Industrial lasers and applications in automotive welding

Makes for interesting  reading

 

Industrial lasers and applications in automotive welding

S T Riches, TWI
This paper was presented on 22 October 1998 at a Make It With Lasers ^TM Workshop entitled Lasers in the Automotive Industry, held at Nissan Motor Manufacturing (UK) Ltd, Sunderland.

Introduction

The use of lasers in automotive manufacture has increased dramatically over recent years to a position where about 15% of all industrial processing lasers are installed in production. Although the lasers are devoted mainly to cutting applications, a significant and growing proportion of lasers is being applied to welding. In a survey in 1992, about 20% of the lasers installed in the automotive industry were used in welding applications. ^[1] Since that time, there has been an explosion of growth in welding applications, particularly involving steel manufacturers, for tailored blank manufacture and in body-in-white welding applications.
This paper will review the industrial laser types which are available for welding applications, then describe the range of current applications used in the automotive industry and highlight areas where developments in processing techniques and equipment are poised to make an impact for laser welding in the future.

Industrial Laser Types

There are two main types of industrial laser of interest to structural fabrication; CO ₂ and Nd:YAG lasers. The lasers have different characteristics that are summarised in Table 1.
Table 1. Summary of characteristics of CO ₂ and Nd:YAG lasers

Property	CO 2 laser	Nd:YAG laser
Lasing medium	CO 2 +N 2 +He	Neodymium doped yttrium aluminium garnet crystal rod
Radiation wavelength	10.6µm	1.06µm
Excitation method	Electric discharge	Flashlamps
Efficiency	5-10%	2.5-5%
Output powers	Up to 60kW	Up to 4kW
Beam transmission	Polished mirrors	Fibre optic cable

High power CO ₂ lasers are predominantly used for the welding of automotive components, such as gears and transmission components, which require circular and annular welds and in tailored blank applications. The majority of lasers have a power of 6kW or less.
High power Nd:YAG lasers are now available at workpiece powers of 4kW, which have fibre-optic beam delivery. The welding applications are concentrated in body-in-white assembly. Higher power equipment is likely to be developed in the next 2-3 years and there are likely to be improvements in the efficiency of Nd:YAG lasers with the arrival of diode pumped Nd:YAG lasers.
Within the laser industry, one of the main advances in the past two years has been in diode lasers (wavelength 0.8-0.9µm), where 2kW systems are now commercially available. However, at the current status of development, the power densities required for welding of sheet materials used in the automotive industry (about 1x10 ⁶ W/cm ²) have not been achieved. Research work is underway in Germany to develop diode lasers and their applications and this situation may change in the next 3 years.

Laser Welding in the Automotive Industry

Welding of Automotive Components

The applications of laser in the manufacture of components cover engine parts, transmission parts, alternators, solenoids, fuel injectors, fuel filters, air conditioning equipment and air bags. An example of a laser welded solenoid is shown in Fig. 1. and gear component is shown in Fig. 2. The attractions of laser welding for these applications are the ability to weld pre-machined precision components with restricted heat input and minimal distortion ^[2] . This enables weight savings to be made through the use of thin walled assemblies and optimisation of the compactness of the component.

Fig. 1. CO 2 laser welded solenoid.

Fig. 2. CO 2 laser welding of gear component.

In industrial production, the advantages of the laser welding process have been established compared to alternatives such as electron beam welding. This is due mainly to the high productivity and small amount of down-time compared to vacuum based systems and the subsequent reduced manufacturing costs ^[3] . However, there is a need to spend a high amount of effort on component preparation and high precision of component handling and beam transmission is required. ^[4]
Most automotive manufacturers have invested heavily in CO ₂ laser technology for these types of applications and most of the issues relating to the production of millions of components have been resolved. Developments have focused on expanding the range of material combinations which can be welded using lasers, including joining of cast irons to steels through the use of wire feed techniques and optimisation of the procedures to harden components whilst avoiding problems with cracking due to high carbon levels in the weld zone.

Welding in Automotive Body Assembly

For the past 40 years, welds in mass produced automotive body parts have been almost exclusively fabricated from pressed steel sheets and joined using resistance spot welding. The potential benefits of implementing laser welding technology are numerous - advantages may be gained in respect of single sided access, reduced flange widths, increased torsional stiffness (thus leading to improved vehicle structural performance and/or down gauging of material thickness), smaller heat affected zones and less thermal distortion, high speed automated processing and design flexibility (eg. in multi-layer joints).
Extensive work has been performed worldwide to realise the potential of laser welding for automotive body manufacture. As the turn of the century approaches, the number of systems installed in production is increasing, where two main types of laser welding are being employed. The first is a direct replacement for resistance spot welding or adhesive bonding, where lap joints or hem flange joints are utilised on pressed components for body-in-white assembly. The second is the laser butt welding of flat sheets (which may be dissimilar thickness or material grade), which are subsequently formed into pressings. The pressings are called "tailored blanks" and the widespread acceptance of this technology has spawned a number of companies and enterprises associated with steel producers to meet the demands of the automotive industry in addition to in-house manufacture.

Body-in-white applications
The implementation of laser welding of sheet assemblies as a replacement for resistance spot welding is growing, where welding of the roof to side panel is one of the most common applications. ^[5,6] This component is normally a two layer lap joint in zinc coated steel, with periodic three layer thicknesses to be welded, over lengths of 2.5-3m. One of the main challenges for laser welding of these types of joints is the presence of the zinc coating at the interface between the sheets, where the low vaporisation temperature of the zinc (906°C), can cause problems with weld consistency due to the formation of blowholes and porosity, if the sheets are tightly clamped together. Various approaches have been adopted to overcome this problem including:

• Roller clamping systems which create a gap at the interface
• Stamping of dimples in the steel pressings of consistent depth

There are both CO ₂ and Nd:YAG lasers in production for this kind of application, and it is forecast that Nd:YAG laser applications will grow due to the flexibility of the fibre optic beam delivery as higher powers become available. A recent review of laser applications at BMW has indicated that 9-11m of laser welding is carried out on certain models ^[7] and the ULSAB project ^[8] has used over 18m of laser welding in its concept structure.

Fig. 3. Lap joint in 1.6mm thick 5754 aluminium alloy sheet welded at 5m/min with CO 2 laser

Laser welding is also being applied in the production of partial penetration welds, for example, in hem flanges often used in doors, bonnets, boot lids and other closures [9]. Although the majority of work performed has been on steel sheet, there have been a number of programmes conducted on aluminium alloys. ^[5] The laser welding of aluminium alloys is more difficult than for steel, due to the high reflectivity and thermal conductivity of the material, the low viscosity of liquid aluminium and the tendency for cracking and porosity in welds in certain alloys. Extensive work over the past 10 years has demonstrated the feasibility of laser welding of aluminium alloys using CO ₂ and Nd:YAG lasers, where once a threshold power density is exceeded for a particular alloy, keyhole welding can be established and seam welds can be produced with similar welding speeds to steel sheets, see Fig. 3.

Fig.4. Tailored blank in door inner panel

Tailored blank welding

The principle of using laser welded tailored blanks in automotive manufacture started with the production of a two piece floor pan for Audi. Since that time, the applications for tailored blanks have grown significantly as it has been gradually realised that tailored blanking is a cost effective method of manufacturing whilst achieving weight savings. The main advantages stem from the ability to join materials of dissimilar thicknesses and grades (thus allowing weight/strength to be placed where required), the improved utilisation of material from steel strip and the high degree of automation possible. These advantages have enabled a reduction in the number of pressing operations through the elimination of reinforcements, for example, which have more than compensated for the cost of manufacture of the tailored blanks. The types of application for tailored blanks are expanding and include rails, panel rockers, panel skirts, door inners (Fig. 4.), body side outers and, in the ULSAB concept vehicle, ne
In the manufacture of tailored blanks, the two main points of debate centre on the preparation of the edges for welding and the choice of laser type. For the edge preparation, there is a desire to use standard blanked edges but this normally requires special clamping or welding systems to ensure that the blanks can be welded consistently. The clamping systems developed include using side pressure on the fixtures or using a roller system to deform the sheet adjacent to the weld to bring the sheet edges into intimate contact (<0.1mm gap). The welding systems employed where the gaps between the sheets are >0.1mm include beam weaving or twin spot welding, but this generally reduces the maximum welding speeds that can be achieved. The use of precision re-shearing of the edge prior to welding within a blank welding system is claimed to improve weld consistency as the cross-sectional area of the joint is guaranteed and the welding speed can be maximised. ^[10]
For the choice of laser type, the established technique is the use of CO₂ lasers and automated production systems normally have a 5-6kW laser. With the development of Nd:YAG lasers with workpiece powers of >3kW, a number of production systems have been installed for tailored blank manufacture, with claimed advantages of increased versatility due to the fibre optic beam delivery, higher welding speeds (due to the improved coupling of the laser wavelength), improved tolerances to gaps and negation of the need for gas shielding. ^[11] In a similar manner to the edge preparation, the economics of manufacture is complex and will have to be resolved on a case by case basis.

Fig. 5. Bulge test in laser welded aluminium alloy tailored blank

Low carbon and high strength steel sheets in thicknesses between 0.7-2mm are used with few reported problems, provided that the welds are located in positions where the weld is not subjected to much movement across the weld line during forming. For chassis and sub-frame components, where thicknesses of steel sheet are generally between 2-4mm and yield strengths are up to 400MPa, there is a similar desire to implement tailored blank technology, but the selection of steel type is more critical to attain the required formability. [12]
There is a growing interest in the exploitation of aluminium alloys for tailored blank application, ^[11] but the laser welding is more critical as the weld line is a zone of weakness, see Fig. 5. Developments have focused on the application of wire feed and twin spot welding to improve weld quality and consistency.

Laser cutting/welding

Lasers are capital intensive tools and, in order to ameliorate the costs, systems should be utilised to maximise their production capacity. One method of achieving this is to use one laser source for cutting and welding. This principle has been demonstrated industrially in the manufacture of a C column, where two zinc coated steel sheets are overlaid in the clamping system and then laser cut. The cut edges are then re-positioned and welded together in a butt joint configuration using filler wire additions. ^[13] For this application, the main advantage is the achievement of a high quality weld which requires minimum finishing to produce a Class A surface.

Future Automotive Laser Welding Applications

Whilst it is predicted that the above automotive laser welding applications will grow, in some cases, at a substantial rate, there are other components and materials that will benefit from the advantages of laser welding. Listed below are some of the topics where laser welding is poised to make an industrial impact:

• Pressed components to hydroformed tubes or extrusions
• Production of tailored hydroformed tubes or other stiffened structures consisting of welded sheet
• Production of node structures in aluminium alloy castings/extrusions or extrusions/extrusions
• Welding of magnesium alloy components

The success of these applications will ultimately depend on the weight, performance and cost advantages that can be demonstrated for high volume manufacturing scenarios.

Concluding remarks

Laser welding has "come of age" for automotive manufacture, where it is an established technique for automotive components, tailored blanks and body assembly. The main advantages to be gained through the use of laser welding include low distortion, single sided access, high torsional stiffness of components, and cost savings through elimination of other manufacturing operations. Most of the applications to date have focused on welding of steels but there is a growth in confidence in laser welding of aluminium alloys.
The advent of high average power Nd:YAG lasers, delivering over 3kW power to the workpiece, will make an increasing impact on the type of laser welding systems installed in production, but there will be considerable pressure to reduce capital costs and improve efficiencies of the laser source. Competition will also be fierce from CO ₂ laser manufacturers and non-laser processes in this significant market.