Laser welding of laser cut edges

TWI Bulletin, March/April 1993

Italian born Carlo Ferlito studied chemical engineering at 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. Whilst at TWI his responsibilities lay in conducting research projects on laser processing with particular attention to laser cutting applications.

For some applications (notably in the aerospace industry), laser cut components need to be welded with high quality edge preparation to produce high performance joints. As Carlo Ferlito explains, this is particularly important in materials such as stainless steel, nickel alloys, titanium alloys and aluminium alloys.

The aim of this work was to evaluate laser welding of laser cut edges using low pressure oxygen, and high pressure inert gas, and the following materials:

• Stainless steel AISI 316, 3mm thick;
• Nickel alloy, 6mm thick;
• Aluminium alloy 2024, 2mm thick;
• Aluminium alloy 5754,1.6mm thick;
• Titanium 6Al4V, 3mm thick.

Laser cutting has become a wholly accepted industrial production process. This has been due mainly to the flexibility of the process and the fine quality cuts which can be produced. A wide range of cutting applications, both metallic and non-metallic, has been developed. For metals the most common fabrication application is cutting components from sheet as finished or semi-finished parts. Industrial acceptance of the laser cutting process can be attributed to a number of factors, not least of which is the achievable component quality and accuracy.

One of the major advantages lasers is their potential to perform more than one process. A laser system capable of cutting and welding would eliminate the need for any post-machining operation, provided that the as-cut edge could be welded with high integrity and consistency. However, using oxygen as an assist gas for laser cutting leaves an oxide layer on the as-cut edge and there may be detrimental microstructural changes in the region where the material has been heated during cutting. Therefore, a high pressure inert gas laser cutting technique which can produce dross and oxide free edges was also evaluated in this programme.

Experimental

Equipment

All cutting trials were carried out using a fast axial flow 1.5kW CO ₂ laser, which generates a TEM 01* beam mode. This laser can operate in continuous wave and pulsed mode, with pulsing frequencies variable between 100 and 1000HZ. The focusing system used for the cutting trials was a zinc selenide meniscus form lens with a 27mm diameter and 127mm focal length. The zinc selenide surfaces of the lens were coated to reduce reflections of the laser beam to a minimum. With low pressure oxygen assist gas a 2.5mm thick lens was used, while a 5mm thick lens was adopted when high pressure assist gas was applied. The beam exit nozzle diameter was 0.8mm and of a circular form. The gas pressure was measured at the nozzle exit. The circularly polarised beam from the laser was directed at the focusing lens after reflection by a single Cu/Au mirror.

Laser welding
Welding trials were carried out using a fast axial flow 5kW CO ₂ laser with a reflective beam focusing system. Parabolic mirrors were used to produce a high power density in the focused spot. For the trials which used powers between 3.5 and 5kW, a TEM 01* beam mode was used. Commercial purity helium was used as assist gas, directed via a shielding device which used a plasma control gasjet and a trailing gas flow to protect the welding area from oxidation.

Procedure

Cutting speed, gas pressure and laser power (continuous and pulsed) were varied to minimise edge oxidation when low pressure oxygen was used as assist gas. With this technique cutting trials were carried out on stainless steel, nickel alloy and a range of aluminium alloys. The same materials plus titanium were also cut using high pressure inert (argon for titanium, nitrogen for the other materials) gas assist, and the laser parameters were varied to determine conditions which had dross and oxide free edges.

Welding of laser cut edges
For all materials investigated, welding parameters were varied to select conditions for the trials. Attention was given to shielding gas parameters to achieve clean, low oxidation welds. When suitable parameters were selected, samples cut by laser were welded in a butt joint configuration. Mechanically cut samples, produced using a bandsaw and guillotine, were also welded for comparison. Samples were produced for assessment of quality.

Quality assessment

All cut edges were visually examined for dross and cracks. Roughness measurements were taken on selected samples of each material. The reference point used was at two thirds of the material thickness from the top surface, except for aluminium (less than 2.0mm thick) where the measuring point was at one half of the material thickness, in accordance with DIN 2320.

Assessment of welded laser cut edges
For each material the welds were visually examined to detect defects such as undercut, oxidation, irregular top and underbead. Selected samples were sectioned, macrographs taken and laser welded samples from edges, both laser and mechanically cut, were tensile tested. Radiographic inspections were carried out to detect porosity and cracks. The hardness of the welded samples was also measured.

Results and discussion

Laser cutting and welding

Table 1 Summary of laser cutting conditions used in experimental trials

Table 2 Results summary for laser welding of laser cut edges

These trials have produced welds on laser cut edges in stainless steels, nickel alloys and titanium alloys without further edge preparation. Laser welding of aluminium alloys previously cut by laser proved to be more difficult and significant further work would be required to improve this situation.

Stainless steels
The advantages of using high pressure inert assist gas have been demonstrated, see Fig. 1 Dross and oxide free edges were obtained on 3mm thick AISI 316 stainless steel with extremely low roughness. The heat affected zone (HAZ) width was negligible and no post-machining of edges was needed. The use of low pressure oxygen assist gas caused the formation of an oxide layer on the surface of the cut edge and an increase of surface roughness. Using low pressure oxygen assist gas, the HAZ was wider, and this could introduce corrosion related problems for some applications.

Welding of cut components in a butt joint configuration was found to be possible where fit-up was good. This was achieved both where edges had been mechanically cut, and high pressure inert assist gas laser cut. Where oxygen was used as assist gas, the fit-up was poor and the weld appearance irregular. Nevertheless, the presence of oxide on the edge of the butt joint did not cause dramatic tensile strength reduction on the welded part. Welds on edges cut with high pressure inert gas were largely porosity free and their tensile performance directly comparable with welds from mechanically cut edges.

The results of the trials on stainless steel were encouraging. The high pressure inert gas assist laser cutting process produced almost square edges with little distortion so subsequent welding of the component was possible. From a first short investigation, clean and oxide free welds were produced, provided that a suitable gas shielding arrangement for the laser welding operation was used.

It was therefore summarised that for stainless steel up to 3mm thick, high pressure inert gas laser cutting can produce dross and oxide free cut edges which can subsequently be laser welded.

Nickel alloys
Nickel alloys normally exhibit similar behaviour to stainless steels when laser cut. Differences became evident at thicknesses over 4mm. The thickness of material used for these trials (6mm) and the presence of high melting point and viscosity alloying elements caused burrs to form and a hard layer of oxide appeared on edges cut using oxygen low pressure assist gas. Such poor edge quality caused fit-up problems for welding trials. The high melting point, high viscosity oxide layer caused undercut and lack of fusion when welded in butt joint configuration.

This result was considered unacceptable and it is thought that low pressure oxygen assist gas is not suitable for producing cut edges for laser welding without post-cut machining. High pressure inert gas assist removed the molten material out of the kerf to produce oxide free edges, but dross could not be completely removed with a sample thickness of 6mm. This was the cause of fit-up problems for some of the laser welded samples. Optimisation of the nozzle design and gas flow in the cut kerf may be able to produce dross and oxide free cut edges.

The weldability of high pressure inert gas assist laser cut samples showed promise in terms of porosity and weld appearance achieved. Comparisons were made with machined edges where a similar weld appearance to the high pressure inert gas assist laser cut edges was obtained (Fig.2). Tensile tests showed that all welded samples, regardless of method of edge preparation, broke at stress values very close to the maximum tensile strength of the material.

Samples welded from laser cut edges failed in the HAZ during tensile tests, while those from mechanically cut edges failed in the parent material. This difference could be related to the small amount of undercut present at the side of the laser weld which causes a reduction in cross-sectional area.

For nickel alloy up to 6mm thick, high pressure inert gas laser cutting is suggested as a technique where subsequent welding of cut components is required. Further optimisation of the laser cutting and welding parameters is required, particularly in thicker sections (> 3mm) to optimise weld appearance and properties. This may need the use of wire feed techniques to eliminate undercut completely.

Aluminium alloys
Results achieved for laser cutting aluminium alloys were encouraging. The cut quality in terms of roughness and visual appearance of the edges were high where laser dross and oxide cut edges were produced. Best results in terms of edge quality were achieved using high pressure inert gas laser cutting. The results of fatigue tests on these produced from 2024 aluminium alloy confirmed that high pressure inert gas produced the best fatigue performances, close to those of mechanically cut and milled edges.

No fit-up problems were encountered during the welding of laser cut aluminium alloy edges. However, trials have demonstrated how difficult it is to produce acceptable welds in aluminium alloys. For example, 2024 alloys showed a 45% reduction in strength after welding due to occurrence of defects in the weld metal such as undercut and porosity. Furthermore, the fact that this alloy is heat treatable makes it sensitive to strength loss from thermal processes. Welds in 5754 alloy also showed reduced ductility in tensile testing but had a similar elongation to failure and tensile strength compared with guillotined edges.

Further work is being undertaken to optimise CO ₂ laser welding procedures for aluminium alloys where encouraging results have been achieved.

Titanium alloys
Results showed that the titanium 6% aluminium 4% vanadium alloy chosen for this work is a good candidate for laser processing. High pressure inert gas laser cutting proved to be successful, fast, and able to produce cut quality comparable with, and even superior to, mechanical methods.

The weldability trials on this material also gave positive results. Parameter studies on the gas shielding system were necessary to produce bright and oxide free welds. The results of quality assessment and tensile tests showed that laser welding of laser cut parts, produced welds of similar appearance and tensile properties to those produced from mechanically cut edges, see Fig.3.

Conclusions

This work has permitted the following conclusions to be drawn:

• Stainless steel 3mm thick, nickel alloy 6mm thick, 2024 aluminium alloy 2mm thick, 5754 aluminium alloy 1.6mm thick, and titanium alloy 3mm thick, were all successfully cut using laser techniques. High pressure inert gas laser cutting gave the best results in terms of cut quality and fit-up for subsequent welding;
• High pressure inert assist gas technique produced oxide and dross free edges on 3mm thick stainless steel, which were subsequently welded to produce joints of acceptable appearance and tensile mechanical integrity;
• Nickel alloy 6mm thick can be cut using high pressure assist gas, however, the process could be improved. The laser cut edges when welded produced welds with some defects but good tensile properties;
• Two aluminium alloys were successfully laser cut with significant improvements in edge quality achieved using high pressure inert gas techniques, when compared with results achieved with low pressure oxygen. Laser welding process produced an unacceptable number of defects and poor tensile mechanical integrity and ductility. More work is needed to improve the general weldability of aluminium alloys using CO ₂ lasers;
• Titanium alloy 3mm thick was successfully cut and subsequently laser welded. Laser welds produced from the laser cut edges had a similar appearance and tensile properties to those produced from mechanically cut edges.

Recommendations

For materials evaluated in this project, C0 ₂ laser welding of laser cut edges should be industrially viable if high pressure inert gas is used for cutting. Laser welding of laser cut edges of stainless steel and Ni and Ti alloys is possible without using post-cutting machining and cleaning procedures. This may have economic implications for manufacture of some aeroengine components, for example, where cost savings could be made.