Trends in laser cutting of advanced materials

TWI Bulletin, September/October 1992

Geoff Street joined TWI in 1965 and has been actively involved with material processing for many years. He joined the Laser Centre six years ago, becoming involved in CO ₂ laser cutting and welding.

At the time of writing, Geoff was particularly interested in developing advanced materials processing techniques and also specialises in laser safety requirements.

Steve Riches joined The Laser Centre from the Microjoining Section of the Ceramics and Precision Processes Department in July 1990. Steve is Head of the Laser Processing Section where he has been involved in, and advised on, all facets of the process technology including cutting, welding and surface treatment of a wide range of metals and non-metallic materials.

Italian born Carlo Ferlito studied chemical engineering in Padova University, Italy and specialised in materials science and laser technology.

He joined TWI in December 1990 after working for the Italian technology research organisation RTM. Now at TWI his responsibilities lie in conducting research projects on laser processing with particular attention to laser cutting applications.

Development of advanced materials including polymer composites, ceramics and metal matrix composites has created problems for conventional contact cutting processes. Geoff Street, Carlo Ferlito and Steve Riches investigate use of CO ₂ lasers to cut such materials.

In parallel with the increased industrial acceptance of laser cutting as a viable economic and technical process, there have been considerable advances in development of plastics, ceramics and metal based materials.

There has been significant development of composite materials, where particulates, whiskers or fibres are embedded in a matrix to produce materials with increased specific strength, higher temperature capabilities and reduced wear rates. However, introduction of the reinforcement adds difficulty to the cutting process, particularly where contact processes are involved, e.g. in milling, sawing or punching techniques. This is because the high strength of the reinforcement causes wear of the machining tool or press tool. As a result, there is an interest in use of non-contacting cutting techniques to profile shapes in these advanced materials.

Although each set of materials examined in this work (polymer composites, ceramics and metal matrix composites) is suited to a number of cutting techniques the two main non-contact techniques which can be considered are laser cutting and water jet cutting.

Abrasive water jet cutting involves forcing pressurised water (up to about 4000 bar) through a small diameter nozzle (0.1-0.8mm diameter) at typical speeds of 800 m/sec and sometimes adding abrasive particulate matter ( e.g. silica garnet or alumina particles 0.25-0.5mm diameter). ^[1]

This work examines the application of laser cutting to a number of advanced materials which can be grouped into the following:

• Polymer composites;
• Ceramics;
• Metal matrix composites.

For each category, the authors review the materials, their general characteristics and properties and the available laser techniques which can be considered for cutting them.

Polymer composites

Polymer composites are based on use of carbon, boron, glass and aromatic polyamide (aramid or Kevlar) fibres, which provide the necessary strength and stiffness, in conjunction with supporting resin systems to produce lightweight structural materials. Many polymer composites have been developed for a range of industrial applications including aircraft, automotive and ship components, and sports equipment such as fishing rods, golf clubs and tennis rackets. The most common fibre used in load bearing applications is glass, but for special high performance requirements carbon and aramid fibres are used. The resins used in these applications include both thermosetting ( e.g. epoxy) and thermoplastic ( e.g. polyetheretherketone (PEEK)) materials.

Addition of a reinforcement phase to a resin can increase wear of contact cutting tools because of the hardness of the fibre ( e.g. glass). In addition, the cutting process tends to pull the fibres from the matrix resulting in a 'fuzzy' edge and voids at the composite edge. For these reasons, difficulties have been encountered in milling, sawing and punching of polymer composites.

One contact technique, which avoids this problem is use of ultrasonically excited knives for thermoplastic composites where the ultrasonic energy is used to soften the material locally which is then displaced. This technique can be used for randomly orientated fibres which are displaced within the softened matrix ^[2] and has been applied to cutting continuous fibre prepreg layer 1mm thick.

Use of lasers for cutting polymer composites has been studied at TWI and elsewhere on aramid, carbon, boron and glass fibres with epoxy resin, polyamide and polysulphone matrices ^[3-6] . Work at TWI has been on CO ₂ laser cutting, using a 1.5kW laser.

In general, the results showed that the fibre type had a significant influence on the cut quality, carbon fibre reinforced materials proving most difficult to cut. The main problem with the carbon fibre reinforced composites was fibre/resin separation. Figure 1 shows cut fibres protruding from the resin matrix. This effect became more severe as the thickness of the composite increased and an example of a laser cut in a 2mm thick carbon fibre/PEEK resin composite is shown in Fig.2.

Fig. 1. Laser cut edge on carbon fibre (PEEK APC2) plastic composite (0.2mm thick) showing protrusion of carbon fibres. CO ₂ laser cutting conditions: 500W power; 4.5 m/min speed; focus position at surface; assist gas at 4bar - A r

Fig. 2. Example of ring cut in carbon fibre/PEEK resin 2mm thick by CO ₂ laser cutting. Laser conditions: 950W power; pulsed: 250µs on 60µs off, 1.0 m/min speed; assist gas N ₂ - 4bar

For glass and aramid reinforced composites, less separation between the fibre and the resin was observed but the cut edges were still characterised by carbon deposits. Fig.3 compares a laser cut and water jet cut. This difference in behaviour can be related to the thermal properties (conductivity, vaporisation temperature) of the carbon, aramid and glass fibres.

Fig. 3. Comparison of typical laser cut (right) and water jet (left) cut edges in aramid fibre/epoxy resin plastic composites

From this work, it was concluded that CO ₂ laser cutting is not capable of producing cuts in carbon fibre reinforced polymer composites without a degree of separation between the matrix and the fibres. If avoiding this separation is considered to be vital to the performance of the components, alternative cutting techniques, such as water jet have to be considered.

Also, the laser cutting process may create hazardous by-products. ^[7] For example, aromatic compounds and polycyclic aromatic hydrocarbons have been identified from the degradation of aramid fibre composites. Further work to examine this topic is necessary, but for all polymer composites effective fume extraction and retention would be required.

Ceramic materials

There is increased interest in use of ceramics in engineering components because of their attractive properties such as wear and corrosion resistance at high temperatures, good insulation and high temperature strength. The main ceramics considered for these types of application include Al ₂O ₃ AIN, SiC and Si ₃N ₄. Applications are being found in the electronics industry for ceramic substrates (Al ₂O ₃ AlN), in the automotive industry for combustion chambers and pistons and as heat exchangers or high temperature burners in the chemical and power generation industries respectively. In addition to the base engineering ceramics, fibre reinforced ceramics are being developed which are showing benefits in terms of strength and toughness. These materials usually consist of ceramic fibre, particles or whiskers, for example of SiC, embedded in a glass ceramic matrix.

Although ceramics can be shaped using conventional cutting and grinding techniques, the process is time consuming, expensive, gives problems with tool wear and has low yields because of the inherent brittleness of the ceramic materials. In addition, these techniques are not suited to production of curved shapes on components.

A laser has several advantages in cutting difficult to machine materials, such as non-contact machining and high precision cutting at high speeds. These advantages have been exploited in laser cutting of alumina substrates (typically up to 1mm thick). The brittleness of the substrate is exploited by only carrying out a laser scribing process (using CO ₂ lasers predominantly) at speeds of up to 15 m/min to weaken the ceramic, which is subsequently broken off to form substrates. It is also possible to produce through-section cuts in this material using CO ₂ and Nd:YAG lasers when a smooth surface is required for metallisation, but with the penalty of reduced production speed.

Although these techniques have been used industrially for over 15 years, adoption of laser cutting of ceramics has not been widespread, particularly with engineering ceramics where thicker sections need to be shaped. The main problem with further exploitation of the technique is the poor thermal shock resistance of the materials and the performance of components with laser cut edges.

In trials undertaken at TWI using a 1.5kW CO ₂ laser, the problem of thermal shock resistance of some ceramic materials (silicon nitride and aluminium nitride) has been approached by use of pre- and post-heating up to 400°C and of pulsed laser energy. The work showed that cracking on the cut edge could be reduced but not completely eliminated, see Fig.4. This shows a microcrack on the cut edge of a 1mm thick silicon nitride sample. Further work is necessary to optimise pre- and post-heating systems or to examine the use of extended laser heat sources to reduce the severe thermal gradients endured by the ceramic materials during laser cutting.

Fig. 4. Laser cut edge on 1mm thick silicon nitride:

Fig.4a) Whole cut surface

Fig.4b) Cut surface showing microcrack. CO ₂ laser cutting conditions: 1400W power; 250µs on, 60µs off pulsing; 1.5 m/min speed; focus position at surface; assist gas N ₂ - 4bar; pre/post-heating - 400°C

Metal matrix composites

Development of metal matrix composites has realised several advantages over their monolithic counterparts including high specific strength and stiffness, lower coefficients of thermal expansion and improved high temperature capabilities. When the particulate reinforcement is between 15-30% by volume the materials become potentially attractive for weight critical structural applications, particularly in the automotive and aerospace industries. If the volume fraction is increased to 40-55% a composite with a low coefficient of thermal expansion and high thermal conductivity results which is suitable for heat sinks in microelectronic packages or for the packages themselves. The majority of work to date has involved evaluation of reinforced aluminium alloys (SiC reinforcement) but Ti, Ni and Cu based alloys are also being developed with SiC and Al ₂O ₃ and other reinforcements.

As with polymer composites, there are problems in removing the matrix and leaving the fibre materials protruding for contact techniques (tool wear) and non-contact techniques ( e.g. laser and water jet cutting).

Limited laser cutting trials have been performed at TWI using a 1.5kW CO ₂ laser on the following metal matrix composites:

• Aluminium-lithium (8090) alloy reinforced with 20%(wt)SiC particulate: for this material, the cut edge of a 1mm thick sample showed only a small amount of dross and a cut surface where no fibres were protruding from the matrix ( Fig.5).
• Ti-6V-4Al alloy with SiC fibre reinforcement unidirectionally aligned: for this material, the cut edges of a 1mm thick sample revealed fibres protruding from the matrix ( Fig.6).

Fig. 5. Laser cut edge on 2mm thick Al-Li-SiC (20% by vol) metal matrix composite:

Fig.5a) Whole cut surface with small amount of dross on underside cut edge;

Fig.5b) Close up of cut edge. CO ₂ laser cutting conditions: 1400W power; 250µs on, 60µs off pulsing; 1.5 m/min speed; focus position at surface; assist gas N ₂ at l0bar.

Fig. 6. Laser cut edge on 1mm thick TiC fibre metal matrix composite:

Fig.6a) Whole cut surface;

Fig.6b) Close up of cut edge showing protruding C-fibre. CO ₂ laser cutting conditions: 810W power; 250µs on, 60µs off pulsing; 3.0 m/min speed; focus position at surface; assist gas N ₂ at 10bar.

From this work, two distinct modes of behaviour in laser cutting of metal matrix composites can be identified. One appears to be where the composites behaved effectively as a metal and the other where fibre and resin separation occurs in a similar way to carbon fibre reinforced polymer composites. However, at present, there is little information available on laser cutting parameters for metal matrix composites with different types of reinforcement.

Other laser systems

In addition to CO ₂ lasers, other laser systems can be considered for cutting advanced materials. The two major industrial laser types are Nd:YAG and excimer.

Nd:YAG lasers, with their shorter wavelength of 1.06µm, have been used industrially for high quality cutting of alumina substrates, where the cut surface is generally considered to be less rough, and therefore more suitable for post-cut metallisation, than that produced using CO ₂ lasers. However, cutting speeds are lower than with CO ₂ lasers. Therefore, application of Nd:YAG lasers may be beneficial for engineering ceramic materials but without pre/post-heating problems with microcracking are still likely to occur.

For polymer composites little information is available on cutting performance at the Nd:YAG wavelength. For metal matrix composites, cuts with a better surface finish and reduced drosses may be achieved especially with a high pressure inert gas assist system, but the problem of different thermal properties of the reinforcement and matrix will remain.

For excimer lasers, the ultraviolet laser emission is fundamentally different from that of the thermal lasers (CO ₂, Nd:YAG) described above. These lasers are capable of processing by photoablation (breaking chemical bonds through application of ultraviolet energy) and produce cut surfaces without thermal damage. This offers significant potential for cutting plastics and ceramics and these lasers are being used for drilling very fine holes (down to 10µm diameter) within complex printed circuit boards. However, although cut surfaces without thermal damage can be produced, there will still be differences in the ablation characteristics of the fibre and the matrix, where one component may be removed faster than the other, leading to the problem of fibre protrusion. For ceramics and metal matrix composites, the main problem of excimer lasers is the lack of power which restricts the rate at which materials can be cut.

In addition to the industrial lasers described above, other types are being developed such as CO lasers and Cu-vapour lasers. The cutting characteristics of these lasers on advanced materials have vet to be established.

Conclusions

Conclusions from the experimental programme on laser cutting of advanced materials were:

CO ₂ laser cutting of polymer composites is dependent on the properties of the fibre reinforcement and the thickness of the composite. For carbon fibre reinforced composites it was difficult to achieve cuts in material thicknesses over 4mm. In addition, separation of the fibre and the resin occurred. For aramid and glass fibres, these problems were reduced but carbon deposits were found on the cut edge. Potentially hazardous by-products may be produced by laser cutting and effective fume extraction and retention are necessary.

CO ₂ laser cutting of the ceramic materials silicon nitride and aluminium nitride is affected by cracking caused by thermal shock. Use of pre/post-heating (400°C) reduced the problem but instances of microcracking still occurred.

CO ₂ laser cutting of a 2mm thick Al-Li-20% by weight SiC (particulate) metal matrix composite produced cuts of a similar appearance to those achievable on a monolithic aluminium alloy.

CO ₂ laser cutting of a 1mm thick Ti-6V-4Al SiC fibre reinforcement (unidirectional) metal matrix composite produced cuts with SiC fibres protruding from the Ti alloy matrix.

Acknowledgements

This work was funded by Industrial Members of TWI and the Minerals and Metals Division of the UK Department of Trade and Industry.

References