Paper presented at IIW Annual Assembly, Osaka, Japan, 11-16 July 2004.
Developments and opportunities for manufacturing with lasers are considered in three broad themes. Process enhancements, in particular hybrid-laser arc welding and twin spot welding are described. Two examples of novel manufacturing processes (laser joining of aluminium to steel and laser direct metal deposition) are considered. Finally, a new laser source (Yb:YAG fibre laser) is introduced as an additional competing processing capability.
The twentieth century was characterised by the harnessing of electrical energy to create broad economic and societal benefits. Examples of developments enabled by electricity include transport, heating, lighting and the recent explosive expansion of information and communication technologies. Looking to the future, innovative applications of energy in the form of light have the potential to shape the twenty-first century.
Some forty years ago, the principle of light amplification by stimulated emission of radiation (giving the acronym laser) was first postulated and since then lasers have matured into reliable and efficient industrial tools. Typical applications include cleaning of artwork and buildings, surgery, automotive manufacturing and shipbuilding. The present paper considers the use of lasers as manufacturing tools.
All manufacturers are continuously striving to gain a competitive edge over their rivals and the advantages and disadvantages of each manufacturing route are thoroughly evaluated before production begins. Fabrication or manufacture with lasers offers several potential benefits:
- Increased productivity - more rapid part manufacture at reduced cost.
- Improved quality - for example, minimal distortion and hence reduced levels of rework.
- Enhanced performance - this can be seen by longer lifetimes or greater resistance to corrosion or high temperature. Additionally, laser repair technologies can give rise to significant life extensions of otherwise exhausted components.
- Novel fabrication routes and components - where alternative technologies simply are not available.
The choice of manufacturing route, however, is essentially an economic one. Consequently, there is a continuous battle between manufacturing technologies to gain a competitive edge that is closely analogous to the competition between individual manufacturers.
To respond to this challenge, developments in laser materials processing are being made to address the limitations of current technology and maximise potential. The present paper describes, somewhat selectively, developments and opportunities for manufacturing with lasers in three broad themes:
- Process Enhancement.
- Novel Manufacturing.
- New Laser Sources.
2. Process enhancements
2.1 Hybrid laser-arc
Due to the narrow deep penetration weld produced, laser welding offers several advantages over other welding processes:
- High joining rates.
- Low consumable costs.
- High reproducibility, as in machine tool processes.
- Low manning levels.
- Low levels of distortion, leading to greater precision in assembly and reduced rectification work.
Laser welding, however, is a demanding process and very tight control of a number of parameters such as fit-up and steel composition is required in order to produce welds of satisfactory quality and performance.
An enhanced range of applicability is available through hybrid laser-arc welding, in which the two welding processes are coupled in a single process zone, as shown in Fig.1. Options available include combining either a CO 2 , Nd:YAG, diode or fibre laser with an arc welding process such as gas metal arc welding, gas tungsten arc welding or plasma arc welding, or even combining two different laser sources.
Fig.1. Typical hybrid laser-arc welding arrangement
Compared with the use of laser power alone, hybrid laser arc welding offers:
- Increased travel speed (x2) or increased penetration (x1.3), see Fig.2.
- Improved tolerance to fit-up gap.
- Ability to add filler material to improve weld metal microstructure, joint quality and joint properties.
- Potentially improved energy coupling.
- Increased heat input and reduced hardness.
Fig.2. Comparison of welding speed and penetration for single and hybrid processes (steel)
There are, however, some drawbacks which include increased complexity ('more things go wrong'); the need to define additional welding parameters; and the requirement to establish the process parameters anew, as these cannot simply be determined from the optimum procedures for the two separate processes.
Nevertheless, hybrid laser-arc welding is now a production process in both the automotive and shipbuilding industries and has been shown to be a candidate process for girth welding gas transmission pipelines.
2.2 Twin spot laser welding
Autogenous laser welding is currently extensively used in the automotive industry for fabricating tailored blanks. The process, however, suffers from two drawbacks; the ability to bridge gaps is poor and the weld bead profile is frequently irregular when steel sheets of dissimilar thickness are joined. To overcome these limitations, methods have been developed to elongate the laser spot whilst keeping the overall spot size sufficiently small to maintain keyhole welding at reasonable speeds.
Elongation of the keyhole perpendicular to the direction of welding improves the capacity for gap bridging and hence enables fit-up tolerances to be relaxed. Analogously, elongation of the keyhole parallel to the direction of welding assists in the escape of trapped gas bubbles and metal vapour. The technique thus reduces porosity and minimises the likelihood of Zn vapour entrainment when welding Zn coated steels.
Elongation of the keyhole can be achieved using two lasers with focussed spots very close to each other, but it is much more advantageous to use a single laser source with a beam splitter to generate two focussed spots at predefined locations. Additionally, the laser energy is not necessarily divided equally between the two spots; the energy can be apportioned between the two spots in any appropriate ratio. This is particularly useful when making butt welds between two sheets of different thickness. Using two spots aligned perpendicularly to the direction of welding, the majority of the energy can be directed to the laser spot on the thicker plate, thus improving joint quality.
Fig.3. Typical examples of spot configurations that could be used to join steel sheet of dissimilar thickness
Recent work at TWI has studied the application of twin spot laser welding for fabricating tailor welded blanks and Fig.3 shows examples of the typical spot configurations examined. As an example of the capability of twin spot welding, Fig.4 shows a typical joint made with a single spot. The edge of the thicker material was not melted which could cause problems in the forming operation. For comparison, Fig.5 shows a joint made with the same materials at the same speed with the same total laser power but with a twin spot technique. The edge of the thicker sheet is clearly melted, giving rise to a tailored blank with better formability and improved appearance.
Fig.4. Typical weld made between 0.74mm and 1.5mm thick steel using a single spot, showing unmelted edge
Fig.5. Typical weld made between 0.74mm and 1.5mm thick steel sheet using a 70/30 twin spot (welding speed and total power as joint shown in Fig.4)
In the more general case, laser welding with tailored energy distributions offers the opportunity to weld nickel based aerospace alloys and other high performance materials by using some of the laser energy to weld whilst employing the remainder, or even the majority of the energy to provide appropriate preheating or post heating.
3. Novel manufacturing processes
3.1 Laser joining of aluminium to steel
The increasing use of aluminium to reduce weight, particularly in the automotive sector has led to a growing need for a robust and reliable method for joining steel to aluminium. Techniques such as mechanical fastening, adhesive bonding, resistance welding and roll bonding have been available for some time, but joints can suffer from poor strength or corrosion resistance.
Conventional fusion welding techniques are difficult to apply when joining steel to aluminium due to the materials' dissimilar expansion coefficients. Additionally there is a tendency to form brittle intermetallic compounds at the joint interface.
To overcome these limitations, a relatively new fusion process, laser joining, has been developed. A defocused laser beam is directed on the steel sheet, causing local heating but not melting. Heat is conducted through the steel and causes local melting of the adjacent aluminium, which has a much lower melting point. The molten aluminium wets the steel and then solidifies, resulting in a bond. The process can be applied to both lap joints and butt joints, as shown schematically in Fig.6, although it is most suited to lap joints. In practice, however, it is very difficult to control heat input sufficiently well to prevent melting of the steel but even when local melting of the steel does take place a relatively strong bond is formed.
Fig.6. Schematic of the laser joining process for both lap and butt joints
Laser joining is clearly applicable to uncoated steel, but the technique can also be used for joining Zn coated steels typically used in automotive body in white fabrication. Figure 7 shows an example of the interface when Zn coated low C automotive steel is laser joined to AA 6111. Clearly intermetallics are formed at the interface but process control measures can be applied to ensure that the intermetallics do not significantly influence joint strength.
Fig.7. Example of interface between laser joined zinc coated steel and aluminium
3.2 Laser direct metal deposition
For high value applications such as aeroengine components it is generally economically attractive to repair worn or locally damaged parts rather than replace them. A candidate repair technology is laser direct metal deposition(DLMD) in which a laser is used to melt powder onto a substrate material. In contrast to approaches in which the powder is placed on the substrate, the powder is fed via a nozzle into the laser spot on the substrate, as illustrated in Fig.8. This results in the formation of a molten pool which, on cooling, leaves a solid deposit of the powder material on the substrate.
Fig.8. A schematic of laser-based deposition using a single, off axis powder feeder
Laser deposition has two main advantages over other powder deposition techniques. Firstly, the operation is a low heat input process which reduces the likelihood of liquation cracking. Secondly, the use of very small spot sizes enables highly accurate and reproducible deposits to be made. Additionally, adaptive control systems have been developed that monitor the characteristics of the molten pool and adjust the process as necessary to maintain deposit quality. Figure 9 shows a multiplayer deposit in an aeroengine alloy.
In addition to repair, however, direct laser metal deposition is suitable for original part build. By varying powder composition as a part is built up, a functionally graded component can be developed with particular performance characteristics directly related to position. The process can also be used for rapid prototyping.
For the process to gain widespread acceptability, however, development is required to increase deposition rates, deposition efficiency and to establish appropriate processing parameters for high accuracy and quality.
Fig.9. Macrograph of a cross section of a multiplayer deposit in an aeroengine alloy
4. Concluding Remarks
This has been a somewhat selective and subjective review of developments in laser materials processing. A similar paper written ten years ago would have identified some developments that have now become commercial manufacturing routes (and some that did not fulfill their early promise). Ten years hence, it is likely that technologies that are now unforeseen will be under development. What is clear, however, is that manufacturing with lasers can provide a competitive advantage both now and in the future.