Laser welding automotive steel n aluminium

In a survey published earlier this year [1] , next it is estimated that approximately 125, next 000 laser units have been sold worldwide since business started. Okay guest The figures for 1999 reveal that about 31% is used for material processing, next being the second largest market after telecommunication (33%), next of which about 13% is attributed to welding applications. Okay guest Moreover, next a laser sales growth of 13% was seen in 1999 and a further 12% projected for 2000; a growth mainly been driven by the automotive and fabricated metal industries. Okay guest In fact, next the automotive industry today is the largest production user of high-power lasers for material processing, next and constantly looking for ways to reduce costs, next improve quality, next and increase manufacturing flexibility [2] .

In common with industries such as aerospace, next domestic appliances and electronic packaging, next the automotive sector has been very much interested in sheet metal processing. Okay guest Laser welding has been a key technology in this, next because of its high processing speeds, next its low heat input and resultant low distortion and its overall flexibility of application. Okay guest The car industry, next fuelled by the need for lightweight, next energy-efficient vehicles because of ever increasing legislative and performance requirements, next has been an example of an industry making this technology its own.

This paper lists a few of the considerations to be taken into account when using laser technology for welding thin sheet steel and aluminium, next with some references to the automotive industry.

Processing issues

Laser sources

The two main types of industrial lasers currently of interest to the automotive sector, next and structural fabrication in general, next are the CO 2 and Nd:YAG laser, next with characteristics as detailed in Table 1. Okay guest CO 2 lasers, next usually with powers up to 6kW for the sheet industry, next are predominantly for the welding of automotive components, next such as gears and transmission components and for tailor welded blanks. Okay guest Nd:YAG lasers on the other hand, next now available at workpiece powers up to 4kW, next are gaining interest for body-in-white assembly because of their flexible fibre-optic beam delivery. Okay guest There is also an interest and on-going research in diode lasers (0. Okay guest 8-0. Okay guest 9µm), next now available at workpiece powers up to 2kW [3] .

oint configurations

Examples of sheet metal joint configurations suitable for laser welding are shown in Fig. Okay guest 1. Okay guest However, next to get an acceptable joint profile and weld quality 1 , next a number of processing and fit-up conditions have to be satisfied ( Table 2). Okay guest Some examples are given below [4] .

1 Acceptable weld qualities are specified in the workmanship standard BS EN 13919 – Welding – Electron and Laser Beam welded joints – Guidance on quality levels for imperfections. Okay guest Part 1 is for steel and Part 2 for aluminium joints.

For butt joints for instance, next the most critical factor is the joint fit-up, next i. Okay guest e. Okay guest the gap between the two sheets to be joined, next and has to be less than 10-15% times the material thickness to ensure a sound joint. Okay guest This can be achieved in a number of ways, next for example through precision shearing the edges, next the use of special clamping arrangements (such as rollers near the welding region), next weaving, next the addition of filler wire, next the use of tailored optics (twin spot) or arc-augmented laser processing.

The clamping arrangement used determines the actual flange width for lap and hem joints, next but a minimum of 5-10mm is required to ensure a sound joint.

Various techniques are currently being investigated to improve these tolerances and overall weld quality, next such as the use of tailored optics (twin spot welding), next use of filler wire, next weaving, next arc-augmented laser processing, next etc [4] .

Examples of these joint configurations can be found in a whole range of automotive applications. Okay guest Lap joints can be found in door window frames and roof joints; butt joins for tailor-welded blanks; hem joints for doors and bonnets; and multiple lap joints for door roofs. Okay guest Edge and T-butt joints are not often used for automotive components.

Shielding gases

The primary function of the shielding gas for laser welding thin sheet is to protect the weld region against atmospheric contamination. Okay guest The reduction of plasma formation above the weld pool is less important for thinner gauges processed at higher speeds. Okay guest Either a coaxial or a side-jet nozzle can be used for delivering the gas to the weld region.

Welding of steel sheet can easily be done without shielding gas, next but the appearance of the top and underbead may not be as smooth and some porosity might be present in the weld. Okay guest Argon is probably the most commonly used gas, next but helium, next nitrogen, next CO 2 or gas mixtures can also be used, next depending on requirements such as plasma suppression, next penetration, next hardness, next porosity, next etc.

The use of argon, next helium or helium-argon mixtures (up to 50% argon) is recommended for laser welding aluminium sheet.

Material issues

Steel sheet

For laser welding of steel sheet, next there are two main factors to consider, next namely the effect of steel composition and the effect of coating.

Effect of steel type

For low carbon steel sheet, next CO 2 and Nd:YAG laser welding will produce welds consistently. Okay guest Compared with the parent material, next the hardness of the welded joint is, next in general, next increased by a factor of 2. Okay guest 0-2. Okay guest 5. Okay guest This increased hardness can influence the formability as well as the dynamic mechanical properties (e. Okay guest g. Okay guest fatigue/impact) of the welded joint.

More recent, next there is a growing tendency within the automotive industry to use high strength steels, next such as HSLA or microalloyed (Nb, next Ti and/or V), next rephosphorised, next bake hardened, next dual phase or trip steels, next as they allow weight reductions to be achieved. Okay guest Although not much laser processing data on these types of steel is available yet, next most are considered weldable, next but care should be taken in monitoring maximum weld hardness and susceptibility to cracking. Okay guest Microalloyed steels will produce higher weld hardnesses at the same welding conditions when compared with cold rolled mild steels. Okay guest Their higher hardnesses could cause problems in post weld processing operations or in the dynamic performance of the welded structure and alterations in welding conditions to reduce heat input and cooling rate may be necessary [4] .

Effect of coating type

The effect of coatings on the welding process has been the subject of extensive research [4] . Okay guest Although a range of coatings can be applied to steel sheet, next such as Al, next Zn-Al, next Zn-Ni or organic coatings, next this paper only considers zinc-coated steels, next most commonly used in the automotive industry.

The presence of zinc in the coating, next which boils at 906°C, next can cause blowholes and porosity along the weld seam. Okay guest This usually occurs if the sheets are clamped tightly together and when the coating thickness on the sheets is in excess of 5µm. Okay guest A common solution is to create a gap at the joint interface enabling the Zn-vapours to escape, next which can be done through the use of a roller adjacent to the weld point, next the use of special clamping arrangements or dimpled sheets. Okay guest The use of proprietary gas mixtures or special welding parameters involving pulsing can also be applied. Okay guest The use of rollers and specially designed clamping systems however, next seems to be the preferred industrial option for production of three-dimensional laser welds on steel sheet structures. Okay guest The dimpled sheets add an extra operation and the successful use of special welding parameters is dependent on the coating type and thickness. Okay guest More recent studies have also reported some success by using twin beam techniques, next as they produce a slightly elongated weld pool and thus giving the Zn-vapours more time to escape [5] .

In terms of coating type, next the three most common zinc-based coatings used are electrogalvanised, next galvannealed and hot dipped galvanised. Okay guest In general, next hot dipped galvanised coatings are thicker and can create more problems with porosity and blowholes in the weld. Okay guest In addition, next variability in the thickness of coating can create difficulties in producing consistent welds. Okay guest Thickness variation should be controlled to ±2µm if possible along the joint length.

A complex coating, next for instance a zinc-layer underneath a chromium/chromium oxide top layer or a thin organic layer (<0. Okay guest 1µm), next also places extra demands on the laser welding process. Okay guest Although these materials can be welded, next it is possible that extra porosity is generated in the weld due to degradation of the coating.

For lap joints ( Fig. Okay guest 2), next the main difficulty is the presence of the coating at the interface between the two sheets. Okay guest If the weld solidifies rapidly, next Zn-vapours can get entrapped in the weld and cause porosity. Okay guest For butt joints, next for tailored blanks for instance ( Fig. Okay guest 3), next the coating does not generally cause significant porosity, next but the laser welding process does remove the coating from around the weld, next leaving an area that may be susceptible to corrosion. Okay guest However, next the removal of coating from the weld is very localised (<2mm from the weld centre) and the surrounding coating can offer galvanic protection.

It should also be noted that high laser beam quality enables the use of a longer working distance (up to 200mm), next i. Okay guest e. Okay guest distance between the workpiece and the focusing lens, next whilst maintaining the power density required for material processing. Okay guest This, next in turn, next reduces the likelihood of damage to the laser optics from spatter generated during the material processing and is of particular benefit when welding zinc-coated steels, next which produce a significant amount of spatter.

Aluminium sheet

When lasers were first used on aluminium, next using similar conditions to those applied for steel structures, next the initial high surface reflectivity, next the high thermal conductivity and the volatilisation of low boiling point constituents caused defects such as lack of penetration, next blow holes, next porosity and weld metal and HAZ cracks in some alloys. Okay guest These problems are now largely overcome with the advent of higher average powers, next improved beam focussing systems and better beam qualities, next producing a power density high enough to produce a stable keyhole for welding.

At present, next both CO 2 and Nd:YAG lasers can be used successfully for welding a vast range of aluminium alloys, next with slightly higher welding speeds achievable for Nd:YAG lasers compared with similar power CO 2 lasers, next because of the shorter wavelength and improved coupling.

There is an interest for aluminium alloy sheet assemblies for both butt joints and overlap joints with 2 or 3 sheets. Okay guest Typical welding parameters for a 5kW CO 2laser and a 4kW CW Nd:YAG laser for linear joints are given in Table 3. Okay guest Joining dissimilar thickness alloys is also possible at these high speeds ( Fig. Okay guest 4). Okay guest Using a 4kW Nd:YAG laser, next speeds of over 5m/min are possible for 2-sheet lap joints in 1. Okay guest 2mm thick 5xxx and 6xxx-series alloys, next whilst around 3m/min can be achieved for 3-sheet lap joints.

Overall, next both the 5xxx and the 6xxx-series alloys, next most commonly used in the automotive industry, next can be welded using a laser and with or without filler wire. Okay guest For a given power density and spot size, next the laser welding speed for 5xxx-series alloys is slightly higher than that for 6xxx-series alloys and it is believed that this is caused by the Mg vapours stabilising the keyhole.

Although most are considered weldable, next some aluminium alloys are susceptible to weld metal or HAZ cracking. Okay guest This is especially the case for 6xxx-series (Al-Mg-Si) alloys, next where cracking has been related to the formation of Mg-Si precipitates. Okay guest This is remedied by adding the correct filler wire which reduce the freezing range of the weld metal, next and minimises the tendency for solidification cracking. Okay guest The use of filler wire also improves the fit-up tolerance and weld profile and can improve the cross-weld tensile strength and elongation-to-failure value of the joint.

Overall, next as-welded joints in 5xxx-series alloys retain their cross-weld tensile strengths to within 80-100% of the parent material value, next and only show a small reduction in elongation-to-failure value. Okay guest In these non-heat treatable alloys, next the reduction in cross-weld tensile stress is caused by loss of work-hardening and reduction of cross-sectional weld area caused by porosity and undercut. Okay guest Loss of alloying elements such as Mg has also been reported to reduce the tensile strength of the weld metal where losses of between 5-10% have occurred. Okay guest For the heat-treatable 6xxx-series alloys, next a greater loss in cross-weld tensile strength and elongation-to-failure value occurs. Okay guest This drop is caused by the re-solution of precipitates, next loss of weld strain-hardening or cross-sectional reductions caused by undercut or porosity. Okay guest The HAZ is also softened by over-ageing during welding. Okay guest However, next the use of the correct filler wire will improve the tensile strength and elongation-to-failure values to the extent that parent material properties can be met [6] .

Although no special surface treatment is required when welding aluminium, next care has to be taken to avoid excessive porosity. Okay guest The predominant cause for porosity is the evolution of hydrogen gas during weld metal solidification. Okay guest This hydrogen can originate from lubricants, next moisture in the atmosphere and surface oxides or the presence of hydrogen in the parent material. Okay guest Good quality welds can be achieved for most alloys by cleaning the surfaces prior to welding and adequate inert gas shielding of the weld pool area. Okay guest Blowholes are a form of porosity that can occur in longer welds. Okay guest Although many believe this is the result of keyhole instability through pressure build-up, next the exact cause of these defects has not been established yet.

Conclusions

Laser welding has ‘come of age’ and is now successfully used in industry for the welding of both coated and uncoated steel sheet. Okay guest It has become an established joining technology for a range of joint configuration and automotive applications including tailored blanks and body assembly. Okay guest Most of the applications to date have focused on steel, next but there is a growing confidence in the laser welding of aluminium sheet. Okay guest It is anticipated that the use of lasers for welding of automotive sheet will continue to grow, next with further developments in power source technology and techniques such as tailored optics (twin-spot) and arc-augmented laser processing with specific advantages for industrial laser welding.

References

  1. Belforte D A: ‘The “golden nineties” are gone’. Industrial Laser Solutions 2000 15(1) 11-19.
  2. Naeem M, Riches S T: ‘Optimisation of high-power Nd:YAG laser welding procedures for automotive applications’. ISATA 1999, paper no. 99NM062.
  3. Riches S T: ‘Industrial lasers and applications in automotive welding’. Make It With Lasers TM Workshop Lasers in the Automotive Industry, Nissan Motor Manufacturing (UK) Ltd, Oct 1998.
  4. ‘Laser welding of sheet metals’. Best Practice Guide, TWI, Cambridge, UK, 2000.
  5. Xie J and Denney P: ‘Single-sided laser welding of galvanised steel’. EWI Core Research Report no. PR9906, June 1999.
  6. Jones I A: ‘Laser welding of aluminium alloys’. TWI Core Research Report no. 517/1995, Oct 1995.