Thermoplastic composites (TPCs) are an integral part of modern lightweight designs in the auto industry.

However, one challenge is how to join TPCs in multi-material assemblies. Mechanical joining processes, such as clinching, are generally suitable for this purpose. Clinching does not add weight to the assembly, and it’s more energy-efficient than spot welding.

The “clinchability” of a joint depends on the ductility and the tensile strength of the parts. If both parts are clinchable, rigid or radial-opening dies can be used. To clinch materials with low ductility or high tensile strength, a pilot hole can be integrated in the less-ductile part, which must be positioned on the die side.

Due to the reduced ductility of TPCs, as well as restrictions from the fibers, conventional clinching methods are challenging for TPC-metal joints. However, heating the polymer matrix can increase its formability. Our research compares three “thermally supported” forming methods for joining TPCs: thermo-clinching, hot-clinching, and insert clinching.

In addition to the design of the joining tools, the process cycle, and the joint geometry, the three technologies differ in terms of the joining direction. In thermo-clinching, the TPC is joined to the metal, while in hot-clinching, the metal is joined to the TPC. Insert clinching can be done in either direction.
 

Thermo-Clinching

The thermo-clinching process combines features of thermoplastic riveting and clinching with a pilot hole, forming a defined fiber-reinforced undercut. The tool consists of a tapered pin and a rigid die with a movable annular anvil. In a preliminary step, the composite material in the joining zone is cut in the thickness direction, and a pilot hole is drilled into the metal sheet.

At first, the joining partners are positioned with the TPC sheet on the punch side and the metal sheet on the die side. Afterwards, the TPC sheet is locally heated above melting temperature to increase the deformability of the fibers inside the thermoplastic matrix. The tools are also warmed up. In the next step, the pin moves downwards to reorient the fibers in the thickness direction through the pilot hole in the metal part. At the end of the joining process, the passed-through TPC material is compressed by the rigid die with an annular anvil to form the final undercut. The process takes less than 1 second.
 

Hot-Clinching

A single-stage joining process, hot-clinching is an adaption of a conventional clinching process with a rigid die. A two-part die consisting of a rigid sleeve and a spring-loaded anvil in combination with thermal support are used. The thermal support is provided via cartridge heaters and improves the formability of the TPC. The TPC is positioned on the die side.

At first, the parts are positioned between the heated split die and the blank holder. The TPC sheet is warmed up by contact heating by the tempered die. Afterwards, the blank holder moves downwards followed by the punch. Due to the downward stroke of the punch, a deformation and offsetting takes place. During the off-setting, the spring-loaded anvil is pressed downwards, thus applying a counter-pressure to the joint. 

In this way, the neck area of the punch-sided sheet is stabilized, which improves the formability. As a result of the applied pressure, the punch-sided material flows in radial direction, whereby an undercut is created. At the end of the process, the finished joint is released.
 

Insert Clinching

Metal inserts can be embedded into TPC during the manufacturing process, using the principle of molding holes. The reinforcing fibers are not cut by punching or drilling, but shifted aside by a tapered pin tool while the TPC is in a plasticized state of the TPC. 

Embedded inserts are suitable as an interface for joining TPC to metal using resistance element welding. Such inserts can also be used as an interface for conventional clinching of hybrid joints. This is known as insert clinching. 

First, the TPC sheet is warmed up above melting temperature of the matrix by an infrared heating device. Afterwards, the TPC sheet is quickly transferred into the open compression mold. Immediately after closing the mold, a tapered pin tool (consisting of pin retainer and tapered pin) is shifted forward, forming a hole by displacing the reinforcing fibers and the still molten matrix. 

The two-part pin tool contains a magnet to attach the clinch insert and the tapered pin to the pin retainer. Subsequently, the pin movement the squeezed-out material is recompressed by a ring-shaped counterpunch, whereby the undercut of the clinch insert is filled with fibers and matrix material. The embedding process takes less than 1 second. 

After solidification, the shaped TPC component with integrated clinch insert is demolded. Afterwards, the composite part can be joined with metallic components in a subsequent clinching process using standard tools. 

Both rigid and opening dies are applicable. The TPC can be positioned punch-side as well as the die-side, which contributes to the flexibility in application. In the clinching process, the clinch insert and the metallic joining partner are deformed, while the TPC is not deformed.
 

Materials and Methods

For the TPC parts, we used glass-fiber-reinforced polypropylene (GF-PP) and glass-fiber-reinforced polyamid 6 (GF-PA6). For the metal parts, we used structural steel, 6016 T4 aluminum alloy, DC04 steel, and S235JR steel.

For the thermo-clinching process, the TPC specimens were cut crosswise and heated up to 200 C in the joining zone. The process is performed on a machine specifically for the purpose. The system is equipped with servo-pneumatic force and displacement control and an interchangeable tool set.

For the hot-clinched joints, we used a hydropneumatic C-frame press from Tox Pressotechnik. The die-side tool holder was modified to accommodate the heating cartridges. The TPC was warmed up to 180 C, just below the melting temperature of the thermoplastic matrix.

The punch velocity during the process depends on the material and clinching machine. For our tests, it was 2 millimeters per second. At the beginning of the process, the anvil protrudes above the rigid sleeve and thus generates a counter-pressure on the joint. The anvil spring in the initial position is not pre-loaded and has a stiffness of 700 newtons per millimeter.

For insert clinching, two types of clinch inserts (V1 and V2) were embedded in the TPC specimens, both rotationally symmetric. The height of the clinch inserts corresponds to the thickness of the TPC. Insert V1 is axially symmetrical, which simplifies feeding. Insert V2 has a larger head diameter on one side, which can be expected to result in higher joint strengths, especially under transverse tensile load.

To make the TPC specimens with integrated clinch inserts, we developed a pilot rig consisting of an infrared heating device and a tempered steel mold with a vertical flash face. The pin tool is pneumatically actuated.

For the clinching process, we used a hydraulic C-frame press from Eckold GmbH & Co. We used conventional clinching tools.

After assembly, we used various imaging technologies to analyze the joining zone. We measured the undercut thickness (tU), the bottom thickness (tB), and the neck thickness (tN). While tN predominantly has an impact on the shear strength of a clinch joint, tU mainly influences the cross-tensile and peel strength.

To investigate the mechanical properties of the clinched joints, we used single-lap shear tests.

Due to the different geometric dimensions of the joining zones, the design of the test specimens and the testing velocities varied. Compared to hot-clinching and insert clinching, the joining zone is larger in thermo-clinching.

Process Phenomena

In general, the main deformation of clinching processes is in the thickness direction. In the thermo-clinching and hot-clinching processes, the forming of the TPC structure takes place during the actual clinching process. In contrast, in the insert clinching process, the TPC is formed during TPC component production and not during the clinching process. Especially for continuous-fiber-reinforced thermoplastics, the forming process changes the local material structure. In all three joining processes, the TPC is formed in a warmed-up condition.

Three main phenomena can be observed during the joining process. The tool penetration and compaction of the joining zone by die or counterpunch lead to a change of the fiber paths, including fiber reorientation both in the thickness direction and in the laminate plane direction. If the penetration and the stroke of the tools lead to an exceeding of the elastic properties of the fibers, fiber failure occurs. The failure modes of the fibers vary between bending or tension in fiber direction.

Micrographic analysis of thermo-clinched joints shows that parts of the textile reinforcement are specifically relocated to the neck and head area of the final joint. This contributes considerably to the load-carrying capacity of the joints.

Through CT analysis, it can be seen that the reorientation of the reinforcing fibers is accompanied by various deformation phenomena. Thus, there is a relocation of the fibers into the form-closed head area, including fiber reorientation both in the thickness direction and the plane direction. As a result, splaying of the roving ends can be observed.

In the hot-clinching process, all three major phenomena can be observed. Fiber reorientations in the thickness direction can be seen, mainly occurring in the heating zone of the die. In the neck area of the joint, the fibers are bent in the motion direction of the punch. Bent fibers next to the formed undercut failed, which means that the critical stress in this area was exceeded. Especially in the bottom area of the joint and in the ring groove of the anvil, excessive fiber failure and radial movement of the fiber fragments occurs.

The material flow of the die-sided joining partner favors the formation of an undercut. The flow pressing of the metal part and resultant undercut forming can explain the fiber failure in the neck area of the joint.

For insert clinching, we analyzed the laminate surface of each TPC specimen photographically before clinching. In addition, we did microscopic examinations of cross-sections of clinched joints.

During embedding of the insert, fibers and the plasticized matrix are initially displaced by the pin movement both laterally in the laminate plane and in the thickness direction. The material displaced in the thickness direction is pressed back into the laminate plane afterwards by the counterpunch. A complete filling of the undercut of the clinch insert with reinforcing fibers and matrix can be achieved during the embedding process as a result of the compression by the counterpunch.

This is possible due to the high temperature in the forming process, which leads to high movability of the reinforcing fibers and the thermoplastic matrix. Thus, the embedding process results in a local complex material structure with a homogeneous three-dimensional fiber orientation and locally varying fiber content.
 

Results

For the joining processes and materials we studied, the maximum shear loads of the joints were comparable (1.3 to 3.3 kilonewtons). Any of the three methods are suitable for nonstructural applications, or, when used in combination with adhesive bonding, for higher strength applications.

Deciding which method is best for a given application cannot be based exclusively on the load-bearing capacities of the joints. Rather, the normalized shear strength should be considered. However, for a valid comparison, aspects such as the materials, sheet thicknesses, joining tool dimensions, and the geometry of the joining zone also must be considered.

Thermo-clinching can join thick TPC and thin metal sheets. Shear loads of 2.6 kilonewtons for a 1-millimeter thick metal can be achieved. Since the metal part is not deformed in the clinching process, even metals with low ductility, such as ultra-high-strength steels, can be joined. 

Since both sheets require local preparation steps, the sheets must be positioned relative to each other with high accuracy.

Hot-clinching is a single-stage joining process that has no special requirements regarding the relative positioning of the sheets. It’s also faster than thermo-clinching. In our tests, it produced joints with a maximum shear load of 2.2 kilonewtons.

Insert clinching shows comparatively high shear loads with metal sheets as thick as 1.5 millimeters. The main advantage of this process is the use of conventional clinching tools. This allows the integration of composites into existing assembly lines without modification of the joining tools or process chains. The process is more suitable for thin TPC sheets (less than 3 millimeters, since thicker clinch inserts are difficult to join without extending the tool geometries.

Clinchability is not limited by the properties of the TPC. In principal, all common clinchable metal-to-metal combinations can be joined.

In contrast to the other clinching methods, both joining directions are applicable. Compared to hot-clinching, insert clinching requires greater positioning accuracy with the parts. The TPC sheet and metal sheet do not have to be positioned exactly in relation to each other, but the clinching tool must hit the position of the clinch insert precisely.

Conclusions

Our research compared three clinching technologies for TPC-to-metal joints. We found that thermal support during the deformation process of the TPC leads to less fiber damage and enables a better fiber reorientation.

Joints made with these processes produce joints with good load-bearing capacities without damaging the fibers or contaminating the clinching tools.

Each process has its advantages and limitations. Thermo-clinching offers the possibility to join ultra-high-strength steels or thick TPC sheets. Hot-clinching enables joining without any preparation step or requirements for positioning accuracy of the joining partners. Insert clinching technology allows standard metal clinching with conventional tools.

Editor’s note: This article is a summary of a research paper co-authored by Juliane Troschitz, Christian Vogel, Robert Kupfer and Maik Gude of the Technical University of Dresden, and Julian Vorderbrüggen and Gerson Meschut of the Laboratory for Material and Joining Technology at Paderborn University in Paderborn, Germany. To read the entire paper, click here.

For more information on composites manufacturing, read these articles:
Assembling Thermoplastic Composites
Ultrasonic Welding of Thermoplastic Composites
Spot Welding Metal-Plastic Composites