Environmental Testing - PCM Joints go on Trial

TWI Bulletin, September - October 1995

Joints between dissimilar materials are very common in everyday life. Roger Wise describes a new technique for making such joints and reports on their environmental resistance.

Roger Wise is a physicist working in the Plastics Joining Group at TWI. He joined the company in 1986 and spent four years in the Electron Beam welding department designing electron optics and high voltage components for high power applications. Since 199O he has been engaged in the development of new technology in the plastics joining field, including joining dissimilar materials, design of ultrasonic welding equipment and in fundamental welding research.

The Polymer Coated Material (PCM) joining technique was first conceived in 1990 as a method for joining dissimilar materials. It involves coating non-thermoplastic components with a thin layer of thermoplastic before joining them, using a thermoplastic welding technique. For example, a joint between a metal component and a ceramic component could be effected by coating both components with a thermoplastic, and then melting the coatings together under pressure so that a weld can form.

To make joints with good mechanical properties, it is important that the coatings adhere very well to the components before welding, and that the welding operation produces a good joint between the coatings.

If such a technique could be optimised, the range of applications would be very wide. Many products contain components made from dissimilar materials which require joining technology to manufacture. The main advantages of using PCM technology over competing technologies such as adhesive bonding or the use of mechanical fasteners could potentially include:

• Rapid assembly using a welding technique
• Joints which can be dismantled
• Elimination of costly mechanical fasteners
• Improved consistency due to joining technique
• On-line continuous processing

Possible uses could include joining thermoplastic body panels to metallic space frames in automotive applications or repairing metallic aircraft fuselages in aerospace.

In most industrial applications where an interface between a polymer and a metal exists, the key parameter governing the applicability of the joining technology is the environmental resistance of the joint. This is a measure of the strength of the joint on exposure to a certain environment and governs the expected design performance of the joint as a function of time. The key element to the prediction of the environmental resistance of the joint is generally its reaction to moisture or salt. This article deals with preliminary studies on PCM joints in a humid environment. The effects of salt have yet to be investigated in detail.

Coating Technology

The PCM technique relies on a good bond between the applied thermoplastic coating and the substrate. A similar problem sometimes occurs during the manufacture of thermoplastic composite materials. ^[1] Various attempts were made to lay down well adhering coatings of thermoplastic on metallic substrates using thermal spraying. ^[2,3] However, coatings generally contained some porosity and adhesion strength was not good.

In 1990, work began at TWI on the development of coating technology based on solvent chemistry. The thermoplastic coating material was dissolved in a suitable solvent and applied to a pretreated metallic substrate. In this way, well-adhering coatings of thermoplastic of approximately 5µm in thickness could be deposited. ^[4] Welding of such coated components to thermoplastic coupons, or to similarly coated components, resulted in joints with initial bond strengths approaching those achievable using epoxy adhesives.

Investigation at TWI originally involved three thermoplastic coating materials, polyethermide (PEI), ^[5] polyethersulphone (PES) ^[6] and polyetheretherketone (PEEK). ^[7] The metal substrate used in early trials was aluminium alloy grade L113 (Table 1)which was anodised using phosphoric acid. This pretreatment is recommended for adhesively bonding aluminium alloy since it is reported to give the best resistance to water of any of the known pretreatments for aluminium. ^[8]

It is now realised that the initial selection of thermoplastics was very fortunate, in that these polymers give very good adhesion to substrates such as aluminium alloy. If a different material such as polyethylene had been selected as a coating material in the early stages of the investigation, the poor results which would have ensued would probably have caused a premature curtailing of the work. However, even polyethylene can be persuaded to adhere to metals provided that the polymer chemistry is specially modified.

Table 1: Material analysis - L113 Aluminium alloy*

*Chemical laboratory report No. CCA Certificate No. 292

Welding Technology

To join components coated with thermoplastic by melting coatings, a polymer welding technology is required. Since some substrates may be sensitive to heat, a technique which produces maximum heat at the joint line is preferred, such as resistive implant welding. With this technique, an electrically conducting implant material is trapped between the two components to be joined and an electric current is passed through it. The electric current causes the implant to heat up and eventually softening or melting of the thermoplastic coatings occurs. With suitable applied pressure, and after sufficient time (predetermined) a weld between the coatings is formed (Fig.1).

Other welding techniques may be applied to PCM components and the selection of the optimum technique will in general be a function of factors such as types of materials to be joined, thermoplastic coating type, component geometry, required production rate, equipment cost.

Preparation of Specimens for Environmental Exposure

The aim of this work was to determine resistance to moisture of the PCM interface between the thermoplastic PE1 ^[5] and phosphoric acid anodised aluminium alloy. For this reason, joints were made in aluminium alloy using PEI as the thermoplastic coating, and welded using a piece of unidirectional carbon fibre reinforced PEEK1 as the resistive implant. In this case, application of electric current through the carbon fibres caused melting of the PEEK and then the PEI coating to form a weld. Fortunately, PEI and PEEK are miscible in molten form ^[9] and produce a mechanically strong polymer blend.

Coupons of LI13 grade aluminium alloy measuring 20mm x 60mm x 1.6mm were anodised in phosphoric acid and then dipped in a solvent solution of PEI. When the solvent had evaporated, the coupons were laid in a jig for welding (Fig.1). The weld was assembled using 150µm of extra PEI material between the carbon fibre implant and the substrate to allow some displacement of molten polymer from the joint during welding.

Welds were made using a direct current of 33 amps which was applied in two stages - a linear ramp up for five seconds followed by a weld time of 30 seconds. A pressure of 2.75 N/mm ² was applied over the 20 x 1Omm joint overlap area for the duration of the heating time and for 60 seconds afterwards.

Welded specimens were divided into five groups as shown in Table 2, each group comprising at least five specimens.

Table 2: Environmental exposure applied to specimen groups

Specimens in group 5 were manufactured in the same way as those in group 1 except that additional PEI was inserted around the carbon fibre implant to prevent any electrical shorting between implant and aluminium alloy substrate during welding.

Environmental exposure

Specimens in groups 3 and 4 were exposed to an environment of 100% relative humidity for 1000 hours where the temperature cycled from 42 to 48°C and back to 42°C in 60±5 minutes. ^[10] Specimens in group 4 were subjected to a mechanical stress of 2kN during this time.

Examination and testing

Specimens were mechanically tested using a tensile testing machine and their single lap shear strength recorded. In addition, some of the failure faces of the tested specimens were analysed using X-ray photoelectron spectroscopy (XPS). This technique produced an elemental analysis of opposite faces of failed specimens. This knowledge allows the determination of failure type. If the elemental analysis is similar on either side of the fracture, then the failure is likely to be cohesive, ie the fracture face ran through a single material rather than at an interface between materials. If the elemental analysis is dissimilar on either side of the fracture, it is likely that failure occurred at an interface (adhesive failure).

Cohesive failures are generally preferred because it is much easier to predict the mechanical properties of joints which fail in a material (whose bulk properties should be obtainable) rather than at an interface whose properties may be difficult or impossible to obtain.

Results

Results of the single lap shear tests of the welded specimens in each of the five groups are shown in Fig.2. Visual inspection of tested specimens showed that failures occurred in mixed mode, ie the fracture face passed through the PEI and implant on most specimens. joint single lap shear strengths corresponding to 47MPa were recorded in one case which was an excellent result (Fig.3). Despite a high degree of scatter in groups 1 to 4, the general trend was that PCM joints exhibited good resistance to moisture over the 1000 hour exposure time. This time had been calculated as being sufficient to allow moisture to diffuse to the centre of the joint. ^[11]

The results of the XPS analysis for one of the specimens from groups 2, 3 and 4 are shown in Tables 3, 4 and 5 respectively. Figures 4 and 5 show single lap shear specimens from groups 3 and 4 respectively.

Table 3: XPS results for a failed specimen left in a desiccator for ten weeks before shear tearing

Table 4: XPS results for a failed specimen exposed to 100% RH at 42 to 48°C for 6 weeks (1000 hours)

Table 5: XPS results for a failed specimen exposed to 100% RH at 42 and 48°C while under a load of 2kN for 6 weeks (1000 hours)

Discussion

The large degree of scatter in all results except group 5 was an obvious cause for concern, although the fact that this was reduced in group 5 indicates that electrical shorting between the carbon fibre implant and the aluminium coupons was a major contributory factor.

Specimens stored in a desiccator for 1000 hours exhibited the highest average single lap shear strength. This can perhaps be explained by reduction in mechanical strength which the thermoplastic PEI exhibits under the influence of moisture. ^[5] However, without detailed analysis of the stress distribution in the single lap shear joint, it is difficult to demonstrate conclusively this effect.

Comparison of these results with those obtained by adhesively bonding aluminium alloy with epoxy resin (Fig.6), shows that the initial lap shear strength of the epoxy bonded specimens was slightly better than the PCM joints. However, after 1000 hours in water at 50°C the reduction in strength of the epoxy bonded specimens was approximately 5MPa. This was a significantly greater reduction than in the corresponding PCM joint strengths after 1000 hours exposure to 100% relative humidity. It is difficult to draw firm conclusions from this comparison however, because the environmental tests and joint geometries were different.

Results of the XPS analyses show that elemental proportions were similar on each side of the fracture surface for each specimen examined. This means that the failures were probably cohesive in each case.

Absence of aluminium from the analysis for specimens from groups 2 and 3 (Tables 3 and 4), indicates that failure took place in the PEI, the PEEK or around the Carbon fibre. However, presence of aluminium in the analysis for the specimen from group 4 (Table 5) means that the failure ran through the aluminium alloy. This is perhaps a surprising result - if the aluminium were detected on one face only the failure would be adhesive, but the aluminium was present on both faces meaning that the failure occurred cohesively in the aluminium alloy. Work by Venables ^[l2] has shown that this can be explained by the fact that moisture can cause hydration of certain pre-treatments such as phosphoric acid anodising. The hydrated layer is physically weak and failure may often occur through this hydrated layer leaving aluminium on both failure faces.

Of eleven specimens analysed using XPS, only two showed small traces (<2.5%) of atomic aluminium on one face only. This would tend to suggest some adhesive failure in these two specimens, although this is not certain due to the complexity of the failure mode.

Practical Significance

Possible applications for the PCM joining technique are many and varied, but all have been limited bv a lack of knowledge concerning the environmental resistance of such joints. This investigation has generated preliminary data. Results generally appear to demonstrate that PCM joints in aluminium alloys can be resistant to moisture. This should allow the development of the technique for certain applications where environmental resistance is an important factor.

Conclusions

• Despite high levels of scatter, PCM joints in aluminium alloy have demonstrated good resistance to moisture.
• Failures in PCM joints exposed to moisture were almost exclusively cohesive.
• One of the main causes of scatter in the single lap shear strength of joints is probably electrical shorting between implant and aluminium alloy coupons during welding.
  

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