Plastics to metals joining - the combined solution

TWI Bulletin, November/December 1998

Roger Wise joined TWI in 1986 and spent four years working on the development of equipment in high power EB welding. He then directed his attentions to plastics joining where he spent six years working extensively on ultrasonic and microwave welding. In 1990 he developed the PCM joining technique for joining dissimilar materials. He is now part of TWI's new technology unit.

Alan Bates worked at TWI during the Summer of 1995 as part of his Mechanical engineering degree which he was later awarded from Birmingham University. His work included joining thermoplastics to metals by a number of methods, including ultrasound, under the supervision of Roger Wise. After gaining his MEng in 1997 he returned to TWI as a PTP associate, as part of a joint venture between Cambridge University and TWI. He is currently studying for a PhD.

The feasibility of joining the thermoplastic polyethersulphone (PES) to aluminium alloy using the PCM joining technique combined with ultrasonic welding, has been investigated by Roger Wise and Alan Bates. As they explain here, two pretreatments were used for the surface of the aluminium alloy, phosphoric acid anodising and silane coupling agents.

The feasibility of joining the thermoplastic polyethersulphone (PES) to aluminium alloy using the polymer coated material (PCM) joining technique combined with ultrasonic welding, has been investigated. Joints with tensile strengths of up to 19MPa were made between aluminium alloy and 30% glass filled PES using ultrasonic welding with a weld time of one second. This could only be achieved with the inclusion of carbon fibres at the joint line to shield the surface of the aluminium alloy from the ultrasound.

A need to build ever lighter structures with improved mechanical properties in industrial sectors such as aerospace and automotive, has led to a re-evaluation of materials joining technologies. For joints between dissimilar materials such as polymers, metals and ceramics, current technology would include mechanical fasteners (such as bolts, screws and rivets) and adhesive bonding. Mechanical fasteners will in general introduce additional weight to structures, and may involve drilling the components to be joined. In addition, mechanical fasteners tend to concentrate mechanical stresses at the fastener.

Adhesively bonded joints may provide an even distribution of mechanical stress provided that the adhesive is evenly applied to the joint. However, they can be time consuming in application and chemical cross-linking or curing may require an elevated temperature for many minutes to produce optimum bond strength. With most adhesive bonding systems the joint strength develops due to a chemical reaction which crosslinks the adhesive material. In order to guarantee consistent joint strength, factors which may affect the chemical reaction must also be consistent within certain tolerances. These may include pre-treatment or surface preparation, ambient temperature, ambient humidity and time for cure.

In 1990, investigations began into a new technique for joining dissimilar materials called the PCM joining technique. This new technology involves precoating non-thermoplastic components with a layer of thermoplastic. These components are then joined by welding the plastic coatings together using a plastics welding technique. The mechanical integrity of these joints relies on the bonding between the thermoplastic coating and the non-thermoplastic substrate, and also the strength of the weld between coatings.

This may appear to be 'doing the job twice' but one of the major advantages is that, unlike adhesive bonding, it is possible to undertake the operation requiring controlled conditions, ie the coating operation, away from, and well before, the final joining operation which may be part of a production line.

Components may be fabricated, coated and stored before being introduced to the production line for final assembly by welding. Since welding generally involves a phase change rather than a chemical reaction the joining operation is simpler to control. PCM joints may also be disassembled on the application of sufficient heat which is also not possible with cross-linked adhesives.

Welding techniques used to make PCM joints between aluminium alloy and thermoplastic in the past have included resistive implant and induction welding. ^[1] In the early stages of the development of the PCM joining technology these welding processes were selected for their suitability in joining thermoplastic composites to aluminium alloy. ^[2] However, typical weld times for these techniques are greater than 30 seconds which is generally a long duration for a thermoplastic weld. The fastest known welding technique for thermoplastics is ultrasonic welding and this paper describes preliminary trials on the application of ultrasonic welding to PCM joints.

Objective

The objective of this investigation was to produce PCM joints with good mechanical properties between PES and aluminium alloy using ultrasonic welding. Achievement of this objective would demonstrate the ability of the PCM joining technique to produce good joints in a weld time of one second or less.

Materials

The aluminium alloy used in all trials was HE15 (Table 1). The polymer used was polyethersulphone (PES) in three forms, powder, film and injection mouldings.

Table 1: Composition of HE-15 aluminium alloy

The powder was Victrex PES Grade 5003P ^[3] the film 100µm thick Victrex Stabar S100 and the injection moulding material was 30% glass filled PES.

The geometry of the PES injection mouldings is shown in Fig.1.

Aluminium alloy specimens were machined to dimensions shown in Fig.2 so that when joined to the PES mouldings, a tensile specimen was formed.

Pretreatment

The PCM joining technique requires that a thin coating of polymer is present on the surface of the aluminium alloy prior to welding. In order to optimise the adhesion of the coating, it is preferable to carry out a surface pretreatment to the aluminium alloy. There are many such available pretreatments but the two selected for this investigation were phosphoric acid anodising and silane coupling agents.

Phosphoric acid anodising creates a hard, porous layer on the surface of the aluminium alloy [4] which is believed to enhance adhesion by providing mechanical keying and improved chemical bonding.

Silane coupling agents produce a chemical link between inorganic and organic surfaces which is known to enhance adhesion. [5]

The procedure for each process was as follows:

Phosphoric acid anodising - specimens were degreased with acetone and then immersed in an alkaline cleaner (NST supplied by Oakite Ltd). An etching procedure was then followed involving immersion in a solution comprising 50g CrO 3, 300g H 2SO 4 and 810cm 3 distilled water at 65°C for 15 minutes. Specimens were then rinsed for two minutes in distilled water before being anodised in orthophosphoric acid: a 10% by weight solution was used and a constant voltage of 10V maintained for 25 minutes. The specimens were rinsed in cool mains water before being dried at 60°C in an air circulating oven for 20 minutes.

Silane coupling agent - specimens were degreased in acetone and then grit blasted using alumina grit grade NK280. The specimens were blasted at an angle of 45° to the grit stream in order to prevent embedment of the particles. The specimens were then degreased once more using acetone. The blasted surfaces were dipped into a 1% (by volume) solution of A-1100 aminosilane γ (-aminopropyltriethoxysilane) to distilled water, for 60 seconds and then dried in an air circulating oven at 100°C. The silane A-1100 was selected for its ability to bond with PES. [5]

Immediately after the surface preparations described above, the specimens were coated with PES solution and left to dry leaving a thin film of PES on the surface. The polymer solution was prepared by combining 20cm 3 of 1, 1, 2 trichloroethane, 20cm 3 dichloroethane, 1.6cm 3 methanol and 4g of PES powder. [6]

Welding

Ultrasonic welding was undertaken using a Branson 7016B series 800 machine operating at 20kHz. The configuration of this equipment is illustrated in Fig.3 and shows that the ultrasound propagated through the PES component to the thermoplastic/aluminium alloy interface. Some additional PES film material was inserted at the joint that was melted and flowed during the weld. The inclusion of this extra material was believed to be important to prevent the PES coating on the aluminium from being displaced during welding and leaving a bare aluminium surface.

Some welds used a piece of loosely woven carbon fibre material (supplied by Courtaulds) or some polyetherimide/carbon fibre prepreg (supplied by Ten Cate NV) in addition to the film material between the components being welded. This had the effect of shielding the pretreated metallic surface while still resulting in rapid localised heating at the joint. The PEI was believed to be melt miscible with PES and should therefore also be mutually weldable.

As a control, some joints in aluminium alloy were prepared using hot bar welding. The configuration used is illustrated in Fig.4. PCM joints were made using a hot bar temperature of 300°C for five minutes and with the aid of 100µm or 200µm PES film at the joint, in materials pretreated using silane and anodising.

As a further control, PES cups were welded together to test the strength of joints between similar materials. All of the ultrasonic welds were made using a welding force of 1500N and the weld time was varied to achieve optimum joint strength.

Mechanical testing

Welded PCM joints were mechanically tested in tension using an Avery Denison Universal Testing machine at a cross head speed of 5mm/min.

Results

The results of the mechanical tests on hot bar welded PCM joints are shown in Table 2. These results show that joints in aluminium alloy having tensile strengths of up to 12MPa can be manufactured using this technique although the thickness of the film at the joint line is important to the strength developed.

Table 2: Results of hot bar welded PCM joints in aluminium alloy. The hot bar was 300°C and the weld time was 5min. All failures appeared to be cohesive in the PEI

The results of the mechanical tests on ultrasonic welds in 30% glass filled PES are shown in Table 3. These results show that 30% glass filled PES coupons can be ultrasonically welded together to give a tensile strength of up to 15MPa in a weld time of 0.2 seconds.

Table 3: Results of ultrasonic welds (at 20kHz) made in 30% glass filled PES using a welding force of 1500N, and with no additional PES film at the joint

The results of the mechanical tests on ultrasonically welded PCM joints between aluminium alloy and 30% glass filled PES are shown in Table 4.

Table 4: Results of ultrasonic welds (at 20kHz) made in PCM joints between 30% glass filled PES and aluminium alloy. All welds made using a welding force of 1500N

Joints made in this way had a maximum single lap shear strength of 1MPa for the anodised aluminium alloy and 6.7MPa for the silane pretreated aluminium alloy. Examination of the tested specimen AN1 showed that there were some regions of discolouration on the joint line in the 30% glass-filled PES specimen (Fig.5). This discolouration was attributed to aluminium oxide which had been stripped from the aluminium component during testing. A similar effect was evident on specimen SI3 but not on specimens SI1 and SI2.

The results of the mechanical tests on ultrasonically welded PCM joints between 30% glass filled PES and anodised aluminium alloy are shown in Table 5. These joints incorporated some carbon fibres at the joint line which were included in order to shield the surface of the aluminium alloy from the full impact of the ultrasonic energy (Fig.6). A joint tensile strength of over 19MPa was achieved in a weld time of one second with the inclusion of some loosely woven carbon fibre material sandwiched between two 200µm thick PES films.

Table 5: Results of ultrasonic welds (at 20kHz) made in PCM joints between 30% glass filled PES and anodised aluminium alloy. All welds made using a welding force of 1500N and contained some composite material at the joints

The results of the mechanical tests on ultrasonically welded PCM joints between 30% glass filled PES and silane pre-treated aluminium alloy are shown in Table 6. These joints incorporated some carbon fibres at the joint line in order to shield the surface of the aluminium alloy from the full impact of the ultrasonic energy. A joint tensile strength of 5.9MPa was achieved in a weld time of 0.8 seconds with the inclusion of some loosely woven carbon fibre material sandwiched between two 200µm thick PES films.

Table 6: Results of ultrasonic welds (at 20kHz) made in PCM joints between 30% glass filled PES and silane treated aluminium alloy. All welds made using a welding force of 1500N and contained some composite material at the joints

Discussion

The results of the hot bar welded PCM joints in aluminium alloy show that joint strengths of over 10MPa are possible with this joint specimen geometry. The effect of bondline thickness is clearly to be considered but was not investigated in this study.

Ultrasonic welds in the 30% glass filled PES produced a maximum tensile strength of 15MPa which was achieved in 0.2 seconds. The results of welding 30% glass filled PES to aluminium alloy show that the tensile strengths achieved are much lower than those achieved in the hot bar welds or the ultrasonic welds in PES. This could be due to differences in the mechanical properties of the specimens producing an uneven stress distribution. However, discolouration on the surface of the PES component of these joints led to the consideration that the surface of the aluminium alloy was being removed during mechanical testing. This could be confirmed by surface analysis.

The results of the mechanical tests of ultrasonic welds between the 30% glass-filled PES and the aluminium alloy showed that joints with tensile strengths of up to 19MPa were produced (Table 6). This figure was produced by dividing the failure load by the area of the joint. It is difficult to make a meaningful comparison between the results of joints in different specimen combinations because the differences in material type and geometry will have a profound effect on the distribution of mechanical stress in each joint. Comparison could be attempted after finite element analysis of each specimen type but this did not form part of the scope of this investigation.

The inclusion of the carbon fibre material at the joint line appeared to assist in the formation of the weld. One possibility is that the ultrasound destroys the continuous oxide layer produced by anodising and the fibres shield this layer from direct exposure.

The results of the mechanical tests on the silane pretreated joints (Table 6) were not as good as those made with the anodised pre-treatment (Table 5) despite having been manufactured in a similar way. The reasons for this are unclear since the pretreatments produced joints of similar strength when welded by a hot bar (Table 2).

Conclusions

Ultrasonically welded PCM joints between 30% glass filled PES and aluminium alloy produce joints with poor mechanical integrity possibly due to the effect of the ultrasound on the pre-treatment to the aluminium alloy. This effect can be dramatically reduced or eliminated, in the case of aluminium pre-treated by anodising, by the inclusion of long carbon fibres placed at the joint line during ultrasonic welding. Under these conditions, joints with in-line tensile strengths of up to 19MPa can be produced in one second. This is a significant improvement over weld times for PCM joints made previously and further optimisation should result in a technique for mass producing joints in dissimilar materials in a very cost-effective manner.

Acknowledgements

The work presented in this paper was funded by the Industrial Members of TWI. The authors would like to thank Dr S D Smith for his help in attempting to interpret the results of the mechanical tests.

References