Thermal Joining Processes Research Articles

Investigations to improve the tool life during thermomechanical and incremental forming of steel auxiliary joining elements

Modern vehicles as well as structures consist more and more often of a multi material mix. In addition to the use of light materials (e. g. aluminium) to optimise lightweight construction potential, higher strength alloys are being used more and more frequently. With the different material combinations, the requirements for the joining technology also increase. Thus (due to restrictions in the area of thermal joining processes) the joining of different material combinations is often realised by mechanical joining technology. Even though different joining tasks (dissimilar material or sheet thicknesses) can be solved by mechanical joining technology in general, it is often required to switch between appropriate auxiliary joining elements and even joining technique. To adapt the auxiliary elements to the individual requirements of each joint an innovative manufacturing process of customisable auxiliary joining elements has been developed. The use of the incremental, thermomechanical manufacturing process called friction-spinning for the manufacturing of adaptive auxiliary joining elements enables to individualise each element for the subsequent joining process. However, this advantage comes at the cost of a high mechanical and thermal loads on the forming tools. These loads also depend on the process design (trajectory, feed, rotational speed) and have to be controlled in order to improve the tool life. For a more profound process knowledge and to better understand the corellation of the individual process variables, a series of tests with varying process parameters were carried out in a partial factorial design of experiments. The results show not only an possible improvement in the tool life, but also the required process design for a higher reproducibility of the auxiliary joining element geometry and the possibility of positively influencing the geometry to achieve a force-closure of the joint.

Influence of the application of ultrasound on the microstructure of cemented carbide/steel joints brazed with Ag49ZnCuMnNi

The poor wettability of cemented carbides by molten metals as well as the different material properties compared to steel makes thermal joining process challenging. In this regard, innovative ultrasonic-assisted induction brazing is capable of joining the materials quickly, cost-effectively and without using environmentally hazardous fluxes. Instead, the oxide scales are broken up within the joining process due to ultrasonic-induced cavitation in the molten filler alloy and hence promote the wetting. However, since the use of ultrasound in brazing steel/cemented carbide joints is still insufficiently investigated, there is still a need to explore the fundamental influences of ultrasound on the brazing process. In this work, the effect of ultrasound on the microstructure of the brazed joints and the diffusion depth of different elements were analysed. The two ultrasonic parameters activation time and amplitude were varied during the experiments. Acicular or dendritic structures that have formed in the braze metal without ultrasonic stimulation were transformed into globular structures, when ultrasound was applied. In addition, a non-eutectic silver-rich phase formed, which was not observed in joints manufactured without the use of ultrasound. Furthermore, the use of ultrasound led to increased diffusion between the base material and the filler material as well as the dissolution and distribution of oxides in the edge regions of the brazing joints.

Hybrid joining of aluminum alloys and thermoplastic CFR‐PA12 manufactured by IR‐irradiation: Influence of pretreatment and aging on the mechanical properties

AbstractIn this work, a thermal direct joining process was applied to obtain hybrid composites of thermoplastic carbon fiber compounds and selected aluminum alloys. With the successful proof‐of‐concept, the aging behavior under hydrothermal and corrosive conditions is also investigated. Before joining, various linear surface topographies with a characteristic aspect ratio ranging from 0.1 to 2.0 were generated on the aluminum using a short‐pulsed near‐infrared (NIR) laser source. By using optimized NIR laser parameters, the achieved shear strength under tensile load can reach the maximum value corresponding to the interlaminar shear strength of the used carbon fiber‐reinforced thermoplastic polymer. In two aging regimes under hydrothermal and corrosive conditions, a rapid and sharp decrease in mechanical strength over time was observed exclusively under corrosive conditions.

Determining the properties of multi-range semi-tubular self-piercing riveted joints

By applying a variety of strategies, including both the development of new drive concepts as well as the use of lightweight constructions, it is intended to meet the climate targets set by the Paris Agreement. Multi-material design is often used when applying lightweight constructions, especially in the mobility sector. Herby, various materials with different properties are combined to well adapt the structure to the application of force in order to reduce weight. For example, this can result in the combined use of (high-strength) steel and aluminium. However, the increasing number of materials used and the number of resulting joints has led to the development of a number of different joining processes. These can only be used to a limited extent for joining multi-material joints and are usually inflexible facing changing boundary conditions. Examples of further difficulties affecting the established processes are metallurgical incompatibilities, which particularly pose difficulties for the use of thermal joining processes. A frequently used mechanical joining process is self-piercing riveting. Due to its high load-bearing capacities, a wide range of application and high process robustness it is often used when joining of dissimilar material is required. However, in case of self-piercing riveting used, the increasing number of multi-material joints and material-thickness combinations leads to the need of a large number of rivet-die-combinations to adapt the joining process to the respective joining task. Since the joining system cannot react to these changes, a tool change or an adjustment of the system is necessary, which leads to a reduction in efficiency and extended process times. To increase flexibility and process efficiency, new, versatile joining technologies are needed that can be adapted to changing boundary conditions. One possibility for this is the use of multi-range capable semi-tubular self-piercing rivets, which are inserted into the joint by using a new joining system with extended punch-sided actuator technology. The increased actuator technology enables the rivet to be set by an inner punch and subsequently to form a rivet head by embossing with an outer punch. All punch movements can be controlled independently of each other, enabling adaptive adjustment of the process parameters. Depending on the rivet geometry used, rivet head formation by the outer punch can be performed both with and without head deformation. The rivet without head deformation consists of a tubular shape with ring grooves in the rivet head area. Using the outer punch, punch-sided material is formed into the ring grooves creating an interlock in the head area of the rivet. The rivet with head deformation is designed differently. It is modified to enable subsequent forming to the respective thickness of the joint by forming the protrusion of the rivet head onto the punch-sided joining part. In the study presented here, the joining process of the versatile self-piercing riveting is presented, analysed and the property profile of the joints is determined on the basis of various material-thickness combinations. Here, both the characteristic parameters of the joints and the joint load-bearing capacities are determined. Finally the property profiles are compared with conventionally manufactured joints in order to identify potential for improvement.

Numerical and experimental investigation of flat-clinch joint strength

The striving for energy savings by lightweight construction requires the combination of different materials with advantageous properties. For joining sheet metal components, clinching offers a good alternative to thermal joining processes. In contrast to thermal joining processes, the microstructure in the joining zone remains largely unaffected. Conventional clinch joints, however, have a protrusion on the underside of the joint, which restricts their use in functional and visible surfaces. Flat-clinching minimizes this disadvantage by using a flat anvil instead of a die. Due to the flatness on the underside, it can be used in visible and functional surfaces. This paper deals with the increase of joint strength by using an auxiliary joining element (AJE) in the second forming stage. To achieve optimum improvement in the joint strength of an aluminum Al99.5 H14 sheet metal joint and to save costs, the AJE was varied numerically in terms of volume, material and basic shape. The geometric parameters (e.g., interlocking f and neck thickness tn) do not allow direct derivation of the joint strength. For this reason, the 2D clinch model was extended for the first time to include 3D load models (cross tension, shear tension). To validate the numerical results, optimized flat-clinch joints with AJE and the associated load tests were implemented experimentally. The numerical models were used to improve the process development.

Data-driven analysis of cold-formed pin structure characteristics in the context of versatile joining processes

Due to increasingly strict emission targets and regulatory requirements, especially for companies in the transport industry, the demand for multi-material-systems is continuously rising in order to lower energy consumption. In this context, mechanical joining processes offer an environmentally friendly and flexible alternative to established joining methods, especially in the field of lightweight design. For example, cold-formed cylindrical pin structures show high potentials in joining multi-material-systems without auxiliary elements. The pin structures are joined either by pressing them directly into the joining partner or by caulking with a pre-punched part. However, to evaluate the strength of the joint and to ensure the joining reliability for versatile processes, such as changing joining partners or batch variations, engineering designers currently have only limited design principles available compared to thermal joining processes. Consequently, the design of an optimal pin joint requires cost- and time-intensive experimental investigations and adjustments to design or process parameters. As a solution, data-driven methods offer procedures for structuring data and identifying dependencies between varying process parameters and resulting pin structure characteristics. Motivated by this, the paper presents an approach for the data-driven analysis of cold-formed pin structures and offers a deeper understanding of how versatile processes affect the pin characteristics. Therefore, the application of an intelligent design of experiment in combination with several machine learning methods enable the setup of a best-fitting meta-model. Resulting, the determination of a mathematical model provides the opportunity to accurately estimate the pin height considering only relevant geometrical and process parameters with a prediction quality of 95 %.

Extension of the Conventional Press Hardening Process by Local Material Influence to Improve Joining Ability

Press hardened structural components are a key factor in lightweight car design and thus in reducing vehicle mass while increasing crash safety. The use of quenched 22MnB5 (Usibor 1500) has been established in hot sheet forming for the production of safety-relevant car body components. In order to expand the field of application for press-hardened components, a process-reliable joining technique is essential. Ultra-high-strength components can be joined with other parts in car bodies using the resistance spot welding process. Here, challenges like uneven welding lens formations with an incorrect connection in multi-sheet joints arise. Mechanical joining processes, for example self-pierce riveting, can only be used to a limited extent due to the high hardness of the hardened parts. For this purpose, an annealing treatment is often carried out in order to reduce the strength of the material after press hardening. Another possibility to create softened areas is the introduction of local deformation in the austenitic material. The phase areas in the continuous cooling transformation diagram are shifted to shorter cooling times, which enables the development of deformation-induced ferrite. The local thinning and softening improves joinability by means of mechanical and thermal joining processes but increases the forming force. Therefore, in this study the process of hot forming and local deformation is first pre-estimated using numerical simulation. The required force for the deformation and an optimal positioning, as well as the possible number of deformation punches, are investigated. Furthermore, the first experimental results of the feasibility of locally thinned and softened sheets are presented. In addition, joining tests by resistance spot welding and self-pierce riveting are carried out on the generated specimen to illustrate the practical effectiveness of the local thinning and the use of deformation-induced ferrite for critical joints.

Metal Polymer Connections: Laser-Induced Surface Enlargement Increases Joint Strength

The trend towards hybrid materials consisting of metals and polymers is strongly driven by requirements such as weight reduction and improved functionality. A crucial step towards new hybrid materials is the advancement of joining technology. While metals and polymers are currently often joined using adhesives, this technology has major drawbacks such as long process cycle time, low robustness with regard to changes of the material composition, and are almost impossible to recycle, a point which is becoming increasingly important. A promising joining method is thermal direct joining of metals and thermoplastic polymers due to its fast cycle time, its robustness of the process and the absence of duroplastic adhesives. Direct joining requires surface treatment of the metallic joining partner prior to the thermal joining process to achieve a sufficiently high contact strength. It is known that laser-induced topologies on the metal surface are beneficial with regard to contact strength of the joints. Designing a joint based on laser structured metals requires a fundamental understanding of the interface interactions. The present paper focuses on the influence of surface enlargement on the joint strength. Laser pretreatment was utilized to generate surface structures with specific surface enlargement on the metallic joining partner. The pretreated metallic parts were subsequently joined with a thermoplastic polymer by injection molding. The joint strength was investigated in single lap shear tests. The key finding is that the contact strength increases almost linearly with the surface area within the tested parameter range, while the structure geometry parameters have only a minor influence. This may help as a design guideline for future adhesive-free hybrid material joining technologies.

A Sustainable Hybrid Inductive Joining Technology for Aluminum and Composites

Abstract Sustainable manufacturing technologies as well as lightweight and multi-material design in production have become key factors for success in industrial sectors such as transportation, automotive production or aerospace. Therefore, an automatable hybrid joining technology is required with a high industrial output. Mechanical, chemical and thermal technologies can be applied to join fiber-reinforced thermoplastics (FRP) with metals. The advantage of induction heating as a thermal joining process is the immediate generation of heat inside the electrically conductive material at the joining surface. Due to the direct conversion of electricity into heat the joining process can be emission-free if the energy is produced by regenerative sources. However, the distance between coil and joining partners, the transport of the workpiece to the pressing tool and the subsequent separate cooling in conventional inductive joining methods reported in the literature result in power losses and extended process times. The unique new hybrid inductive contact joining (ICJ) process is characterized by the integration of heating, pressing and cooling into one hybrid process step which is performed by one tool. The conducted experiments and finite element simulations provide data for the main process parameters during joining, joint strength as well as the impact of surface preparations and demonstrate the advantages of the ICJ process in comparison with conventional thermal joining processes.

Combined deep drawing and fusion bonding of structural FRP-metal hybrid parts

Abstract This research describes the development of a combined process of forming and thermal joining without adhesives of high strength steels and organosheets. In order to achieve cycle times below 20 s the temperature of the forming tool remains constant at 110 °C throughout the process. Thus, the removal of the component can take place immediately after the forming process without cooling of the forming tool. The thermal energy required to assure the thermal joining process at 280 °C at the beginning of the intermediate forming step is applied by means of heating the steel in a furnace before the transfer to the forming tool. By choosing certain coating depended heat treatment parameters during the heating step, galvannealed rough surfaces of the steel blanks can be generated in order to increase the joint strength. The paper includes a surface analysis and lap-shear testing results to quantify the respective joint strength to be achieved with the proposed surface treatment. In addition tensile test results of the heated DP800 steel to characterize the thermal influence on the mechanical properties of the heated high-strength steels will be shown. Furthermore the paper presents a tool concept with a graded blank holder to perform stretching of the metal on the one hand and draping of the organosheet on the other hand as well as preliminary forming results.

Laser-based plastic-metal-joining with self-organizing microstructures considering different load directions

Innovative multi-material lightweight construction enables reducing dead weight while maintaining and preferably boosting the components' performance. The implementation of multi-material parts (e.g., plastic-metal-components) requires reliable joining processes since a direct connection between these materials is not feasible due to their different physical and chemical properties. In order to avoid additional weight through adhesive bonding, riveting or fasteners, thermal direct joining with a modified metal surface is a promising approach. Within a first process step, the metal surface is modified by laser-microstructuring. To enlarge the boundary surface and create undercut structures, random self-organizing micro- and nanostructures are generated with ultrashort pulsed laser radiation on stainless steel samples. In the subsequent direct thermal joining process, both joining partners are clamped together. The metal is heated up with diode laser radiation, and through heat conduction, the polymer melts and flows into the generated cavities. After cooling-down, a firm joint between both materials is created, which is based on mechanical interlocking and increased specific adhesion between the joining partners. The mechanical strength of the joint depends strongly on the load direction. In the presented contribution, the strength of the joint between stainless steel and glass-fiber-reinforced and non-reinforced thermoplastics (PP) is investigated for three different load directions (tensile shear, tensile and peel).

Polygon scanning system for high-power, high-speed microstructuring

A key requirement in automotive industry is reducing weight through innovative lightweight construction concepts. For multi-material construction the manufacture of plastic-metal hybrids poses a particular challenge due to the chemical and physical dissimilarity. At Fraunhofer ILT, a process chain has been developed for connecting thermoplastics with metals. First, the metal surface is microstructured with laser radiation to create undercut structures for the interlocking of polymer and metal, which are connected afterwards via a thermal joining process. Although laser-microstructuring is already a fast process, for automotive cycle times higher processing speeds are required. In this contribution a new polygon-based scanning system is combined with a 2 kW single mode fiber laser for a continuous high-speed microstructuring process of metals as pretreatment for local UD-tape-reinforcements. The scanner setup and first microstructuring results on different metals are presented.

Combustion synthesis of reactive nickel-aluminum particles as an innovative approach for thermal joining applications

Reactive systems, which are widely used in combustion synthesis, represent a promising solution for challenging joining tasks. They are able to undergo a self-sustaining, highly exothermic reaction when exposed to an external energy source. Reactive foils are the only systems that are currently commercially available. However, their industrial use is limited due to the brittle nature of the material and the restriction to planar geometries. Reactive particles represent a more flexible format, but are currently not commercially available. Therefore, a two-step electroless plating process has been developed to synthesize nickel-aluminum core-shell structures. These structures function as microreactors, which provide the energy for the thermal joining process. Ignition tests with electromagnetic waves were performed in order to investigate the overall reactivity. Energy input and particle size significantly influence the activation and the reaction behavior of the core-shell structures. Furthermore, a general approach to use reactive particles as a heat source in joining applications is proposed.

Optimization of laser welding of tri – metal joint via response surface methodology

 Laser welding input parameters play a major role in determining the quality of a weld joint. In the nuclear power plants, hybrid structures of nickel and steel alloys offer an advantage in comparison to conventional materials, e.g. in heat exchanger tube areas. Due to demand in the nuclear industry for new material combinations based on commercially available and qualified materials, research into thermal joining of dissimilar materials has been initiated. The use of laser for joining mild steel / nickel with 316L austenitic stainless steel filler material and structures offers some advantages compared with usual thermal joining processes. The main aim is the control of phase formation, which occurs during thermal joining of mild steel to nickel. In this research work microstructure study and optimization of laser welding of mild steel / nickel sheets with wire feeding was done using Central Composite Design(CCD) and Response Surface Methodology (RSM) are used to build the mathematical model. By means of the laser power, welding speed and pulse width on the tensile strength model was developed and tested by analysis of variance method (ANOVA), the relationship between process parameters and output response and interaction among the process parameters are analyzed and discussed in detail. The scanning electron microscopes (SEM) with energy dispersive X-ray spectroscopy (EDS) technique were used for microstructure study of the bi-metal and tri-metal joints of the weld.

Resource-efficient development of thermally highly resistant engine components of hybrid metal composites – experiments and numerical analysis

Abstract Requirements for protection of environment and climate, increasing energy cost and security demands form the basis for research activities in the maritime sector. Within the scope of the joint project »INKOV – development of innovative piston and valve solutions for composites in ship engines« metallic composites are developed and investigated. Their application in large engines powered by heavy fuel oil shall reduce nitrogen oxide emissions. The investigations of the publicly funded research project intend to generate high-strength components made of a hybrid composite material while simultaneously increasing resource efficiency. The focus of the research and development activities lies on components for large engines which are subject to extremely high dynamic, thermal and corrosive loads. Using conventional heat-resistant and wear-resistant steels, the wear related to the conditions of the application site can hardly be controlled. High-performance alternatives are necessary. The goal of the approach is to specifically apply a nickel-base alloy to a locally limited area by using a selected thermal joining process. Thus, a layer composite is to be produced, which corresponds to the solid material of a nickel-base alloy regarding its properties of thermal resistance and corrosion resistance. Currently, criteria for excluding the integration of these materials include high prices as well as their partly limited availability despite of their reported suitability for high-temperature applications. Using hybrid materials should open up their considerable potential in terms of increasing strength and high-temperature resistance while simultaneously reducing wear-related downtimes to a minimum. An aim of the project is to implement forming processes of hybrid composites by means of simulation as a basis for optimizations in terms of joining materials, joining processes or geometrical dimensions. Furthermore, the basic behavior of hybrid materials and their interface shall be examined in hot bulk metal forming by means of the generated FEM models.

Method for efficient determination of material properties for modelling aluminium welding processes

The trend in the automotive and aircraft industry is moving towards customized designs by using compound structures, which provide an enlarged potential for weight and cost savings. Particularly, thermal joined lightweight structures made of aluminium alloys are characterized by moderate material and processing costs. However, the short and locally focused heat input during welding leads to a degradation of material properties, particularly strength in the heat affected zone (HAZ) of the weld. The need of minimizing of rejections and rework leads to high demands on the welding process. This often requires time‐ and cost‐intensive part testing including accurate determination of the mechanical properties after welding. The prediction of the mechanical properties of welded joints, by means of welding simulation, enables a significant reduction of this time‐ and cost‐intensive testing effort. However, the application of such a model requires a sufficient base of material data, which has usually to be generated by an extensive testing procedure for each single alloy. Against this background, a new method for an efficient determination of material properties for the simulation of thermal joining processes of age‐hardenable aluminium alloys was developed within the programme of the research project.

Process-Integrated Projection Welding during Deep Drawing

Within deep drawing processes, welding represents an innovative approach to optimising the branched process chains which entail uneconomic process steps in production and transport lines. Previous applications of thermal joining processes in presses required a downstream process step for joining standardised functional elements such as nuts. Within the scope of this publication, a weldable tool system is presented which offers the possibility of welding a deep drawing component to an automatically added non-standardised holder in a single-step deep drawing process without additional dwell time in the bottom dead point. In order to realise this innovative tool system, the interdependencies of deep drawing and projection welding are considered to enable a splash-free welding on flat and curved component areas, such as the rounding of a punch edge. Based on experimental research a special concept for the tool kinematics of welding electronics is drawn up which is based on press kinematics. In addition, this article also deals with electric insulation and the forming forces which have an impact on the welding electrodes integrated into the active surface of the forming tool. Thus, the joining process becomes independent from the type of press.

Tolerances and Measuring Strategies in the Virtual Process Chain for Spot Welded Structures; Substitute Modeling and Automating the Calibration Procedure

This paper presents a method with whose help it is possible, to quickly and precisely predict the influence that thermal spot-shaped joining processes has on the dimensional stability of complex component structures even in early planning phase. The welding distortion is calculated in the context of reduced computing time, based upon an experimentally calibrated mechanical substitute model. This expands existing approaches of substitute models and defines both an experimental and numerical procedure for creating adequate calibration samples. In turn, this makes use of the potential obt ained for standardizing the experimental basis for calculating and modelling the distortion to automatically carry out painstaking calibration processes in simulations and experiments in future based upon mathematical model functions. Finally, the limits to applying the substitute spot welding model are verified with reference to its predictability using a complex joining situation of a car body construction.

Magnetic Pulse Welding by Electromagnetic Compression: Determination of the Impact Velocity

The implementation of multi-material concepts and the manufacturing of modern lightweight structures, for example in automotive engineering, require appropriate joining technologies. The ability to join dissimilar materials without additional mechanical elements, chemical binders, or adverse influences of heat on the joining partners is key in reaching the desired weight reduction in engineering structures. The Magnetic Pulse Welding (MPW) process meets these demands, making it a viable alternative to conventional thermal welding and mechanical joining processes. The present paper focuses on the analytical determination of the impact velocity as one of the key parameters of MPW processes. On the basis of experimentally recorded data concerning the course of the discharge current and geometrical parameters of the welding setup, the respective velocity is determined. A comparison with measurement data gained by Photon Doppler Velocimetry is performed.

A Method for the Prediction of Process Parameters for Minimal Distortion in Welded Frame Structures Using a FE-simulation

Welded frame structures are often subject to unintended distortions due to the thermal joining process. In order to precisely quantify and reduce the distortion of welded frame structures using Finite Element (FE) simulation, a fast and reliable method is required, especially for industrial applications. This paper presents a methodical, simulation based and time optimised framework for the prediction of appropriate process parameters for minimal component distortion of complex welded frame structures by means of e.g. a variation of the process parameters or the weld seam sequences. To achieve a minimal distortion of the final structure, different optimisation algorithms will be used in combination with a database.