Make It With LASERS TM Workshop, July 2000
Paper presented at meeting on 'Lasers in the automotive and sheet metal industries', TWI, Great Abington, UK on 13 July 2000
Introduction
In a survey published earlier this year [1] , it is estimated that approximately 125,000 laser units have been sold worldwide since business started. The figures for 1999 reveal that about 31% is used for material processing, being the second largest market after telecommunication (33%), of which about 13% is attributed to welding applications. Moreover, 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. In fact, the automotive industry today is the largest production user of high-power lasers for material processing, and constantly looking for ways to reduce costs, improve quality, and increase manufacturing flexibility [2] .
In common with industries such as aerospace, domestic appliances and electronic packaging, the automotive sector has been very much interested in sheet metal processing. Laser welding has been a key technology in this, because of its high processing speeds, its low heat input and resultant low distortion and its overall flexibility of application. The car industry, fuelled by the need for lightweight, energy-efficient vehicles because of ever increasing legislative and performance requirements, 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, 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, and structural fabrication in general, are the CO 2 and Nd:YAG laser, with characteristics as detailed in Table 1. CO 2 lasers, usually with powers up to 6kW for the sheet industry, are predominantly for the welding of automotive components, such as gears and transmission components and for tailor welded blanks. Nd:YAG lasers on the other hand, now available at workpiece powers up to 4kW, are gaining interest for body-in-white assembly because of their flexible fibre-optic beam delivery. There is also an interest and on-going research in diode lasers (0.8-0.9µm), now available at workpiece powers up to 2kW [3] .
Table 1 CO 2 and Nd:YAG laser characteristics
| Property | CO 2 | Nd:YAG |
|---|
| lasing medium |
gas |
solid |
| excitation |
electrical |
flash lamp/diode |
| transmission |
mirror |
fibre-optic |
| power efficiency |
5-10% |
3-5/10% |
| wavelength (µm) |
10.6 |
1.06 |
Joint configurations
Examples of sheet metal joint configurations suitable for laser welding are shown in
Fig.1. However, to get an acceptable joint profile and weld quality
1 , a number of processing and fit-up conditions have to be satisfied (
Table 2). 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. Part 1 is for steel and Part 2 for aluminium joints.
Fig.1. Thin sheet joint configurations
Table 2 Processing factors for different joint configurations
| Factors | Joint configuration |
|---|
| Lap | Multi-layer | Butt | Hem | Edge | T-Butt |
|---|
| Tolerance to gap between sheets |
<0.1t |
<0.1t for each layer |
<0.1t |
<0.1t |
<0.1t |
<0.1t |
| Tolerance to beam-joint misalignment, mm |
>1 |
>1 |
<0.3-0.5 |
>1 |
<0.3-0.5 |
<0.3-0.5 |
| Tolerance to beam focus position, mm |
±1 |
±1 |
±1 |
±1 |
±1 |
±1 |
| Tolerance to edge preparation |
Avoid burrs |
Avoid burrs |
<0.5t |
Avoid burrs |
<0.1t |
<0.1t |
| Tolerance to coatings (e.g. Zn) |
Low |
Low |
Medium |
Low |
Medium |
Medium |
For butt joints for instance, the most critical factor is the joint fit-up, i.e. the gap between the two sheets to be joined, and has to be less than 10-15% times the material thickness to ensure a sound joint. This can be achieved in a number of ways, for example through precision shearing the edges, the use of special clamping arrangements (such as rollers near the welding region), weaving, the addition of filler wire, 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, 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, such as the use of tailored optics (twin spot welding), use of filler wire, weaving, arc-augmented laser processing, etc [4] .
Examples of these joint configurations can be found in a whole range of automotive applications. 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. 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. The reduction of plasma formation above the weld pool is less important for thinner gauges processed at higher speeds. 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, but the appearance of the top and underbead may not be as smooth and some porosity might be present in the weld. Argon is probably the most commonly used gas, but helium, nitrogen, CO 2 or gas mixtures can also be used, depending on requirements such as plasma suppression, penetration, hardness, porosity, etc.
The use of argon, 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, there are two main factors to consider, namely the effect of steel composition and the effect of coating.
Effect of steel type
For low carbon steel sheet, CO
2 and Nd:YAG laser welding will produce welds consistently. Compared with the parent material, the hardness of the welded joint is, in general, increased by a factor of 2.0-2.5. This increased hardness can influence the formability as well as the dynamic mechanical properties (e.g. fatigue/impact) of the welded joint.
More recent, there is a growing tendency within the automotive industry to use high strength steels, such as HSLA or microalloyed (Nb, Ti and/or V), rephosphorised, bake hardened, dual phase or trip steels, as they allow weight reductions to be achieved. Although not much laser processing data on these types of steel is available yet, most are considered weldable, but care should be taken in monitoring maximum weld hardness and susceptibility to cracking. Microalloyed steels will produce higher weld hardnesses at the same welding conditions when compared with cold rolled mild steels. 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] . Although a range of coatings can be applied to steel sheet, such as Al, Zn-Al, Zn-Ni or organic coatings, this paper only considers zinc-coated steels, most commonly used in the automotive industry.
The presence of zinc in the coating, which boils at 906°C, can cause blowholes and porosity along the weld seam. This usually occurs if the sheets are clamped tightly together and when the coating thickness on the sheets is in excess of 5µm. A common solution is to create a gap at the joint interface enabling the Zn-vapours to escape, which can be done through the use of a roller adjacent to the weld point, the use of special clamping arrangements or dimpled sheets. The use of proprietary gas mixtures or special welding parameters involving pulsing can also be applied. The use of rollers and specially designed clamping systems however, seems to be the preferred industrial option for production of three-dimensional laser welds on steel sheet structures. The dimpled sheets add an extra operation and the successful use of special welding parameters is dependent on the coating type and thickness. More recent studies have also reported some success by using twin beam techniques, as they produce a slightly elongated weld pool and thus giving the Zn-vapours more time to escape [5] .
In terms of coating type, the three most common zinc-based coatings used are electrogalvanised, galvannealed and hot dipped galvanised. In general, hot dipped galvanised coatings are thicker and can create more problems with porosity and blowholes in the weld. In addition, variability in the thickness of coating can create difficulties in producing consistent welds. Thickness variation should be controlled to ±2µm if possible along the joint length.
A complex coating, for instance a zinc-layer underneath a chromium/chromium oxide top layer or a thin organic layer (<0.1µm), also places extra demands on the laser welding process. Although these materials can be welded, it is possible that extra porosity is generated in the weld due to degradation of the coating.
For lap joints ( Fig.2), the main difficulty is the presence of the coating at the interface between the two sheets. If the weld solidifies rapidly, Zn-vapours can get entrapped in the weld and cause porosity. For butt joints, for tailored blanks for instance ( Fig.3), the coating does not generally cause significant porosity, but the laser welding process does remove the coating from around the weld, leaving an area that may be susceptible to corrosion. However, the removal of coating from the weld is very localised (<2mm from the weld centre) and the surrounding coating can offer galvanic protection.
Fig.3. Steel tailor welded blanks
It should also be noted that high laser beam quality enables the use of a longer working distance (up to 200mm), i.e. distance between the workpiece and the focusing lens, whilst maintaining the power density required for material processing. This, in turn, 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, which produce a significant amount of spatter.
Aluminium sheet
When lasers were first used on aluminium, using similar conditions to those applied for steel structures, the initial high surface reflectivity, the high thermal conductivity and the volatilisation of low boiling point constituents caused defects such as lack of penetration, blow holes, porosity and weld metal and HAZ cracks in some alloys. These problems are now largely overcome with the advent of higher average powers, improved beam focussing systems and better beam qualities, producing a power density high enough to produce a stable keyhole for welding.
At present, both CO 2 and Nd:YAG lasers can be used successfully for welding a vast range of aluminium alloys, with slightly higher welding speeds achievable for Nd:YAG lasers compared with similar power CO 2 lasers, because of the shorter wavelength and improved coupling.
Table 3 CO 2 and Nd:YAG laser welding capabilities for linear butt welds in aluminium sheet
Material thickness,
mm | CO 2 laser | Nd:YAG laser |
|---|
Laser power,
kW | Travel speed,
m/min | Laser power,
kW | Travel speed,
m/min |
|---|
| 2 |
5 |
6 |
2 |
1 |
| |
|
|
4 |
6 |
| 6 |
5 |
1 |
4 |
0.5 |
| |
10 |
6 |
|
|
There is an interest for aluminium alloy sheet assemblies for both butt joints and overlap joints with 2 or 3 sheets. Typical welding parameters for a 5kW CO 2laser and a 4kW CW Nd:YAG laser for linear joints are given in Table 3. Joining dissimilar thickness alloys is also possible at these high speeds ( Fig.4). Using a 4kW Nd:YAG laser, speeds of over 5m/min are possible for 2-sheet lap joints in 1.2mm thick 5xxx and 6xxx-series alloys, whilst around 3m/min can be achieved for 3-sheet lap joints.
Fig.4. Nd:YAG laser welding sheet alloy
Overall, both the 5xxx and the 6xxx-series alloys, most commonly used in the automotive industry, can be welded using a laser and with or without filler wire. For a given power density and spot size, 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, some aluminium alloys are susceptible to weld metal or HAZ cracking. This is especially the case for 6xxx-series (Al-Mg-Si) alloys, where cracking has been related to the formation of Mg-Si precipitates. This is remedied by adding the correct filler wire which reduce the freezing range of the weld metal, and minimises the tendency for solidification cracking. 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, as-welded joints in 5xxx-series alloys retain their cross-weld tensile strengths to within 80-100% of the parent material value, and only show a small reduction in elongation-to-failure value. In these non-heat treatable alloys, 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. 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. For the heat-treatable 6xxx-series alloys, a greater loss in cross-weld tensile strength and elongation-to-failure value occurs. This drop is caused by the re-solution of precipitates, loss of weld strain-hardening or cross-sectional reductions caused by undercut or porosity. The HAZ is also softened by over-ageing during welding. However, 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, care has to be taken to avoid excessive porosity. The predominant cause for porosity is the evolution of hydrogen gas during weld metal solidification. This hydrogen can originate from lubricants, moisture in the atmosphere and surface oxides or the presence of hydrogen in the parent material. 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. Blowholes are a form of porosity that can occur in longer welds. Although many believe this is the result of keyhole instability through pressure build-up, 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. It has become an established joining technology for a range of joint configuration and automotive applications including tailored blanks and body assembly. Most of the applications to date have focused on steel, but there is a growing confidence in the laser welding of aluminium sheet. It is anticipated that the use of lasers for welding of automotive sheet will continue to grow, 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
- Belforte D A: 'The "golden nineties" are gone'. Industrial Laser Solutions 2000 15(1) 11-19.
- Naeem M, Riches S T: 'Optimisation of high-power Nd:YAG laser welding procedures for automotive applications'. ISATA 1999, paper no. 99NM062.
- 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.
- 'Laser welding of sheet metals'. Best Practice Guide, TWI, Cambridge, UK, 2000.
- Xie J and Denney P: 'Single-sided laser welding of galvanised steel'. EWI Core Research Report no. PR9906, June 1999.
- Jones I A: 'Laser welding of aluminium alloys'. TWI Core Research Report no. 517/1995, Oct 1995.