Lasers make advances in welding of automotive aluminium alloys

TWI Bulletin, March/April 1995

Ian Jones graduated from Jesus College, Cambridge with a degree in Natural Sciences, specialising in Materials Science. He joined The Laser Centre at TWI in 1989, and since then has worked on a wide range of laser applications in the Laser Processing Section. Specifically these have included development of high power laser welding of steel, nickel-based, titanium and aluminium alloys, as well as plastics processing. Ian is also involved in EUREKA initiatives for the advancement of laser processing in European industry.

Laser welding of aluminium sheet with filler material gives improved strength and formability. Ian Jones describes the techniques used and the materials selected to give the best results.

Laser welding offers a rapid, flexible, low distortion, automated manufacturing route for many automotive components. Laser welding is already being carried out for steel and coated steel applications, such as butt welding of sheets for tailored blanks, welding of pre-machined gear components and for lap joints in sheets as an alternative to resistance spot welding.

Two main types of laser can be considered for sheet metal welding:

• CO ₂ lasers;
• Nd:YAG lasers

They are both available as sources giving either continuous or pulsed power output.

The use of aluminium alloys as sheet, extrusions or castings in automotive industries is increasing with the continuing drive towards reduced weight, improved efficiency and recyclability. Resistance spot welding of aluminium is difficult, so there is considerable interest in other processes, such as laser welding, especially for sheet metal processing. ^[1-4] However, aluminium alloys have been notoriously difficult to join using lasers due to problems related to their high surface reflectivity, high thermal conductivity, and for some alloys, low boiling point constituents. These, and other material-related issues, have led to weld and HAZ cracking, porosity, degradation in mechanical properties and inconsistent welding performance. Understanding and controlling such problems have been uppermost in the recent research on butt welds in automotive aluminium alloys. The objective of the present work was to allow easier selection of the laser source, alloy type and filler wire for a given application.

Crack sensitivity

Two forms of weld-related cracking occur; HAZ liquation cracking and solidification cracking. In this work, the occurrence of solidification cracking was quantified in relation to material composition and welding conditions using a self-restraint cracking test. ^[5-7] The test involved the use of a tapered specimen as a development of an earlier test used by Houldcroft.

Mechanical properties

The performance of the laser welds, with and without filler, was evaluated by both tensile and formability testing. The formability evaluation is particularly important to the potential use of aluminium for tailored blank application. A biaxial bulge test, in which the sample was hydraulically stressed under biaxial loading, followed by measurement of the height of deformation before failure, was used in this work as a measure of formability.

Porosity

Porosity occurs in three forms: throughthickness blow holes, large irregular pores and small spherical pores. X-ray radiography was used to quantify the occurrence of porosity.

Results

A comparison was made between butt welds made using a continuous output 5kW CO ₂ laser and a pulsed output 2kW Nd:YAG laser. The CO ₂ laser was used to show the effect of adding filler material to the welds. The welding conditions and a summary of the weld property results are shown in Tables 1 and 2.

Table 1 - Properties of aluminium sheet alloys CW CO ₂ laser welded with and without filler wires (all are 2mm thick except 1.6mm thick 6061 alloy).

Table 2 - Properties of Nd:YAG laser welded aluminium alloy sheets (2mm thick unless stated), using 2kW average power and 100Hz pulse frequency.

CO ₂ laser welding

Typical CO ₂ laser butt joints are shown in Fig.1 and 2, and indicate the effects of adding filler wire to a close butted joint. A slightly wider and more voluminous bead was associated with increased amounts of filler wire, weld properties varied with base metal and wire composition. The hot crack susceptibility of aluminium alloys was especially prone to changes in alloy content and Al-Mg-Si alloys (6061, 6082) showed a higher tendency to cracking than Al-Cu (2219) and Al-Mg alloys. The Al-Mg alloys are considered separately in Fig.3, which shows a clear trend of increasing cracking up to about 2%Mg and then a reduction as magnesium is further increased. If these results are extended to butt weld specimens (rather than the self-restraint test) cracking is only apparent in the most susceptible Al-Mg-Si alloys, and this could be reduced by using Al-12%Si wire in the welds.

When other weld properties are considered, formability (evaluated by tensile tests and a biaxially stressed bulge test, Table 1), was greatest in the 3%Mg alloy (5754) and improvements resulted from the addition of Al-5%Mg filler wire (5556A), giving tensile failure away from the weld fused zone. Failure strengths and elongation were lower compared with the parent material in the Al-Mg-Si alloys, but were improved slightly by the addition of Al-12%Si filler (4047A). Porosity was not a major problem when welding this thickness of material, and there were generally fewer than 15 pores (0.5mm diameter) or groups of smaller pores in 100mm of weld.

Pulsed Nd:YAG laser welding

The weld profile made using the pulsed Nd:YAG laser was slightly wider and smoother than the CO ₂ laser welds (see Fig.4). The hot crack susceptibility was also similar to that of the CO ₂ laser welds.

In terms of mechanical properties, formability was increased compared with CO ₂ laser welding, and maximum formability occurred at about 2-3%Mg content in the Al-Mg alloys (see Table 2 and Fig.5). Porosity levels were as low as the CO ₂ laser welds, except for two alloys (1200 and 6082).

Discussion

Laser welding of aluminium alloys has been successfully demonstrated previously, but the influences of laser type and alloy composition on mechanical properties had not been fully evaluated. This work has indicated clear trends in crack susceptibility, ductility, formability and tensile strength, with changes in these variables. Improved results have also been achieved for welding of alloy types and thicknesses of interest in automotive application by using a variety of techniques.

There were some tensile test failures away from the weld region, indicating that the requirements of failure in the parent material for welding of steel sheet for the automotive industry are close to being met in laser welding of aluminium. Improvements in formability have been achieved by using filler material and different weld profiles and weld thermal cycles, more improvements might be expected following further optimisation of these parameters.

Selection of 5000 series alloy

In general, the range of 5000 series (Al-Mg) alloy sheets welded to date, tended to give butt welds with low crack susceptibility and low levels of flaws. Peak formability and ductility appeared to be achieved at approximately 3wt%Mg, whereas peak weld strength and lowest crack susceptibility was achieved by increasing the magnesium content to at least 4wt%. The effects of other alloying elements were not studied in detail; this remains as an area where further developments might be achieved.

Selection of 6000 series alloy and filler wire

When the 6000 (Al-Mg-Si) alloys were welded, the welds formed had lower strength ( cf. base material), lower ductility and much higher crack sensitivity than the 5000 series alloys welded. This might be expected from these precipitation strengthened alloys, which would regain some of their original properties on post-weld heat treatment, if the weld-related cracking were controlled. Of all the as-welded properties, crack susceptibility was the simplest to control using filler wire. A 12wt%Si containing filler (4047) reduced cracking significantly.

Selection of laser source

Currently available CO ₂ and Nd:YAG laser sources were used and the following observations may be made about their potential application. The Nd:YAG laser has the capability of being delivered to the workpicce by flexible fibre optics, whereas the CO ₂ laser requires a more precisely mounted mirror-based system. This would make the Nd:YAG laser potentially cheaper to apply to complex applications. The Nd:YAG laser is, however, available only in powers up to about 2.5kW, so processing speed is restricted to 0.8-1.6 m/min when welding aluminium sheet (2mm thick) compared with 5.0-7.5 m/min with the 5kW CO ₂ laser. Possibly related to these differences in power and speed, and hence weld width differences, are the differences in mechanical performance. In most materials tested, the ductility and formability of the welds were higher using the Nd:YAG laser, while the crack susceptibility and tensile strength were often similar. It may, therefore, be advantageous in certain applications, to use the slower Nd:YAG laser welding. The results also suggest that either development of high power Nd:YAG lasers or solutions giving wider welds from CO ₂ laser processing might lead to further improvements in mechanical properties.

Conclusions

Laser welding of 1.6-2.0mm thick aluminium alloys using a 5kW continuous wave CO ₂ laser and a 2kW pulsed Nd:YAG laser followed by assessment of various mechanical properties, has led to the following conclusions:

• Laser welding can be applied to 1.6-2.0mm aluminium sheet, giving reliable butt and lap welds at processing speeds of around 6 m/min, using a 5kW CO ₂ laser and 1 m/min using a 2kW Nd:YAG laser.
  
  
• Solidification crack sensitivity measured using a self-restraining crack test, was highly dependent upon the composition of the alloy used, but not so dependent on the laser type. In 5000 series (Al-Mg) alloys, there was an increase in cracking up to ~2%Mg, followed by reduced cracking with further increases in Mg. Much higher crack sensitivity was observed in 6000 series (Al-Mg-Si) alloys, except when 4047 filler wire (Al-12wt%Si) was used.
  
  
• Formability (measured using a biaxial bulge test) and ductility (from a transverse tensile test) were greatest for Al-Mg alloys containing approximately 3wt%Mg. Up to 15% elongation was shown by 5754 alloy in CO ₂ and Nd:YAG laser welding. Tensile properties of the weld were high for these alloys, with over 95% of the parent material strength, especially when high Mg (~5wt%) filler wire was used. The 6000 series alloys in the as-welded condition, gave rise to lower tensile strengths (58-72% of parent) and elongations (0.7-3.0%) than the 5000 series alloys.
  
  
• Porosity levels ranged from 1 to 24 small pores and groups of pores per 100mm of weld bead, but showed no clear association with alloy, laser or mechanical properties achieved.
  
  
• Careful selection of alloy type and laser type is required for any given application. Special attention will need to be given to productivity, application complexity, weld strength, weld formability and cracking tendency in the requirements of any given design or manufacturing route.

References

Acknowledgements

Much of the technical content of this work is from projects funded by the Information and Manufacturing Technology division of the Department of Trade and Industry, the Industrial Members of TWI and the Core 2 project of EU194 (Industrial Applications of High Power CO ₂ Lasers).