Joining Partners Research Articles

Influence of Tool Wear on the Load-Bearing Capacity of Shear-Clinched Joints

Shear-clinching allows the joining by forming of dissimilar materials with high differences between their mechanical properties without additional fasteners. Since the lower joining partner is indirectly shear cut during the process, even ultra-high strength materials can be joined. However, the cutting of the high-strength materials as well as the extrusion of the upper joining partner leads to high process forces and therefore to high tool loads. This applies in particular for the die, which is highly stressed during the cutting phase and therefore plastically deformed. Within the scope of this work, the influence of the occurring wear on the formation of the joint and its load-bearing capacity is analyzed for a scope of 500 strokes. For this purpose, press hardened 22MnB5 is used as lower joining partner. Its high strength leads to the plastic deformation of the cutting edge, which increases within the first 200 strokes. Afterwards only minor changes occur. Yet, no effect of the occurring wear on the joint formation and the joint strength, which was tested under shear and tensile load, could be determined. Functioning joints could still be produced for more than 500 strokes as the load-bearing capacity remained on a comparable level.

Interface Formation during Collision Welding of Aluminum

Collision welding is a high-speed joining technology based on the plastic deformation of at least one of the joining partners. During the process, several phenomena like the formation of a so-called jet and a cloud of particles occur and enable bond formation. However, the interaction of these phenomena and how they are influenced by the amount of kinetic energy is still unclear. In this paper, the results of three series of experiments with two different setups to determine the influence of the process parameters on the fundamental phenomena and relevant mechanisms of bond formation are presented. The welding processes are monitored by different methods, like high-speed imaging, photonic Doppler velocimetry and light emission measurements. The weld interfaces are analyzed by ultrasonic investigations, metallographic analyses by optical and scanning electron microscopy, and characterized by tensile shear tests. The results provide detailed information on the influence of the different process parameters on the classical welding window and allow a prediction of the different bond mechanisms. They show that during a single magnetic pulse welding process aluminum both fusion-like and solid-state welding can occur. Furthermore, the findings allow predicting the formation of the weld interface with respect to location and shape as well as its mechanical strength.

Interface Formation during Collision Welding of Aluminum

Collision welding is a high-speed joining technology based on the plastic deformation of at least one of the joining partners. During the process, several phenomena like the formation of a so-called jet and a cloud of particles occur and enable bond formation. However, the interaction of these phenomena and how they are influenced by the amount of kinetic energy is still unclear. In this paper, the results of three series of experiments with two different setups to determine the influence of the process parameters on the fundamental phenomena and relevant mechanisms of bond formation are presented. The welding processes are monitored by different methods, like high-speed imaging, photonic Doppler velocimetry and light emission measurements. The weld interfaces are analyzed by ultrasonic investigations, metallographic analyses by optical and scanning electron microscopy, and characterized by tensile shear tests. The results provide detailed information on the influence of the different process parameters on the classical welding window and allow a prediction of the different bond mechanisms. They show that during a single magnetic pulse welding process aluminum both fusion-like and solid-state welding can occur. Furthermore, the findings allow predicting the formation of the weld interface with respect to location and shape as well as its mechanical strength.

Modelling and thermal simulation of absorber-free quasi-simultaneous laser welding of transparent plastics

The growing demands on the quality of plastic components and the trend towards miniaturisation are posing a great challenge on plastics processing technology. As many complex components can no longer be manufactured in a single step, joining processes such as laser transmission welding are gaining in importance. In classic laser transmission welding, the joining partners have different optical properties. The upper joining partner is transparent in the laser wavelength range, whilst the lower partner is absorbent due to the addition of absorber materials. In medical and biotechnological applications, the addition of absorber materials is often undesirable due to strict biocompatibility requirements. If, on the other hand, radiation sources are used which emit radiation in the area of the natural absorption of the plastic (λ = 1600–2000 nm), untreated transparent plastics can also be welded. In this work, a theoretical model will be presented to calculate the temperature distribution and progression during quasi-simultaneous welding using a thulium fibre laser (λ = 1940 nm). A sensitivity analysis is carried out to investigate the influence of different parameters on the heat affected zone (HAZ). The simulated HAZ is then compared with the HAZ from the experimental work.

Particle Ejection by Jetting and Related Effects in Impact Welding Processes

Collision welding processes are accompanied by the ejection of a metal jet, a cloud of particles (CoP), or both phenomena, respectively. The purpose of this study is to investigate the formation, the characteristics as well as the influence of the CoP on weld formation. Impact welding experiments on three different setups in normal ambient atmosphere and under vacuum-like conditions are performed and monitored using a high-speed camera, accompanied by long-term exposures, recordings of the emission spectrum, and an evaluation of the CoP interaction with witness pins made of different materials. It was found that the CoP formed during the collision of the joining partners is compressed by the closing joining gap and particularly at small collision angles it can reach temperatures sufficient to melt the surfaces to be joined. This effect was proved using a tracer material that is detectable on the witness pins after welding. The formation of the CoP is reduced with increasing yield strength of the material and the escape of the CoP is hindered with increasing surface roughness. Both effects make welding with low-impact velocities difficult, whereas weld formation is facilitated using smooth surfaces and a reduced ambient pressure under vacuum-like conditions. Furthermore, the absence of surrounding air eases the process observation since exothermic oxidation reactions and shock compression of the gas are avoided. This also enables an estimation of the temperature in the joining gap, which was found to be more than 5600 K under normal ambient pressure.

Mechanical joining behavior of Cu–Fe dissimilar metallic foils in laser shock clinching

Laser shock clinching is a novel joining method derived from laser shock forming in which the metal foil is plastically deformed under the pulsed laser induced shock wave, and then two or more metal foils can be joined together based on plastic deformation. However, the present researches are less concerned with the mechanical joining behavior of metal foils during incremental impacts of multiple laser pulses. In the present study, the mechanical joining behavior of pure copper foil and pre-pierced stainless steel sheet in laser shock clinching was investigated. A finite element model was established to analyze the material flowing and clinching behavior of metal foils under multiple laser pulses. Based on the validated model, the deformation stages, thickness change, and shock wave propagation features were studied. The temperature rise during clinching was assessed considering both the compression by shock wave and plastic deformation at high strain rates. It is revealed that the laser shock clinching process can be divided into three deformation stages, that is, free bulging forming, radial expansion, and formation of interlock. Both experimental and numerical results prove that the formation of clinched joints relies on the plastic deformation of the joining partner I. The thinnest region of the joint locates at the material of the joining partner I in contact with the upper corner of the joining partner II. In addition, there is no obvious influence of temperature increase on the mechanical properties of joining partners. Moreover, the shock wave propagation characteristics along axial direction and the influence of laser power density on interlock value and thickness distribution were also discussed.

Influence of process parameters on forming quality of Cu-Fe joints by laser shock hole-clinching

Laser shock hole-clinching is a novel mechanical joining process in which the shock wave pressure induced by a high-energy pulsed laser is employed as a punch. This process is quite suitable for the joining of two dissimilar materials with significant differences in plasticity and strength. However, the effect of key process parameters on the forming quality of joints by laser shock hole-clinching is less concerned. In this study, the influence of laser power density, spacer height, number of laser pulse, and initial grain size of metal foils on the forming quality of Cu-Fe joints was experimentally investigated. The characteristics of interlock value and thinning rate distribution under various process parameters were systematically examined. It is revealed that laser shock hole-clinching process can be divided into the confined bulging and interlock formation stages. Based upon the examination of substandard joints, the defects in laser shock hole-clinching process are classified which contains no interlock, nonuniform interlock, bottom and neck fracture. It is found that the thinnest position of joints locates at the region in contact with the upper corner of the joining partner II in which extensive plastic deformation occurs related to the force acting during clinching process. The interlock value increases significantly as the enlargement of initial grain size, whereas the deviation also becomes larger due to the inhomogeneous plastic deformation for the metal foil with coarse grains. Moreover, the ratio of spacer height to the thickness of the joining partner II affects the thinning rate distribution and interlock value of joints. The spacer height equaling to the thickness of the joining partner II is recommended within the given parameters.

Vertical continuous compound casting of copper aluminum bilayer rods

A vertical continuous compound casting technology for the production of rotationally symmetric bilayer rods consisting of brass CuZn37 and aluminum AA7075 was developed. The interface of the metallurgically bonded semi-finished products was composed of four intermetallic layers and an anomalous eutectic area of Al2Cu (θ-AlCu), Al solid solution (α-Al), and other intermetallic precipitates. The intermetallic layers were CuZn (β-CuZn), a suggested two-phase region of Al3Cu5Zn2 (τ-AlCuZn) and Cu5Zn8 (γ-CuZn), Al4Cu3Zn (τ′-AlCuZn), and AlCu (η2-AlCu). The correlation between the process parameters casting speed, cladding temperature, and tool tempering and the interface formation was studied. The experimental investigations were accompanied by a numerical FEM analysis. The casting speed and the cladding temperature were found to be key factors, that influence the formation of a cohesion between the joining partners.

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.

Description of the Boundary Layer Behavior of an Aluminum\u2013Carbon-Fiber-Reinforced Polymer Hybrid Compound Using a Cohesive Zone Model

Compounds of light metals and fiber composites have a large potential in the field of lightweight construction. In order to fully exploit the properties of both materials, joining technology is a major challenge. One of the reasons for this is the electrochemical contact corrosion in these materials. By using a high-temperature resistant thermoplastic (PEEK) as a separating layer between the joining partners, it is possible to produce a material composite using the aluminum die casting process, which exhibits both electrochemical decoupling and high composite strength. The description of the failure behavior of this composite plays an important role in the application of this type of joint to structural components. One of the most widely used methods to describe the failure behavior of a composite using the finite element method is the cohesive zone model. With this model, the initiation of a crack, its evolution and finally the failure of the component can be described by means of a bilinear law. In this paper, a methodology to determine the necessary parameters for fracture mode I using a modified wedge test for the cohesive zone model is described.

Laser transmission welding of absorber-free semi-crystalline polypropylene by using a quasi-simultaneous irradiation strategy

Unlike other joining techniques, laser transmission welding offers unique advantages such as selective and contactless energy deposition. This enables the fabrication of flexible seam geometries at low mechanical and thermal stresses. However, the use of absorbing additives for the lower joining partner such as carbon black is crucial as most polymers are transparent in the spectral range of typical beam sources (800–1100 nm). A novel approach is the application of beam sources emitting radiation within the polymeric intrinsic absorption bands between 1500 and 2000 nm. This enables absorber-free laser welding of transparent polymers for medical or microfluidic applications such as Lab-on-a-Chip devices. The main drawback on the other hand is the large heat affected zone (HAZ) due to the volume absorption which is extending over the entire cross section. A possible way to overcome this disadvantage is a quasi-simultaneous irradiation strategy. It could be proved in the past that the HAZ of polycarbonate (PC) can be reduced in the vertical direction by up to 30% compared with contour welding. Since the effects of light scattering on the absorber-free quasi-simultaneous irradiation strategy are still unknown, the beam propagation was simulated in polypropylene (PP). Based on the results, a thermal simulation of the welding process was carried out using the finite element method (FEM). The simulation model was then evaluated by comparing the results with experimental trials.

Casting methods for the production of rotationally symmetric copper bimetals

ABSTRACT Multiple-material products are characterised by a complex property profile which is achieved by combining the particular advantages of at least two different materials. Bimetal casting is an energy- and material-efficient technology for the production of multi-metallic objects. This paper describes the development of a semi-continuous casting process for the formation of a rotationally symmetric bimetal with a cohesive bonding character at the interface of a copper–tin alloy (CuSn6) and pure copper (Cu99.5). Initial experiments are conducted by static casting to evaluate the thermal process window. Based on the results of the initial experiments, a vertical semi-continuous compound casting process is developed. A stable cohesive bond between the joining partners is accomplished by forming a solid solution at the interface. This paper is part of a Thematic Issue on Copper and its Alloys.

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.

Joining by die-less hydroforming with outer pressurization

Joining by hydraulic expansion is a well-investigated process to create force- and form-fit joints. In order to extend the application possibilities of this technology, the process of joining by die-less hydroforming with outer pressurization is developed. In this process, the force-fit is a result of an elastic springback restriction of the inner joining partner, caused by an elastic-plastic compression of the outer joining partner. The influence of the chosen materials on the establishment of a force-fit connection is investigated both numerically and experimentally. In this context, the ratio of the maximum elastic strains λ is introduced. For values of λ close to 1, the interference pressure is rather a result of the difference in the hardening gradient than of a restricted elastic springback. For values of λ close to 0 in turn, the interference pressure is a result of a restricted elastic springback. The obtained numerical data is validated with the experimental results for tube-to-tube connections of the aluminum alloys EN AW-2007 with EN AW-6060 and EN AW-6060 O, respectively.

Experimental Analysis of Interference-Fit Joining of Aluminum Tubes by Electromagnetic Forming

This study investigates the main parameters that influence the strength of tubular component joints fabricated by electromagnetic compression, more specifically, interference fit by crimping tubes into mandrels. The mechanical parameters are the residual stress on the mandrel, the joining area, and the interfacial friction coefficient between the tube and mandrel. These parameters have a considerably complex relationship with electromagnetic compression process parameters, starting with the charging energy and distribution of magnetic pressure pulse that trigger the forming process, the initial gap between joining partners, the material components and geometry, and the shape and surface roughness of mandrel. This work investigates the interference-fit joining of AA6082-O aluminum alloy tubes with mandrels made of different materials (AISI 1045, AA6082-O, and AA6082-T6) and fabricated under different metallurgical conditions. The process is gradually analyzed by systematically considering the influence of other process parameters, such as the initial gap between joining partners, energy pulse, field shaper geometry and its resulting magnetic pressure, and mandrel material and surface roughness in order to fabricate joints with maximum strength. The derived results indicate, apart from the initial gap, the importance of the mandrel surface roughness and the field shaper geometry on the joint strength and its associated failure mechanism. These are presumed to be important contributions to the development and industrial implementation of EMF technology.

Investigation of reactive bimetallic Ni-Al particles as a heat source for microwave-assisted joining

Abstract Thermal joining of materials with dissimilar thermo-physical properties requires an efficient and adaptable energy input that does not lead to thermal damage of the joining partners. Reactive particles provide a promising solution for this purpose as they represent a tailorable heat source by being able to undergo an exothermic, self-sustaining reaction. A sophisticated method of initiating the reaction is the use of microwaves, since they allow high heating rates, volumetric heating, and may influence the reaction even after the activation. In this way, particles consisting of nickel and aluminium can generate temperatures up to 1550 K within a few hundred milliseconds. However, high reaction rates, elevated temperatures, and still not fully understood microwave-specific phenomena complicate the derivation of cause-effect relationships. Therefore, an experimental set-up was developed, in which the reactive particles can be activated either in the predominant magnetic or electric field due to a customised resonant cavity. Elaborate high-temperature and several power measurement methods were implemented to characterise the activation and reaction behaviour of particles with a lamellar intrinsic structure. Both the electromagnetic field and the particle size influenced the resulting temperature-time profiles and arcing phenomena. In order to demonstrate the potential of reactive particles, they were also used as a new heat source in microwave-assisted adhesive bonding. An empirical study showed that microwaves and reactive particles interacted in an advantageous way with epoxy resins, promoted the cross-linking of the polymers, and allowed the fabrication of polypropylene joints in the microwave cavity.

Production of aluminum AA7075/6060 compounds by die casting and hot extrusion

Abstract The process chain of die casting and hot extrusion for the fabrication of cohesively bonded aluminum bilayer billets was investigated. It represents an energy- and material-efficient link between casting and forming technologies for the production of load-adjusted lightweight constructions. Commercial aluminum AA7075 and AA6060 were initially jointed by die casting. The influence of various process parameters, such as die temperature and cooling conditions, on the interface formation was analyzed. Remelting and recrystallization processes as well as the diffusion-driven interaction between the joining partners were identified as major bonding mechanisms. As-cast bimetal billets were subsequently post-processed using hot extrusion. The present paper shows the interface evolution over the process chain of casting and massive forming.

Influence of the Structuring-Tool Geometry on Form-Fit Joining by Use of Pin-Like Structures in Vibration Welding

Joining often assumes a central position in the manufacturing process. Vibration welding is characterized by its process flexibility, its high energy-efficiency as well as its short cycle times. Polymer welding processes, however, are restricted due to the necessary adhesion compatibility of the joining partners. Therefore, the industrial application of welding is strongly limited for multi-material joints. Generating a form-fit connection by means of the vibration welding process enables an innovative approach for joining adhesion incompatible polymers. For this purpose, pin-like structures are embedded into at least one joining partner using a structuring-tool. Subsequently, these structures are filled by the further joining partner and a form-fit connection is achieved.Within these investigations, the influence of the structuring-tool geometry on the pin-like structures and the resulting form-fit connection is investigated. Tools with similar undercut volumes but different pin geometries (mushroom-like, trapezoidal) were used. Polyamide 66 (PA66) was used as structured-partner and polypropylene (PP) as well as polymethylmethacrylate (PMMA) as bonding-partner. In addition to the resulting shape of the pin-like structures, the mechanical resistance and fracture behavior of the multi-material joint were analyzed depending on the tool geometry and the bonding-partner. The results show that the selected tool geometry has only a minor influence on the maximum achievable force. The fracture behavior of the multi-material combination PA66-PMMA, however, is mainly determined by the structuring-tool geometry. While trapezoidal structures show predominantly adhesive failure, mushroom-like structures show cohesive failure due to fracture of one joining partner. The combination of PA66-PP, show a predominantly adhesive failure for all tool geometries. Thus, the investigations revealed that, in addition to the structuring parameters, the structuring-tool must also be adapted to the selected material combination.

Investigation of the melt dynamics during the laser impulse metal bonding process with keyhole measurement

The laser welding process requires a comparable aspect ratio between the material thicknesses of joining partners due to the penetration depth and the high energy absorption. The high energy absorption during the joining process is commonly not tolerated for the sensitive PCB-substrates below the joining partners. However, the Laser Impulse Metal Bonding (LIMBO) process enables a laser based weld joint between an interconnector and a metallization on sensitive substrate. The main approach is the spatial separation of both joining partner combined with the temporal beam modulation for the energetic separation between the melting and the joining phase. Consequently, a low penetration depth is achieved and the laser welding process is applicable for the interconnection and packaging of power electronic devices. In this paper the keyhole depth during the LIMBO process is investigated with an optical coherence tomography. Subsequently, a correlation between the keyhole depth and penetration depth during the LIMBO process is further investigated.

New cross-rolling process for joining of hybrid components

The proposed innovative cross-rolling process can be used to form hybrid components like shaft hub connections and electric drive parts. The advantages as well as the challenges are discussed in detail, especially the resulting notch geometry and possible approaches for mitigating adverse effects. While the shaft ideally should be a high strength steel, generally any other metal, ceramic or composite material can be used as a joining partner. This paper will present a numerical analysis of the joining process regarding work roll path and geometry as well as an experimental evaluation of the achievable properties and the resulting transmissible loads.