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Polymer coated material (PCM) joining technology for manufacture and repair of dissimilar material structures (March 1999)

   

R J Wise and K Gosai

Paper presented at International Conference: Joining and Repair of Plastics and Composites, 16 - 17 March 1999, London

Synopsis

The Polymer Coated Material (PCM) joining technique involves the use of thermoplastics as structural adhesives where the final assembly operation is a polymer weld. In the manufacture of a joint between a thermoplastic component and a dissimilar material component (for example metal), the non-thermoplastic component is first coated with the same thermoplastic before both components are welded together. This approach has a number of advantages over conventional adhesive bonding with thermo setting adhesives, such as:

  • The final joining operation (welding) can be very rapid and would typically be less than 30 seconds.
  • Joints can be disassembled for repair, re-use or component recycling on the application of sufficient heat.
  • The interface between adherends can be achieved away from the area of final joint assembly and so 'clean activities' can be segregated.

This paper describes some of the work undertaken on this new technology and involves the joining of polymers to aluminium alloys. It also describes how the technology can be applied locally for rapid repair of structures or scaled up for mass production of structures including joints between dissimilar materials. This article concentrates on the use of induction welding for PCM joining of dissimilar materials.

The mechanical properties of PCM joints in aluminium alloy are presented in this paper, specifically the result of single lap shear tests on as-welded specimens and specimens exposed to salt spray and high humidity.

1. Introduction

Exploiting and combining the technical and commercial benefits offered by light-weight engineering materials such as plastics and composites with the more traditional engineering metals, is possible today mostly due to current joining techniques such as mechanical fastening, adhesive bonding and hybrid joining.

Fabricators of engineering structures in the transport (aerospace, automotive, marine, rail), defence, building and construction industry sectors are currently able to assemble plastics and composite parts to steel, aluminium and titanium components successfully using high performance adhesives.

Examples of structural bonding include modern aircraft which comprise riveted and bonded 2xxx and 7xxx series aluminium alloys, rubber-to-metal bonds in the engine, transmission and exhaust mounting in automobiles and railway bogiesuspensions, and of the highly acclaimed Aberfeldy bridge which is constructed solely from bonded GRP (glass reinforced plastic) sub-structures. Without adhesives and adhesion science, the only alternative technique for joining dissimilar materials is mechanical fastening. The advantage of using adhesives being that it does not introduce holes which act as stress raisers which can subsequently cause premature failure particularly in composite structures. [1]

Polymer coated material (PCM) joining technology is essentially a quick assembly technique, which welds parts coated with thermoplastic together using just heat and pressure - perceived advantages are:

  • PCM presents a potential cost and weight saving over riveted structures
  • PCM joint strengths are similar to high performance epoxy adhesives - around 30MPa for metal-to-metal joints
  • PCM joints have shown resistance to extremes of environmental exposure and testing and maintained joint integrity
  • To repair, add a thermoplastic-coated patch, apply heat and weld
  • To disassemble, remove plastic by heat and separate material types.

This means that material suppliers could market material precoated with polymer for joining which will deliver cost savings to assemblers and offers a route to improved product consistency/quality.

2. Polymer coated materials (PCM) joining technology

2.1 Introduction

The PCM technique involves the surface pretreatment of one component by one or a combination of several methods; solvent degrease, grit-blasting, acid etching, anodising for metallic substrates. Plastics/composites are pretreated by abrading, grit blasting, corona discharge, plasma or flame treated. Ceramics are usually abraded or grit blasted due to their inert nature.

This pretreated surface is then coated with a thermoplastic. This can be achieved in a variety of ways:

  • Coating from solution by dipping, brush application or spraying and allowed to 'dry', i.e. allowing the solvent to evaporate*.
  • Using water based colloidal systems
  • Introducing the monomer and polymerising in-situ
  • Frictional deposition of polymer.

* Most of the work done to date has been with polymers dissolved in solvents. Current research is concentrating on the use of chemically modified and low molecular weight polymers to aid adhesion to the substrate. The solvent thenevaporates leaving a polymer coating on the substrate.

When dry, this plastic coated component is placed in contact with the component to which it is to be joined, which may have also undergone a similar pretreatment in the required joint configuration or design such as butt or lap.Using an additional thermoplastic polymer interlayer (polymer film) at the joint line provides the sacrificial 'adhesive' which melts and bonds the two substrates together. The structure of a typical joint is shown in Fig.1.

Fig.1. Schematic cross section through a PCM joint
Fig.1. Schematic cross section through a PCM joint

2.2 Background

Previous work [2,3] has shown that it is possible to join dissimilar materials such as aluminium alloy to polyetheretherketone (PEEK)/carbon fibre composite materials together or to themselves using a coating and interlayer of the thermoplasticpolyetherimide (PEI). However, the glass transition temperature (T g ) of PEI is approximately 215°C [4] and the actual melt processing temperature for PCM joints probably exceeded 300°C. At this temperature, the aluminium alloys employed in this work underwent an undesirable change in microstructure. For this reason,thermoplastic polymer systems were sought which had lower melt processing temperatures. One material investigated in this study was polyvinylidene fluoride (PVDF) which had been modified by grafting carboxylic acid to improve its adhesion to metal. The melting point of this semicrystalline polymer was approximately 170°C which was sufficiently high for most operational use but not high enough to cause undesirable changes to the aluminium alloys. To join the two plastic coated components together, a conventional plastics welding technique such as induction welding is used.

This technique offers the ability to apply heat and pressure, quickly and effectively over a short time period and produce welds in thermoplastic and dissimilar materials.

3. Experimental procedure

3.1 Materials

  1. Aluminium sheet alloy 2024T3 (unclad) of 1.6mm thickness was used for the experimental trials.
  2. Modified PVDF powder with good melt adhesion properties was manufactured and supplied by Elf Atochem.
  3. N-methyl pyrrolidone (NMP) solvent was used to dissolve the PVDF powder to prepare the coating.
  4. PVDF film of 100 microns thickness was used as the thermoplastic interlayer ( Fig.1).

3.2 Sample preparation

Surface pretreatment

Coupons measuring 25.4mm x 100mm were cut from sheet alloy and linished. Previous work at TWI had shown linishing to be an important stage before anodising to improve the joint geometry of the lap shear configuration.Orthophosphoric acid anodising pretreatment was carried out on the aluminium alloy coupons.

Coating

Modified PVDF powder was dissolved in N-methyl pyrrolidone (NMP) industrial solvent in the ratio 1:10. Warming the mixture to 40°C aided the dissolution of powder. The coating was applied by dipping the anodised aluminium coupons in the solution. The solvent was allowed to evaporate by placing the coated coupons in an oven at 40°C, after which they were placed in a desiccator.

Film

Unmodified PVDF film was cut into strips 40mm wide and 100mm thick to serve as the thermoplastic interlayer.

4. Induction welding equipment

Previous work at TWI had shown that deliberately shaping the joint geometry would reduce the stress concentration over the joint line and hence special jigging and a shim were made to support the lap shear specimens. The shim was shaped with a 45° angle to allow molten plastic to take up this form during welding.

Induction welding equipment consisted of a water-cooled copper tube workcoil embedded in an epoxy resin press-foot. The workcoil was electrically connected to a 450kHz/6kW high frequency generator. The press-foot was attached to apneumatic press which was required to apply pressure to the joint during welding as shown in Fig.2.

Fig.2.
Fig.2.

Induction welding relies upon the interaction of a dynamic induction field with an electrical conductor, in this case aluminium, to produce eddy currents. As a result, the conductor heats up and any thermoplastic in direct contact may melt if sufficient heat is generated.

4.1 Welding procedure

The dried coated coupons were placed in a lap shear configuration in specially prepared jigging. Two pieces of PVDF film were placed at the joint line. The press, which was calibrated to apply 0.35 or 0.7MPa was used to apply pressure to the lap joint. A series of initial joining trials were conducted and acceptable results were achieved using 40 and 50 seconds joining time. These initial trials gave an indication of the ranges of conditions to be employed for the experiments. Following the definition of the matrix of experiments, trials using time, power and pressure as the 3 factors were conducted to identify the limits of the processing window and establish the optimum conditions using statistical analysis.

4.2 Weld assessment

In order to assess the results of the work, the joints were subjected to the following methods of analysis:

  • Tensile testing
  • Sectioning and optical microscopy
  • Hardness testing
  • Environmental testing
  • Thermal analysis.

5. Results & discussion

5.1 Tensile testing

The highest tensile strength result obtained was 30 ±2 MPa. Interesting features included the formation of the shear bands in the joint area indicating ductile failure in the thermoplastic interlayer. The mode of failure in the joint was cohesive as polymer was obviously present on both sides of the tested lap joint.

5.2 Sectioning and optical microscopy

A comparison of untreated and post-welded aluminium alloy microstructure showed the disappearance of some of the strengthening precipitates (CuAl 2 ) from the post-welded aluminium microstructure as a result of reversion. The study also revealed the presence of voids in the thermoplastic reinforcement created by the shim.

5.3 Hardness testing

As-received aluminium alloy gave a hardness value of 126Hv. Corresponding hardness tests on the post-welded alloy showed a slight decrease in value to 100Hv. This effect can be attributed to the changes in microstructure as revealed and observed by the sectioning and optical microscopy.

5.4 Environmental testing

Joints which had been kept in the salt spray cabinet for 6 weeks experienced the formation of a pink grainy deposit. On drying, this deposit became blue. The formation of these pink hydrated salts can be attributed to the leaching of copper from the aluminium alloy during salt spray testing. Tensile testing of the 6 week old joints showed a very slight reduction in strength from 28 to 23MPa.

5.5 Thermal analysis

The results of the differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) analysis suggested that some of the NMP solvent may have been present in the coating, however, this would have evaporated during the welding process. Evidence of this is shown in the voids present in the thermoplastics reinforcement created by the shim. A more rigorous drying procedure after coating could be adopted to eliminate this problem.

6. Implementation of PCM joining technology

6.1 Why PCM joining?

There are three key aspects to the PCM approach which make it a strong competitor to adhesive bonding technology:

  • The critical interface between the metal and polymer is made in a controlled environment and away from the area where structures are assembled, which will typically be a more difficult environment to control.
  • The operation which makes the structural joint is rapid (typically seconds).
  • The joints can be disassembled with heat (which facilities recycling).

In other respects, mechanical properties, environmental resistance etc. PCM joining technology is believed to be at least as good as conventional adhesive bonding technology, and in some cases better.

6.2 Coating technology

The application of polymer from a liquid having low viscosity (in this case a solution), is vital to the formation of joints having high mechanical strength because excellent wetting (intimate contact between polymer and pretreated metal) can be achieved via this route. [5] The coating technology, as a critical step, can be handled in several ways including local application and precoating the sheet. Precoated aluminium alloy sheet would constitute a material having added value which could be prepared under carefully controlled conditions by material manufacturers. One possible sequence of manufacture for the material in coated metal coil form is shown in Fig.3.

Fig.3. A scheme for coating metal coil
Fig.3. A scheme for coating metal coil

For less complete coverage with polymer, station 6 can be replaced with a spraying facility incorporating solvent recovery. It is believed that a coating of no more than 10µm is required to achieve the mechanical properties described in this article. This figure can be used as an upper limit in calculations of additional weight and implied cost associated with aluminium coil coating. The manufacture of joints, of course requires more polymer to be added but this occurs during the joining operation.

Local application of coatings is possible using several techniques such as dipping, brushing, spraying and screen printing. However, the use of the solvent must be carefully controlled in the usual way, and conditions forpretreatment and drying should be carefully reproduced to guarantee consistent joint characteristics.

6.3 Joining technology

The main advantages of the PCM joining approach are perceived to be:

  • Rapid joining of dissimilar or similar materials using polymer welding
  • Repairability
  • Rapid disassembly for recycling.

To fully exploit these advantages, welding technology permitting rapid localised heating is required. Techniques applied so far have included induction, resistive implant and ultrasonic welding, of which ultrasonic processing [6] offers the shortest processing time (approximately 5 seconds).

Induction welding is the preferred welding technique for PCM joining, involves the generation of high frequency electric current in the aluminium components by the action of a dynamic magnetic field provided by a work coil. The work coil is shaped to deliver a magnetic field of the appropriate distribution to produce an even flux density to the component. As the magnetic field fluctuates with time, electric current is induced in the surface of the component and if this causes sufficient resistive heating the polymer coating melts and a joint is formed under the influence of some externally applied pressure.

Resistive implant welding, a technique which has also been used for PCM joining, uses an electrically resistive implant placed in the joint as the source of heat. As electric current (often DC or low frequency AC) passes through this implant, heat energy is dissipated by Joule heating and if this occurs at a suitable rate then it can cause the thermoplastic coating to melt. A weld forms in this manner under the influence of some mechanical pressure. The implant remains in the joint after welding and so is often made from a material which is compatible with the polymer coating. Some of the first PCM joints were welded with a resistive implant comprising a piece of unidirectional carbon fibre thermoplastic composite, where the welding current passed through the carbon fibres. [2]

Ultrasonic welding is another process which can be used for PCM joining and involves the use of high frequency sound energy to soften or melt the thermoplastic at the joint line. Parts to be joined are held together under pressure and are then subjected to ultrasonic vibrations usually at a frequency of 20kHz or 40kHz which generally causes melting of the thermoplastic in less than one second. The mechanical oscillations produced by an ultrasonic weld can cause failure of the pretreatment in PCM joints and so to make joints successfully, a layer of fibres were placed in the joint which appeared to protect pretreatments by absorbing and scattering the ultrasound. [6] PCM joints could be produced in a few seconds using ultrasonic welding.

6.4 PCM joint manufacturing technology

It is worth considering the possibility offered by the PCM joining technique integrated into manufacture of parts made from aluminium alloy, and one such scheme is shown in Fig.4.

Fig.4. Manufacture and disassembly of aluminium alloy components using PCM joining technology
Fig.4. Manufacture and disassembly of aluminium alloy components using PCM joining technology

In this way, components or whole products may be assembled and disassembled rapidly using heat. It should be emphasised that there are a number of issues in the development of this technology which must be resolved before the full benefits can be exploited, including:

  • Effects of bending/forming on the mechanical properties of pretreatments and coatings (step 3a).
  • Environmental resistance of all joint types and pretreatments to humidity and salt spray. Preliminary results of environmental tests look very promising [2] but further work is required.
  • Optimisation of welding technologies to suit the selected coating and component geometries.
  • Identification of optimum polymer systems for coating and joining of structural materials. This will be a function of in-service environment, material types to be joined and cost.
  • Formulation of joint designs for the optimum management of in-service loads.

7. Conclusions

The following conclusions can be drawn from the results of this work:

  1. Single lap shear joints having strengths of 30 ± 2 MPa can be produced using PCM technology for aluminium: aluminium components using PVDF interlayer
  2. A change in microstructure of the aluminium alloy caused by the thermal effects of welding led to a slight reduction in hardness.
  3. Surface formation of hydrated copper (II) salts and a slight reduction in strength occur as a result of environmental testing under salt spray.
  4. Statistical analysis of designed experiments can be used to determine the optimum conditions for induction welding.
  5. PCM technology can be exploited using induction welding to rapidly manufacture structural components for industry.

8. Recommendations

This study has demonstrated the feasibility of using PCM technology for the future manufacture of aluminium alloy structures. Further work in the areas of assessment of distortion, likely cost of scale-up, full implication of technology and prediction of in-service requirements of joints over design life need to be conducted.

9. Acknowledgements

This work was sponsored by Short Brothers plc.

10. References

  1. G C McGrath: 'Aspects of joining composite structures'. Maritime and Offshore Use of Fibre Reinforced Composites Conference, Gateshead, 2 - 3 June 1992.
  2. R J Wise and M N Watson: 'A new approach for joining plastics and composites to metal' 2113-2116, 1992, SPE ANTEC.
  3. R J Wise 'Polymer coated material joining technique: preliminary environmental testing of joints' Plastics, Rubber and Composites, Processing and Applications, 1996, 25, 2, 55-63.
  4. 'Ultem Design Guide', GE Plastics.
  5. F N Cogswell: 'Thermoplastic aromatic polymer composites',1-261; 1992, London, Butterworth-Heinemann.
  6. R J Wise and A D H Bates, 'Ultrasonic Welding of PES to Aluminium Alloy',1203-1207,1996, SPE ANTEC.

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