Clinching (metalworking)

Clinching also referred as "Press-Joining" is a bulk-sheet metal-forming process aimed at joining thin metal sheet without additional components, using special tools to plastically form an interlock between two or more sheets. The process is generally performed at room temperature but in some special cases the sheets can be pre-heated to improve the material ductility. Clinching is characterized by a series of advantages over competitive technologies:

• Reduced joining time (the joining time is less than a second);
• Reduced cost and weigh: the process does not involves additional elements such as screws of rivets or adhesives
• Reduced cost of the machine;
• No pre-holes are required;
• Can be adopted to join different materials including metals, polymers, composite and wood;
• Can be easily automated and does not require qualified workers;
• Eco-friendly: it does not require pre-treatments (solvents, acids, etc)
• the mechanical behavior of the metal material near the joint is generally improved since the work-hardening;
• Cleaness: the process does not produce flashes of fumes;
• Repeatibility;
• Flexibility: the same tools can be employed for a wide series of materials
• Reduced Joining Forces

File:G5151Clinching Phases from Lambiase "Clinch joining of heat-treatable aluminum AA6082-T6 alloy under warm conditions".png

Clinching Phases

Tools

Because of the process involves relatively low forces (ranging from 5 to 50 kN depending on the material to join, type of tools and sheet thicknesses), clinching generally involves reduced size (often portable) machines. The tools consist typically of a punch and a die. Different tools have been developed so far, which can be classified in round and rectangular tools. Round clinching tools include: fixed grooved dies, split dies (with 2-4 movable sectors) and flat dies. Such tools produce round joints which show almost identical mechanical behaviors in all plane directions. when round tools are adopted, the integrity of the sheet in the joint must be guaranteed in order to preserve good mechanical behavior of the joints.

File:Clinching Dies from Lambiase - "Mechanical behaviour of polymer-metal hybrid joints produced by clinching with different types of dies".png

Clinching dies

On the there hand, rectangular clinched joints exhibit behaviors which depend on the loading direction and both sheets are intentionally sheared along the "long direction" in order to produce the interlock. The choice of the tools is highly influenced by:

• Material ductility
• Loading direction
• Thickness of the sheets

In addition, the choice of the clinching tools highly affect the joining strength and absorbed energy of a clinched connection other than the joining force e.g. rectangular tools require lower joining force than round tools since the material shearing, while among the round clinching tools split dies require the minimum joining force and the largest interlock.

One benefit of clinching is the capability to join prepainted sheet metal commonly used in the appliance industry without damaging the painted surface. Clinching is an important means of fastening aluminum panels, such as hoods and decklids, in the automotive industry, due to the difficulty of spot welding aluminum.

Main Advantages as compared to Welding

Clinching is used primarily in the automotive, appliance and electronic industries, where it often replaces spot welding. Clinching does not require electricity or cooling of the electrodes commonly associated with spot welding. Being a mechanical joining process, clinching can be used also to join materials showing no electrical conductivity such as polymers, or composites. In addition, it does not require the substrate preparation such as pre-cleaning of surfaces as that required by welding process which contributes to reduce the joining cost and environmental impact (since chemical cleaning is not required). Clinching does not generate sparks or fumes. The strength of a clinched joint can be tested non-destructively using a simple measuring instrument to measure the remaining thickness at the bottom of the joint, or the diameter of the produced button depending on the type of tools employed. Life expectancy for clinching tools is in the hundreds of thousands of cycles, making it an economical process. A recent study demonstrated that clinched connections performed on aluminum sheets had higher fatigue life as compared to spot welding.

Main advantages as compared to Adhesive Joining

Clinching does not require pre-cleaning of surfaces as that needed before applying adhesives. Clinching is almost an instant joining process (the required joining time is lower than a second) while adhesive joining often require a much longer time mainly owing to the curing of the joint (up to many hours). Clinched joints are less affected by environmental agents and effect of aging.

Main Limitations

Being a joining method based on the plastic deformation of the sheets, the main limitation of clinching is mainly represented by the sheet material formability (ductility). Since metal ductility increases with temperature, a number of researchers developed heat assisted clinching processes which to increase the material ductility and extend the clinching "join ability". In addition, increasing the joining temperature comes with a reduction in the material yield stress and consequently a reduction in the required joining force. Nevertheless, if prolonged heating is adopted, an increase in the grain size as well as metallurgical changes of alloys may affect the mechanical behavior of the material at the joint neighbor, as reported by Lambiase. To this end different heating systems have been employed to heat the sheets before clinching:

• convective heating: this represents the cheapest solution and it is suitable for a wide range of materials including metals, polymers (thermoplastics). It was used by Lambiase to join aluminum sheets and also polymer sheets
• Inductive heating: this represents a fast heating solution which allows to concentrate the heat flux in a reduced area: Nevertheless it can be applied only on metal sheets. It was used by Neugebauer to join magnesium alloy sheet
• Flame heating

File:Convection Heating from Lambiase - "Joinability of different thermoplastic polymers with aluminium AA6082 sheets by mechanical clinching".png

Convection Heating System

Materials

Although clinching has been widely employed for joining ductile metals including:

• low carbon steel,
• aluminium,
• copper

it has recently extended to:

• magnesium;
• aluminium with reduced ductility (AA6082-T6);
• high strength steel;
• titanium alloys;

other than to join metal to other material sheets including:

• polymers

• fibre reinforced plastics (composite): GFRP, CFRP;
• wood + metal