New developments in advanced welding

Nd:YAG laser welding of different metals

Interest in welding applications for high-power Nd:YAG lasers is growing as the availability of average power, pulsed and CW models of these lasers increases. These lasers offer processing rates and capabilities that can compete with the industry standard CO2 laser welder, but have the added benefit of flexible fibre optic beam delivery. Pulsed and CW Nd:YAG laser-welding processes differ in performance characteristics, weld shapes and applications, even though both types of lasers produce energy at the same wavelength, i. e. 1.06 mm. This section will review typical laser welding capabilities of both pulsed and CW lasers in a range of materials.

5.7.1 Steels

The largest market for laser welding is the automotive industry where it is being applied to the welding of thin, typically 0.7 to 3 mm thick, coated and uncoated steels, transmission components and the fabrication of sub-assemblies. Perhaps the most significant laser welding application is laser welding of tailored blanks.20,21 The process involves welding sections of steel sheet of different thickness together first and then stamping the product to form a part. It offers significant cost and environmental benefits over the traditional approach in which a range of techniques are applied to preformed parts which are then subsequently welded; it is now transforming the approach to car body manufacture. Other significant areas where laser welding is making an impact include the electronics and aerospace industries where the applications include hermetic sealing of electronics packages, joining difficult - to-weld electronic materials and the fabrication of components made from high-performance alloys.

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, CO2 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 recently, there is a growing tendency within the automotive industry to use high strength steels, such as high strength low alloy (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 little laser processing data on these types of steel is yet available it is considered possible to weld most, 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 in comparison 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.

Effect of coating type

The effect of coatings on the welding process has been the subject of extensive research. Although a range of coatings can be applied to steel sheet, such as

Al, Zn-Al, Zn-Ni or organic coatings, only zinc-coated steels are considered here; they are 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 5 mm. A common solution is to create a gap at the joint interface enabling the Zn vapours to escape, which can be done with 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 the 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, because they produce a slightly elongated weld pool and therefore give the Zn vapours more time to escape.

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 ± 2 mm 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 (< 0.1mm), 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, the main difficulty is the presence of the coating at the interface between the two sheets. If the weld solidifies rapidly, Zn vapours can be entrapped in the weld and cause porosity. For butt joints, for tailored blanks for instance, 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 much localised (< 2 mm from the weld centre) and the surrounding coating can offer galvanic protection.

5.7.2 Aluminium alloys

Aluminium alloys are used in a wide range of industrial applications because of their low density and good structural properties. Laser welding has been identified as a key technology that can offer distinct advantages over conventional joining techniques such as TIG, MIG (tungsten and metal inert gas, respectively), resistance spot welding, mechanical fasteners and adhesive bonding.

The main problems associated with laser welding of aluminium alloys in general are the high surface reflectivity, high thermal conductivity and volatilisation of low boiling point constituents. These and other material- related difficulties can lead to problems with weld and HAZ cracking, degradation in the mechanical properties and inconsistent welding performance. These problems are now largely overcome with the advent of higher average powers, improved beam qualities giving a power density high enough to produce a stable keyhole for welding. At present, both CO2 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 CO2 lasers, because of shorter wavelength (1.06 mm) and improved coupling.

The greatest recent drive towards use of aluminium-based structures has arisen mainly from the automotive industry (e. g. Audi A8, new Audi A2). The requirement for reduced vehicle weight has led to the development of aluminium frame structures (assembled from cast and profile materials), aluminium alloy sheet assemblies and the use of lightweight cast components. Both the 5xxx (aluminium-magnesium) and 6xxx (aluminium-magnesium - silicon) series aluminium alloys are candidate materials and these alloys can be welded with or without filler wire. For a given power density and spot size, the laser welding speed (Fig. 5.15) for 5xxx series alloys is slightly higher than that for 6xxx series alloys and it is believed that this is caused by magnesium vapours stabilising the keyhole.

Although it is possible to weld most aluminium alloys, some are susceptible to weld metal or HAZ cracking. This is especially the case for the 6xxx series alloys, where cracking has been related to the formation of Mg-Si

precipitates. This cracking can be reduced or eliminated by addition of correct filler wire during welding (Fig. 5.16) which reduces the freezing range of the weld metal and minimises the tendency for solidification cracking. The use of filler wire also improves the fitup tolerance and weld profile and can improve the cross-weld strength and elongation to failure value of the joint. Typical welding and wire speeds for 6xxx series alloys are shown in Tables 5.2 and 5.3 respectively. Figure 5.17 shows tensile strength results for different shield gases (6181 aluminium alloy).

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 from 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.

Whereas high power CW Nd:YAG is best suited for welding aluminium alloy sheet metal up to 3 mm for automotive applications, pulsed Nd:YAG is better suited for the welding of electronic packages. This is because its pulsing capabilities can deliver the power to the workpiece with minimal heat input. When a designer requires a lightweight, corrosion-resistant, heat- dissipating, robust, and economical package, aluminium is usually the first choice. Aerospace packages for microwave circuits, sensor mounts, or small- ordinance imitators are the most common examples of aluminium components that can be laser welded. Aluminium alloy type 6061-T6 is the material of choice because of its rigidity, ease of machining and economic considerations. However, the material cannot be successfully laser welded to itself, because the partially solidified melt zone cannot withstand the stress of shrinkage upon solidifying and cracks are formed (termed ‘soldification cracking’ or ‘hot cracking’). The solution to this problem is to improve the ductility of the weld metal by using aluminium with a high silicon content such as alloy 4047 (Al 12% Si). This alloy is very ductile as a solid and difficult to machine into small complex shapes. Therefore, 6061 is usually employed as the package component with intricate features, and 4047 is used as a simple lid that is relatively thin (typically less than 1 mm). A 4047 ribbon can be inserted between 6061 components to produce excellent welds, but this requires a very labour intensive step, unless round washers or other simple preform geometries can be employed.

Alloy 2xxx (Al-Cu) and many other popular aluminium alloys are also weldable using 4047 filler metal. So far, there has been no experience indicating that 2xxx can be welded to itself without the use of filler material. The only alloys that can be welded with low heat input and with any filler material are 1000 and 1100 series alloys. These commercially pure alloys have the metallurgical characteristics that enable them to avoid hot cracking, but their poor mechanical and machining properties usually prohibit use in most applications.

5.7.3 Stainless steels

Stainless steels are chosen because of their enhanced corrosion resistance, high temperature oxidation resistance or their strength. The various types of stainless steel are identified and guidance given on welding processes and techniques that can be employed in fabricating stainless steel components without impairing the corrosion, oxidation and mechanical properties of the material or introducing defects into the weld. The unique properties of the stainless steels are derived from the addition of alloying elements, principally chromium and nickel, to steel. Typically, more than 10% chromium is required to produce a stainless iron. The four grades of stainless steel have been classified according to their material properties and welding requirements:

• austenitic

• ferritic

• martensitic

• austenitic-ferritic (duplex and super-duplex).

When laser welding these steels care is required in the selection of gases and gas-shielding arrangements to produce clean, oxide-free welds.

Austenitic stainless steels

These steels are usually referred to as the 300 series and are generally suitable for pulsed and CW laser welding. Slightly higher weld penetration depths or increased weld speeds can be achieved when compared with low carbon steels (Fig. 5.18) due to the lower thermal conductivity of most stainless steel grades. The high speeds of laser welding are also advantageous in reducing susceptibility to corrosion. This corrosion is caused by precipitation of chromium carbides at the grain boundaries and can occur with high heat input welding processes. In addition, laser welding of these grades results in less thermal distortion and residual stresses compared with conventional welding techniques, especially in those steels having 50% greater thermal expansion than have plain carbon steels. The use of free machining grades should be avoided because these steels contain sulphur that can lead to hot

cracking. An excellent example of stainless steel welding with a pulsed Nd:YAG laser is the manufacture of the Gillette sensor razor (Fig. 5.19). Over a hundred lasers with fibre optic beam delivery systems are in operation, producing over 50000 spot welds and approximately 1900 cartridges each minute.

Austenitic stainless steels are used in applications requiring corrosion resistance and toughness. These steels find wide ranging applications in the oil and gas, transport, chemical, and power generation industries and are particularly useful in high temperature environments. There are a number of potential benefits that result from using high power Nd:YAG laser welding of stainless steels, including productivity increases. The low heat input of the laser welding process reduces the width of the HAZ, thus reducing the

140 New developments in advanced welding 16

Spot size = 0.45 mm Spot size = 0.60 mm

Thickness (mm)

5.20 Material thickness vs. welding speed for 304 stainless steel (3.50 kW, CW).

region that may be susceptible to pitting corrosion. A graph of welding speed against material thickness is shown in Fig. 5.20. The tensile tests on the samples produced an average tensile strength of 98 % of the parent material values.

Ferritic stainless steels

These 400 series steels do not possess the good all-round weldability of the austenitic grades. Laser welding of the ferritic grades in some cases impairs joint toughness and corrosion resistance. The reduction in toughness is due in part to the formation of coarse grains in the HAZ and to martensite formation which occurs in the higher carbon grades. The heat-affected zone may have a higher hardness due to the fast cooling rate.

Martensitic stainless steels

These steels are the 400 series and produce poorer quality welds than do either austenitic or ferritic grades. The high carbon martensitic grades (> 0.15%C) can cause problems in laser welds due to the hard brittle welds and the formation of HAZs. If carbon contents above 0.1% must be welded, use of an austenitic stainless steel filler material can improve the weld toughness and reduce the susceptibility to cracking but cannot reduce the brittleness in the HAZ. Pre-heating or tempering at 650-750 °C after laser welding may also be considered.

Duplex stainless steel

The introduction of duplex stainless steel for tube and pipe work has created a number of difficulties for the more conventional arc welding processes in
terms of achieving the desired phase balance and mechanical/corrosion performance. For tubes or pipework, the manipulation capabilities of fibre optic beam delivery associated with Nd:YAG laser technology and the possibility of remote welding makes high power CW Nd:YAG laser potentially attractive for welding thin-walled duplex stainless pipes. The steel composition, laser parameters and type of gas shielding can influence phase distribution in the weld metal. The low heat input associated with Nd:YAG laser welding can reduce the proportion of austenitic material present in the weld metal and HAZ, which may impair the corrosion properties of the joint.

Titanium alloys

The high strength, low weight and outstanding corrosion resistance possessed by titanium and its alloys has led to a wide and diversified range of successful applications in chemical plant, power generation, oil and gas extraction, medical and especially aerospace industries. The common problem linking all of these applications is how best to weld titanium parts together or to other materials. High power CW Nd:YAG laser welding is one technique that is finding increasing application for titanium alloys. The process, which offers low distortion and good productivity, is potentially more flexible than either TIG or electron beam for automated welding and the application is not restricted by a requirement to evacuate the joint region. Furthermore, laser beams can be directed, enabling a wide range of component configurations to be joined using different welding positions. The welds are usually neat in appearance and have low distortion when compared with their arc-welded counterparts. The fusion zone width and the grain growth can usually be controlled according to laser power at the workpiece and the welding speed used. Although these alloys can be welded without difficulty using lasers, special attention must be given to the joint cleanliness and the gas shielding. Titanium alloys are highly sensitive to oxidation and to interstitial embrittlement through the presence of oxygen, hydrogen, nitrogen and carbon. Laser welding, as arc welding, requires the use of an inert shield gas to provide protection against oxidation and atmospheric contamination. The most frequently used cover gases are helium and argon. Figure 5.21 shows typical welding speeds for Ti-6Al-4V alloy with argon shield gas. Full penetration welds can be produced up to 12 mm thick. The results show that with optimum laser and processing parameters it is possible to produce porosity and crack free welds in both alloys.

While the high power CW Nd:YAG process is mainly used for welding thick sections, pulsed Nd:YAG lasers are largely used for small components that require very little heat input. One such component is the heart pacemaker as shown in Fig. 5.22. The pacemaker is made of titanium alloy package fabricated from two disk-like halves after pacemaking electronics have been

sandwiched between them. The sealing weld has to provide a very high quality hermetic seal to prevent body fluids entering the package and causing the pacemaker to fail. The weld also has to take place within 1mm of some of the circuitry and batteries, which should not be subjected to temperatures above 50 °C.

Structural steels

Conventional Nd:YAG laser welding applications have been high speed or precision welding of thin-section materials. However, the introduction of Nd:YAG lasers with 4-5 kW of power opened up many new opportunities for the welding of thicker sections in industries that had not previously considered the process viable. Such applications can be found in shipbuilding, off-road vehicles, power generation and petrochemical industries. For these industries, distortion due to welding is being increasingly recognised as a major cost in fabrication. This realisation led to at least three shipyards introducing high power CO2 laser welding in an attempt to reduce distortion substantially and improve overall fabrication accuracy. High power CW Nd:YAG laser welding with the benefit of fibre optic beam delivery offers even more potential benefits for thick section welding and a programme has been carried out to asses the potential for the welding of structural steel.

Interest lies mainly in butt welds and T-joints in various linear and circular forms depending on the application. Owing to the limitation at present of typically 3.5-4 kW at the workpiece, the welding speeds for single pass welding of butt joints in 10 mm thick steel are not very high (typically 0.3 m/min). For T-joints, which can be welded from each side, higher speeds are possible.

The main difference between laser welding at pulsed laser power (600W) and CW laser power (4kW) is the greater amount of laser-induced plasma/ plume emanating from the deep penetration keyhole in the former. This plasma/plume can reduce weld penetration and cause instabilities in the vapour-filled keyhole at the centre of the weld pool, resulting in coarse porosity particularly for materials > 4 mm thick. In order to produce defect - free welds it is essential to suppress the plume/plasma during welding. This usually is done with a helium shield gas ejected through a nozzle.

5.7.4 Nickel-based alloys

These alloys can be laser welded either by pulsed or CW Nd:YAG lasers in a similar manner to that carried out on stainless steels. Comprehensive gas shielding is needed and welding speeds may need to be modified to avoid the occurrence of solidification and HAZ cracking for selective alloys.

5.7.5 Copper-based alloys

These alloys have high reflectivity and thermal conductivity which restricts the penetration capability of laser welds to 1-2mm. Pulsed Nd:YAG lasers are suitable because of their high peak powers. Laser welding of brass can also suffer from porosity due to vaporisation of zinc.