For some time, Port Melbourne based Advanced Composite Structures Australia has endeavored to develop step-change composite joining and assembly technology that eliminates fastening and secondary bonding. Now patented, the Thermoset Composite Welding (TCW) process has proven to reduce composite component assembly costs as well as enable rapid assembly. TCW also results in a joint that has better environmental resistance than many adhesive joints, as the thermoplastic also has high performance in chemical and ultraviolet environments.
The TCW Technology
To enable the welding process, a bespoke thermoplastic polymer layer is integrated onto the surface composite thermoset parts during the fabrication process. During the heating and curing process, the thermoplastic layer and the epoxy resin become mutually soluble and a controlled amount of diffusion takes place across the interface. The thermoplastic surface becomes well attached to the carbon-epoxy laminate. The interface between the thermoplastic layer and the epoxy becomes stronger than the epoxy resin itself. TCW-ready components engineered with thermoplastic surfaces can then be welded together under moderate heat and pressure, with welding times in the order of minutes.
The TCW process allows the use of innovative tooling and heating arrangements, and therefore provides greater flexibility in the design and scheduling of the assembly process. Welding can be done using a variety of heating and tooling systems, such as an oven or autoclave, or using direct weld-line heating methods often used for joining thermoplastic composites (such as resistance or induction heating). In many cases, the team at ACS Australia has found that the most efficient heating method for TCW appears to be simple local contact heating through one of the laminates to be joined, combined with local application of pressure.
The Benefits of Welding Composites
The performance of the TCW joint has been inspected and assessed using many types of mechanical and chemical tests. In most cases, comparative tests have been carried out on adhesive-bonded joints made with a standard high-temperature epoxy film adhesive. The mechanical properties of carbon-epoxy TCW joints have been found to be equivalent or superior to those of adhesively bonded joints in nearly all cases.
Fatigue and impact behaviour of TCW joints were better than adhesive joints, as the thermoplastic used for the weld material is a tough engineering thermoplastic.
Another heartening aspect of the TCW process is that it can be automated to produce high quality, rapidly assembled parts. Studies on TCW in automated, series production have revealed significant cost advantages made from savings in assembly costs as well as from large reductions in assembly time.
Demonstrating the Technology
The team at ACS Australia has manufactured aircraft control surface structure prototypes with the skin, ribs and spar cured separately with thermoplastic interlayer, assembled into a self-supporting substructure, and subsequently welded together with the aid of simple tooling. The welding was conducted using flexible heaters with multiple heater zones, proving the viability of the process for assembly of complex geometry structures. The simple and rapid welding process confirmed the practicality of local through-thickness heating. With appropriate tooling and an optimised process, it was possible to weld many joints simultaneously, reducing the assembly cycle time to just over 20 minutes. Ultrasonic inspection of the welds verified the joints to be sound.
The Future of Composites Assembly
The TCW process has great potential for significant cost and time savings in assembly of composite structures in a large variety of applications, from aerospace and automotive structures to consumer goods and the oil & gas industry. Further applications and adaptations of the technique are also being developed at present, which could revolutionise the assembly of composite structures in the near future