing reliable and hermetical sealing without adhesives or particle formation. The main advantages of laser-based energy input - high precision, no additional adhesive and no particle emission - are essential for medical and optical applications. To obtain sufficient absorption in visually transparent polymers, thulium fibre lasers emitting in the polymers’ intrinsic absorption spectrum are used. Optics with high numerical aperture provide large intensity gradients inside the specimen, enabling selective fusing in the joining zone. Although the basic feasibility has already been demonstrated, the welding process lacks stability and productivity.Aim of this work is the determination of optimized settings for a fast and reliable welding process. Thus, the interplay of process parameters as well as their impact on the seam are analysed by thermal simulations. Calculated settings are verified by welding tests with COC, PA6 and PETG, using a fixed-focus optics and a 3D laser scanner with up to 200 mm/s feed rate.

Influence of subsurface properties on the application behavior of hybrid components

Abstract In the manufacturing of massive components, demands such as weight reduction and increasing functional integration become more and more important. By combining different materials, for example high-strength steel and aluminium, hybrid massive components with specifically adapted properties for a required application behavior can be producd. The application behavior of hybrid components is especially influenced by its surface and subsurface properties, like e.g. residual stresses, surface roughness or material structure, which consequently determine the life span of such components. The previous machining process significantly influences these properties. The modifications of the above-mentioned subsurface properties (residual stresses and material structure) in the joining zone of hybrid components during deep rolling and longitudinal turning are analysed. In order to examine the influence of these properties on the life span of hybrid components, rotating bending tests and in situ scanning electron microscope (SEM) investigations during tensile tests are conducted. The investigations show that subsurface properties in the joining zone have a deep impact on the application behavior of hybrid components.

Influence of the laser-based surface modification on the bond strength for friction press joining of aluminum and polyethylene

Friction press joining is an innovative joining process for the production of plastic-metal joints without additives, in an overlap configuration. In order to achieve a high bond strength, the metallic joining partner is pretreated with laser radiation. Subsequently, heat is induced by friction and pressure during the joining process, causing the thermoplastic material to melt and adhere to the metallic joining partner. In this work, the temperature distribution during the process in the composite is analyzed and characterized. It was found that the occurring temperatures and temperature differences are not only dependent on the rotational speed, but also on the feed rate. It is also shown that the friction surface temperature can be used as an indirect control variable for a model-based, closed-loop control. Based on these findings, various surface modifications forthe metallic joining partner were investigated and analyzed with regard to the maximum strength of the joint. It was observed that the highest tensile shear strength can be achieved with a quasi-chaotic nano structure. In addition, the joining compound was characterized by a thin section, facilitating the identification of specific zones in the joint. These investigations show the high potential for friction press joining of plastics and metals, and form the basis for a model-based control of the joining zone temperature.

Measurement of core temperature through semi-transparent polyamide 6 using scanner-integrated pyrometer in laser welding

Predicting the core temperature during welding is an ambitious aim in many research works. In this work, a 3D-scanner with integrated pyrometer is characterized and used to measure the temperature during quasi-simultaneous laser transmission welding of polyamide 6. However, due to welding in an overlap configuration, the heat radiation emitted from the joining zone of a laser transmission weld has to pass through the upper polymer, which is itself a semi-transparent emitter. Therefore, the spectral filtering of the heat radiation in the upper polymer is taken into account by calibrating the pyrometer for the measurement task. Thermal process simulations are performed to compare the temperature field with the measured temperature signal. The absorption coefficients of the polymers are measured, in order to get precise results from the computation. The temperature signals during welding are in good agreement with the computed mean temperature inside the detection spot, located in the joining area. This is also true for varying laser power, laser beam diameter and the carbon black content in the lower polymer. Both, the computed mean temperature and the temperature signal are representing the core temperature. In order to evaluate the spatial sensitivity of the measurement system, the emitted heat radiation from both polymers is calculated on basis of the computed temperature field. Hereby it is found, that more than 90 percent of the detected heat radiation comes from the joining area, which is a crucial information for contact-free temperature measurement tasks on semi-transparent polymers.

Microstructure and mechanical properties of YG8/IN718 alloy joints prepared by vacuum brazing

YG8/IN718 joint was prepared with AgCuNiLi filler alloy by vacuum brazing, the microstructure and mechanical properties were investigated. The interfaces, fracture, composition and phases of as-prepared YG8/IN718 joint were analyzed by scanning electronic microscopy (SEM) with an energy disperse spectroscopy (EDS) and X-ray diffraction (XRD), as well as the shear strength and microhardness of joints were tested by universal testing machine and microhardness tester, respectively. The analyses on the interfacial microstructure indicate that the joint is compact and the solid solution of Ag (s, s), Cu (s, s) and (Cu, Ni) can form in the joining zone by element interdiffusion and reaction. The shear strength of 127.5 ± 2.1 MPa is obtained under the process parameters of 870 °C, 30 min, and fracture occurs in the interlayer, which presents a brittle characteristic.

Simulation‐aided development of a robust thermoclinching joining process for hybrid structures with textile reinforced thermoplastic composites and metallic components

AbstractIn order to take advantage of the specific structural and functional properties inside a multi‐material assembly, it is necessary to provide adapted joining technologies. Therefore, the new joining technology “thermoclinching” was developed to join endless fiber reinforced thermoplastic composites and metallic joining partners. Previous experimental and numerical studies demonstrated the potential of the novel joining technology. In experimental and numerical studies essential geometrical and material parameters influencing the forming behavior are identified and evaluated. Thereby, the inline‐thermoclinching technology is developed by integrating the pre‐processed steps of cutting and heating into the actual joining process. For structural analysis of the joining zone as well as for quality analysis and validation of the process simulations computer tomography (CT) is used. Furthermore, experimental characterizations of the failure behavior of thermoclinched joints and the development of simulation models as well as strategies for numerical strength analyses are carried out.

Investigation of the joining zone displacement of cross-wedge rolled serially arranged hybrid parts

Within the Collaborative Research Centre (CRC) 1153 “Tailored Forming “the manufacturing of hybrid bulk components is investigated. Therefore, a process chain consisting of joining, forming, milling and quality control has been established by multiple subprojects. Within subproject B1 of the CRC forming of hybrid parts by the incrementally forming cross-wedge rolling (CWR) process is investigated. The superior aim is to determine process limits and capabilities, when forming parts consisting of different materials joined by varying technologies. In this paper, the investigation of cross-wedge rolling of serially arranged hybrid parts made of steel and aluminium is described. The focus of the research presented in this publication is the displacement of the joining zone of hybrid parts due to the cross-wedge rolling process. Therefore, finite element simulations have been developed, that allow the investigations of hybrid solid components. After simulation of various variations of the cross-wedge rolling process, i.e. differently shaped tools and forming velocities, experimental trials were carried out with identical parameter sets. A comparison of simulation and experiment, showed that the simulation model is capable of describing the cross-wedge rolling process of hybrid parts. The standard deviation of the displacement of the joining zone between simulation and experimental trials is 8.8% with regard to all investigated cases.

Resistance element welding of magnesium alloy and austenitic stainless steel in three-sheet configurations

The Mg alloy and two austenitic stainless steel sheets were joined together by a metallurgical bond across the rivet and two austenitic stainless steel sheets. More heat was generated at the austenitic stainless/austenitic stainless interface than at the rivet/ austenitic stainless steel interface, leading to larger nugget size at the austenitic stainless steel /austenitic stainless steel interface at all welding currents. Thus, the nugget size at the austenitic stainless steel /austenitic stainless steel interface mainly influenced the transition from interfacial to pullout failure modes. The fusion zone microstructure consisted of ferrite and austenite. The microstructure in the edges of the nugget (both in the rivet and ASS) consisted of fine columnar dendritic grains. Owing to variation of temperature gradient and solidification growth rate, the grains morphology changed from columnar dendritic to equiaxed dendritic in the nugget center. The fine grains resulted in high fusion zone hardness. Digital image correlation analysis revealed that the joints could experience joining zone rotation/out-of-plane displacement during lap-shear tests, which reduced the magnitude of strain sustained by the joints in the loading direction. The joint configuration that did not undergo joining zone rotation and failed via pullout failure in the austenitic stainless steel sheet exhibited superior lap-shear performance.