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Joining polyethersulphone to aluminium by ultrasonic welding (January 1999)

   
R J Wise and A D H Bates, TWI

Based on 'Ultrasonic welding of PES (Polyethersulphone) to aluminium alloy', presented at 'Plastics - Racing into the future', ANTEC May 1996.

Introduction

A continuing need to build structures with reduced weight and improved mechanical properties in industrial sectors such as aerospace and automotive, has led to a re-evaluation of materials joining technologies.

In 1990, investigations began at TWI into a new technique for joining dissimilar materials - Polymer Coated Material (PCM) joining. This involves precoating non-thermoplastic components with a layer of thermoplastic. These components are 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.

A major advantage is that, unlike adhesive bonding, it is possible to carry out the coating operation (which requires controlled conditions) separately, away from, and well before, the final joining operation which may be part of a production sequence. 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 application of sufficient heat.

Welding techniques employed 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 development of 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 sec, which is generally long for a thermoplastic weld. The fastest welding technique for thermoplastics is ultrasonic welding and the remainder of this document describes preliminary trials on applying ultrasonic welding to PCM joints.

The objective was to produce PCM joints with good mechanical properties between polyethersulphone (PES) and an aluminium alloy using ultrasonic welding.

Trials

Materials

The aluminium alloy used in all trials was HE15 (Table 1); the polymer was PES in three forms, powder, film and injection mouldings. The powder was Victrex PES Grade 5003P; [3] the film was 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.

Table 1: Composition of HE-15 Aluminium Alloy


ElementCSiMnSAlCrCuFeMgNiTiZrPbSnZn
% comp. 0.003 0.60 0.73 <0.001 bal 0.06 4.61 0.29 0.79 <0.01 0.05 <0.01 <0.01 <0.01 0.06
b3963f01.gif
 

Fig.1 Geometry of axisymmetric PES specimens (dimensions in mm).

  
b3963f02.gif

Fig.2 Geometry of axisymmetric aluminium alloy specimens (dimensions in mm).

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 relies on the presence of a thin coating of polymer on the surface of the aluminium alloy prior to welding. To optimise adhesion of the coating, it is preferable to surface pretreat the aluminium alloy. The two pretreatments selected for this investigation were phosphoric acid anodising and silane coupling agents.

Phosphoric acid anodising creates a hard, porous layer on the surface of aluminium alloy [4] . This 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 immersed in an alkaline cleaner. Etching involved 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 rinsed for 2 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 and dried at 60°C in an air circulating oven for 20 minutes.

Silane Coupling Agent - Specimens were degreased in acetone and grit blasted using alumina grit grade NK280. The specimens were then degreased once more using acetone. The blasted surfaces were dipped into a 1% (by volume) solution of A-1100 aminosilane ( δ-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 surface preparation the specimens were coated with PES solvent solution [6] and left to dry leaving a thin film of PES on the surface.

Welding

Ultrasonic welding used a Branson 7016B series 800 machine operating at 20kHz, see Fig.3. The ultrasound propagated through the PES component to the thermoplastic/aluminium alloy interface. Additional PES film material was inserted at the joint which then subsequently melted and flowed during the weld. This extra material was believed to prevent displacement of the PES coating on the aluminium during welding which can lead to regions that are free of any coating.

b3963f03.gif
 

Fig.3 Configuration of equipment for ultrasonically welding PES to aluminium alloy (axisymmetric).

  
b3963f04.gif

Fig.4 Equipment configuration for hot bar welding aluminium alloy specimens as a control

Some welds employed a piece of loosely woven carbon fibre material or some polyetherimide (PEI)/carbon fibre prepreg, in addition to the film material between the components being welded. This shielded 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 the aluminium alloy were prepared using hot bar welding, see Fig.4. PCM joints were made using a hot bar temperature of 300°C for 5 min 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 the ultrasonic welds were made using a welding force of 1500N and the weld time was varied to achieve optimum joint strength.

Tensile Testing

Welded PCM joints were tested at a cross head speed of 5mm/min.

Results

Results of tensile tests on hot bar welded PCM joints are shown in Table 2. These results show that joints in aluminium alloy having tensile strengths 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.

The results of the tests on ultrasonic welds in 30% glass filled PES are shown in Table 3.

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


SpecimenPretreatmentThickness of PES
film at the joint
(µm)
Failure
load
(kN)
Tensile strength
of joint
(N/mm 2)
HB1 Anodised 100 8 12
HB2 Anodised 200 6 9
HB3 Silane 200 7 11
HB4 Silane 100 1 1.5
These show that 30% glass filled PES coupons can be ultrasonically welded together to give a tensile strength up to 15 MPa in a weld time of 0.2 sec.

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

Table 3: Results of tensile tests on 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.

SpecimenWeld time
(sec)
Failure load
(kN)
Tensile
strength
(N/mm 2)
PES1 0.2 2.1 15.1
PES2 0.4 1.7 12.5
PES3 0.8 1.5 11.0
PES4 1.0 1.8 12.7

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. Specimen AN1 showed that there were regions of discolouration on the joint line in the 30% glass-filled PES specimen. 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.

Table 4: Results of tensile tests on 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.

SpecimenPretreatmentWeld time
(seconds)
Additional
PES film
(µm)
Load to
failure
(kN)
Tensile
Strength
(N/mm 2)
AN1 Anodised 0.4 100 0.1 1.0
SI1 Silane 0.4 100 0.5 3.7
SI2 Silane 0.2 100 0.9 6.7
SI3 Silane 1.0 100 0.1 0.7

The results of tensile tests on ultrasonically welded PCM joints between 30% glass filled PES and anodised aluminium alloy are shown in Table 5. These joints incorporated carbon fibres at the joint line, to shield the surface of the aluminium alloy from the full impact of the ultrasonic energy. A joint tensile strength over 19MPa was achieved in a weld time of 1 sec with the inclusion of some loosely woven carbon fibre material sandwiched between two 200µm thick PES films.


Table 5: Results of tensile tests on 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 composite material at the joints.


SpecimenWeld time
(seconds)
Additional PES film (µm) and compositeLoad to
Failure
(kN)
Tensile
Strength
(N/mm 2)
AN3 1.0 PEI/carbon fibre prepreg 0.2  
AN4 2.0 PEI/carbon fibre prepreg and 200µm PES film 1.9 13.6
AN5 2.0 PEI/carbon fibre prepreg and 100µm PES film 1.2 8.5
AN6 1.5 100µm PES + PEI/carbon fibre prepreg + 100µm PES 1.8 13.1
AN7 1.0 200µm PES + loose woven carbon fibre + 200µm PES 2.7 19.4
AN8 1.0 100µm PES + PES/carbon fibre prepreg + 100µm PES 1.3 9.3
AN9 1.5 100µm PES + PES/carbon fibre prepreg + 100µm PES 1.7 12.3
AN10 1.0 200µm PES + PES/carbon fibre prepreg + 200µm PES 1.6 11.6
AN11 1.0 100µm PES + loose woven carbon fibre + 100µm PES 2.6 19.0
AN12 0.8 200µm PES + loose woven carbon fibre + 100µm PES 2.3 16.5
AN13 1.5 200µm PES + loose woven carbon fibre + 100µm PES 2.2 16.1
AN14 1.0 100µm PES + fine layer of carbon fibre + 100µm PES 2.1 14.8

 

The results of tests on ultrasonically welded PCM joints between 30% glass filled PES and silane pretreated aluminium alloy are shown in Table 6.

These joints similarly incorporated carbon fibres at the joint line 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 sec with inclusion of loosely woven carbon fibre material sandwiched between two 200µm thick PES films.

Table 6: Results of tensile tests on 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 composite material at the joint.


SpecimenWeld time
(seconds)
Additional PES film (µm) and compositeLoad to
failure
(kN)
Tensile
strength
(N/mm 2)
SI4 1.0 PEI/carbon fibre prepreg 0.65 4.7
SI5 1.0 200µm PES + loose woven carbon fibre + 100µm PES 0.64 4.6
SI7 1.0 100µmm PES + PEI/carbon fibre prepreg + 100µm PES 0.11 0.8
SI8 0.8 100µmm PES + loose woven carbon fibre + 100µm PES 0.81 5.9

Discussion

The results from hot bar welded PCM joints in aluminium alloy show that strengths of over 10MPa are possible with this joint specimen geometry. Ultrasonic welds in 30% glass filled PES had a maximum tensile strength of 15MPa, achieved in 0.2 sec. The results of welding 30% glass filled PES to aluminium alloy show that the tensile strengths achieved are much lower than those in the hot bar welds or the ultrasonic welds in PES. This could be caused by 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 suggested that the surface of the aluminium alloy was being removed during mechanical testing.

Tests of ultrasonic welds between 30% glass-filled PES and the anodised aluminium alloy showed that joints with tensile strengths of up to 19MPa were produced (Table 5). This figure was produced by dividing the failure load by the area of the joint. It is difficult to compare the results from joints in different specimen combinations because differences in material type and geometry have a profound effect on the distribution of mechanical stress in each joint.

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

Results of the tests on the silane pretreated joints (Table 6) were not as good as those made with the anodised pretreatment (Table 5) despite having been manufactured in a similar way. The reasons 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 a HE-15 aluminium alloy produce joints with poor mechanical integrity, possibly because of the effect of the ultrasound on the pretreated aluminium alloy. This effect can be dramatically reduced or eliminated, in the case of aluminium pretreated by anodising, by inclusion of long carbon fibres at the joint line during ultrasonic welding. Under these conditions, joints with in line tensile strengths of up to 19MPa can be produced in 1 sec. This is a significant improvement over previous weld times for PCM joints, and further development should result in a technique for mass producing joints in dissimilar materials cost-effectively.

 

* * *

The work described was funded by the Industrial Members of TWI. Thanks are due to Dr S D Smith for his help in attempting to interpret the results of the mechanical tests.

References

AuthorTitle 
1 Wise R J 'New Technique for Joining Dissimilar Materials'. Welding Review International, February 1993, pp 40-42. Return to text
2 Wise R J 'The Polymer Coated Material (PCM) Joining Technique: Preliminary Environmental Testing of PCM Joints'. Plastics, Rubber and Composites: Processing and Applications, to be published. Return to text
3   Victrex PES Data for Design. ICI Advanced Materials, Reference VS4/1089, 1989. Return to text
4 Ahearn J S, Davis G D, Sun T S and Venables J D 'Correlation of Surface Chemistry and Durability of Aluminium/Polymer Bonds from Adhesion Aspects of Polymeric Coatings, Ed K L Mittal, Plenum Press, 1983. Return to text
5 Ranney M W, Berger S E and Marsden J G from 'Composities Materials' Volume 6, Ed E Plueddeman, Academic Press, London 1974.  
6   Victrex PES, Chemical Resistance Data, ICI Advanced Materials, 1989. Return to text

 

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