Potential of Joining Dissimilar Materials by Cold Formed Pin-Structures

https://doi.org/10.1016/j.jmatprotec.2020.116697Get rights and content

Highlights

  • Basic research on the joining of dissimilar metals via cold formed pin-structures.

  • Cold formed pin-structures are suitable to join dissimilar metals.

  • Caulking enables higher joint strengths compared to direct pin-pressing.

  • Significant increase of joint strength by using multi-pin-structures.

  • Inhomogeneous material flow complicates the forming of multi-pin-structures.

Abstract

Joining dissimilar materials brings established joining techniques to their limits due to different thermal expansion, unequal stiffness/strength and chemical incompatibilities. The use of pin-structures has proven an appropriate strategy to join multi-material-systems in laboratory scale. At present, the industrial application of this joining technology is limited due to the time-consuming pin manufacturing process and the inadequately researched achievable joint strengths. In this study, extrusion of the pin-structures from sheet metal is used to investigate an approach that can be directly integrated into existing production processes and does not require auxiliary joining elements. With direct-pin-pressing and caulking, two different joining strategies are analysed to join multi-material systems of sheet steel (DC04, t0 = 2 mm) and aluminium (EN AW 6016-T4, t0 = 2 mm). The basic feasibility of the joining technique is experimentally proven by using single-pin-structures. The joint strength is analysed by tensile shear tests, micro sections and failure patterns. Based on the experimental results, a failure criterion for the joint is defined. Parallel to the experiments, numerical finite element models (FE-models) for the single-pin-joint are developed and validated. In a further step, numerical investigations for multi-pin-structures are carried out in order to derive predictions on achievable joint strengths with this joining technique. It is shown that the form-fit is crucial for the joint strength. Thus, 93 % higher shear strength is achieved for caulked joints. By multi-pin-structures the strength can be increased considerably. For a 5 × 3 pin-structure, shear strength of 9.9 kN can be achieved.

Introduction

The reduction of fuel consumption and greenhouse emissions are key challenges in the transport sector. Especially the automotive industry is confronted with increasingly stricter CO2 targets from politics and customers. By lowering the vehicle mass, considerable fuel savings can be achieved. Thus, intelligent lightweight construction is in the focus of current research. The aim is to use the right material in the right place in terms of function, safety, life time, production and costs. This results in a variant-rich multi-material mix of high-strength steel, aluminium alloys and endless fibre-reinforced plastic composites (EFRPC). This trend is a major challenge for the joining technology. To join similar materials, a variety of different joining technologies such as welding, brazing, gluing, screwing or clinching can be used. But the established joining techniques are limited with regard to the joinability of hybrid compounds made of different metals or metals and composites due to different thermal expansion, unequal stiffness/strength and chemical incompatibilities. Furthermore, additional auxiliary joining parts and/or substances are required which lead to an increase of the weight, production time and complicate both a media-tight connection and corrosion protection. Thus there is a great demand of high-strength, versatile joining techniques. The use of pin-structures has proven an appropriate strategy to join multi-material-systems in laboratory scale (Eberl et al., 2017).

The incompatibility of materials plays a minor role for joining techniques based on form closure and/or force fit, which makes them advantageous to join multi-material-systems in terms of a fast production and easy disassembly. Numerous studies have been published for joining dissimilar materials using pin-structures, especially for hybrid metal-EFRPC components. The pin-structures are inserted into the EFRPC component during fabrication or moulding. For this purpose, dry (Smith, 2005) or thermoset (Parkers et al., 2014), or thermoplastic (Thakkar and Ucsnik, 2014) pre-impregnated semi-finished fibre fabrics are draped on, or pressed into the pin-structures. The pin-structures penetrate into, or between the fibre bundles, which results in a formfit. Dry semi-finished fibre fabrics are then infiltrated with a thermosetting plastic and subsequently cured. For pre-impregnated fibre semi-finished products, the curing process follows the pin insertion when using a thermosetting plastic or the reconsolidation and cooling process when using thermoplastics. On double-shear shear test specimens, significant increases in the maximum transferable force of up to 650 % (Parkers et al., 2014) and energy absorption of up to 3000 % (Ucsnik et al., 2010) were observed in the shear test by using pinstructures compared to pinless references. Eberl et al. (2017) demonstrated that a rearrangement of the fibres around pins during the joining process results in an up to 31.8 % higher joint strength compared to pins inserted after fibre-destructive drilling. Furthermore, a more progressive failure, i.e. no sudden failure of the joint, has been observed by Smith (2005) for samples with pin-structures compared to the pinless reference. Plettke et al. (2014) developed a new process chain for joining hybrid metal-EFRPC components with thermoplastic matrix using a three-stage joining concept. In this process chain, the pin-structures were built up additively on the metallic joining partner by laser beam melting (LBM). The upper part of the pin-structures was conically shaped at a 45° angle in order to achieve a fibre-friendly penetration into the EFRPC. Additionally, a hollow cross section was created in the upper part of the pin-structure, which serves as a weak point during the joining process in order to achieve a wider pin head. This increases the strength of the joint and prevents the joining partners from detaching. The insertion of the pin-structures into the thermoplastic EFRPC was carried out by a local thermal support. Due to the local melting of the thermoplastic matrix during pin insertion, the fibres can move freely which enables reorientation around the pin-structures (Kraus et al., 2019a). Finally, the joint was caulked to realize an undercut over the EFRPC. In tensile shear tests forces of over 2000 N were measured. Thus, the principal joinability of loadable metal-EFRPC joints with thermoplastic matrix was demonstrated.

The joining via pin-structures has besides the joining of metal-EFRPC-systems also a big potential for joining dissimilar metals. Kraus et al. (2019a) has shown that it is possible to join steel with aluminium by direct pressing in an unperforated or caulking with a pre-drilled joining partner using cold formed pin-structures. In the single lap shear test, a shear force of 702 N was achieved for caulked joints and 363 N for directly pressed joints using single-pin-structures. At present there are no further systematic studies for joining dissimilar metals with pin-structures. Comparable approaches for joining without pre-punching with a pin-like auxiliary joining part can be found in the building industry for tack-setting (Kuhlmann, 2011) or in car body fabrication by high-speed joining (Draht, 2006). The significant process characteristic is the insertion of the auxiliary joining part at high speed. As a result, the kinetic energy is converted into heat due to the plastic deformation of the joining partners, which facilitates the penetration of the tack. The connection is force-fit by the joint pressure which locks the auxiliary joining part and possibly also adhesive bonded, since cold welding may occur (Kuhlmann, 2011). Depending on the design of the auxiliary joining part, in particular if it has a pattern, microform-fit occur which increase the joint strength. A variation of the process, known under the product name "Metal Tack", relies on the formation of a form-fit between the auxiliary joining part and the material accumulated around the penetration area. The material is displaced into an undercut of a circular groove. However, this requires a direct contact between the head area and the joining partner (MetalTack, 2019).

The pin manufacturing is a central challenge for joining with metallic pin-structures. Numerous manufacturing technologies can be used for this purpose. In current studies, the pin-structures are fabricated by metal powder injection moulding (MIM), wire arc additive manufacturing (WAAM) e.g. cold metal transfer (CMT), arc percussive micro-welding (APMW) or additively built by powder-bed-based processes. A high material utilization, a wide selection of materials and a high reproducibility are the advantages of MIM. Disadvantages are the complex process chain, a reduced component density (>96 %) and a low surface quality. With regard to flexibility of the joining technique, MIM has further disadvantages, since the pins and the joining partner must be cast in one piece (Freistauer et al., 2016). This limits the MIM process to small parts and makes it unsuitable for joining large sheet metal parts. Further limitations exist in the joining strategy. For injection-moulded pin-structures, only direct insertion into a softer joining partner is recommended, since caulking would lead to fractures due to the substantial reduction of breaking elongation during sintering (Schatt et al., 2007). Another technique for the production of pins is the WAAM by using the CMT-process, which has been further developed from conventional MIG/MAG welding. In this process, the welding wire is actively integrated into the welding regulation. This enables a large design flexibility of the pin geometry (Trommer and Nitsch, 2008). Furthermore, the CMT process is suitable for a wide range of different materials. Due to the long production time of approximately three seconds per pin (diameter = 1.2 mm; height = 1.6 mm), an industrial application of this technology is only partially economical (Hopmann et al., 2017). Thus, numerous research projects concentrate on increasing the flexibility, accuracy and efficiency of the process. Kerber et al. (2018) has shown that the pin manufacturing by CMT is possible on angled surfaces, which enables a wide field of applications. Reisgen et al. (2019a), Reisgen et al. (2019b) have proven that optical image processing can be used to control the height of the welding torch and the workpiece. Furthermore, the chemical composition of the molten pool can be modified by varying the wire feed speeds during the process. This allows a locally targeted adjustment of the mechanical properties (Reisgen et al., 2019a; Reisgen et al., 2019b). An even faster pin-structure manufacturing can be achieved by the APMW-process, in which prefabricated, tailored pin-structures are welded onto the joining partner. The welding process per pin-structure takes less than 5 ms (Oluleke et al., 2013). This technique allows the joining of parts from different materials. As a result of the very short welding process, the melting zone develops a compositionally heterogeneous, partially martensitic microstructure, which favours a failure at the weld seam and is therefore to be mentioned as a major disadvantage of the process. Different techniques are used for the powder-bed-based additive manufacturing of joining pin-structures but they all use the same basic principle, the layer-by-layer manufacturing of the pin-structure on substrate by sequential addition of material layers. The metal powder processing allows a free design of the pin geometry without damaging the surface of the joining partner. Two common used techniques are selective laser melting (SLM) and laser metal deposition (LMD). The production of a pin with a diameter of 1 mm and a height of 3 mm takes approximately 10 s with the LMD process (Graham et al., 2014). The additive manufacturing of pin-structures offers numerous advantages due to the design flexibility. The major disadvantage for the industrial use is the long production time.

The pin manufacturing techniques currently used in laboratory scale have a great potential with respect to the design flexibility, but unfortunately they do not meet the requirements of an industrial production in terms of short production times, stress optimized part properties and robust processes. Thus, joining via pins is presently not widely used. For a broad industrial application, the manufacturing process must be capable to produce the pin-structures in the required quality at a high output rate reproducible with an advantageous geometry for the joining process. Furthermore, it is important that the manufacturing process can be integrated into existing manufacturing processes and flexibly adapted to changing requirements. Compared to the processes mentioned above, the manufacturing of pin-structures by cold bulk forming offers technological, economical and ecological advantages (Schneider, 2004). Cold bulk forming is a promising pin-structures manufacturing technology due to the strain hardening, the high surface quality, the energy efficiency and the short cycle times since the entire pin-structure can be formed in one stroke. As the required material is extruded directly from the joining partner, the product mass is reduced compared to other pin manufacturing and joining techniques. The fundamental feasibility for joining multi-material systems with cold formed pin-structures has already been proven on single-pin-structures for joining DC04 with EN AW-6016 and DC04 with GF-PP (Kraus et al., 2019a).

Fig. 1 illustrates the working principle of joining dissimilar materials by cold formed pin-structures. With the cold extrusion of the joining pins from sheet metal (Fig. 1a), a promising pin manufacturing approach is investigated which can be directly integrated into existing sheet metal forming processes. The extruded pins can be subsequently used to join dissimilar materials by caulking (Fig. 1b) or direct pin-pressing (Fig. 1c). For caulking, the choice of material for the joining partner is almost unlimited, as the pin is inserted through a pre-drilled hole. For thermoplastic joining partners, the material can also be melted by local heat supply in the penetration zone, which eliminates the need for pre-drilling. After insertion, the pin head is compressed to achieve a form-fit joint. By direct pin-pressing, the pin is pressed into an unperforated joining partner. Thus, the material of the joining partner must be softer than the pin. During the insertion, the pin is compressed, which enables the forming of an undercut. By a suitable selection of the process and material parameters, a form-fit joint can also be realized this way (Kraus et al., 2019a).

Within this paper, the fundamental joinability of two dissimilar metals by using pin-structures is analysed. Fig. 2 shows the methodological approach of this work. Initially, the pin extrusion process and the joining processes, by direct pressing into an unperforated joining partner and caulking with a perforated joining partner, are investigated experimentally by using single-pin-structures. The joint strength is analysed qualitatively by micrographs and quantitatively by tensile shear tests according to DIN EN ISO 12996 (2013). Parallel to the experiments, FE-models for the pin manufacturing process, the joining processes and the tensile shear test are developed and validated with the experimental data. The objective of this contribution is to evaluate methodically the suitability of FE-simulation for a fundamental analysis of the complete process chain for joining dissimilar metals by using formed metallic pin-structures. The validated models are subsequently used to identify the challenges when forming multi-pin-structures. Furthermore, the FE-models are used to calculate the joint strength depending on the number of pins and the pin-arrangement. This allows predictions on the achievable joint strengths with this technique.

Section snippets

Materials

In this study, tests are carried out with mild steel DC04 and aluminium alloy EN AW 6016-T4. These materials are commonly used for car body construction in the automotive industry. Due to its good cold formability and low corrosion resistance, the mild steel DC04 is mainly used for painted car bodies. EN AW 6016-T4 is a precipitation-hardenable aluminium-silicone alloy of the 6xxx series. This alloy is used for body-in-white construction due to strain hardening and a further increase in

Validation of the FE-models

In order to ensure the numerical models quality for the joint pin design and the prediction of the joint strength, the accuracy of the presented FE-models is validated by the experimentally determined findings.

Challenges and potentials for joining with multi-pin-structures

The experiments have proven the principle joinability of steel-aluminium specimens by using cold formed pin-structures. In addition, it is demonstrated that the FE-simulation is suitable for the fundamental analysis of this new joining process. The validated FE-models are used to identify the challenges of forming multi-pin-structures and to estimate the strengths of multi-pin joints.

Conclusion and outlook

This article presented a new technique for joining dissimilar metals using pins produced by forming technology. It was proven experimentally that the joining of dissimilar metal-metal systems made of steel and aluminium with pin-structures is possible by caulking them with a pre-drilled joining partner and direct pin-pressing them into an unperforated joining partner. The achievable joint strength of the different joining strategies is analysed in tensile shear tests. The formation of an

CRediT authorship contribution statement

Martin Kraus: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Visualization. Marion Merklein: Conceptualization, Methodology, Writing - review & editing, Resources, Supervision, Project administration, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) within the scope of the Transregional Collaborative Research Centre on “Method Development for Mechanical Joinability in Versatile Process Chains” - TRR 285 - Project-ID 418701707 (Subproject C01).

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