Al-Li alloys - fusion welding

TWI Bulletin, May/June 1994

Aluminium-lithium alloys are lighter and stiffer than conventional aluminium alloys and are seen as useful aerospace materials. As with all structural materials, joining is of paramount importance if they are to be accepted for widespread use. Mike Ellis looks at fusion welding of some common Al-Li alloys.

The late 1950s heralded the development of the first Al-Li alloy (Al-4.5Cu-1.1Li-0.5Mn-0.2Cd), designated 2020. The Soviet alloy 01420 (Al-5.5Mg-2.1Li-0.2Mn-0.18Zr) was patented in the 1960s and was claimed to be the only weldable Al-Li alloy in widespread commercial use. ^[3] A Swiss alloy (Al2.5Mg-0.6Li-0.55Mn-0.14Ti) developed in 1980, ^[4] was also claimed to have good weldability as well as excellent formability, moderate strength and good elevated temperature strength.

Table 1 Current Al-Li alloys and compositions ^[5]

At present, there are a number of other Al-Li alloys in use ( Table 1). Most of these alloys contain 2-3wt%Li, however, an Al-Cu-Li alloy, Weldalite 0.49 ^TM (Al-5.4Cu-1.3Li-0.4Ag-0.4Mg-0.14Zr) is also available. More recent developments have led to spray deposited UL40 ^TM (Al-4.33Li-0.11Zr) which has 5% lower density and 5% higher modulus compared with the earlier 8090 alloy (Al-2.5Li-1.2Cu-0.7Mg-0.1Zr) which is made by a more conventional production route. ^[5]

This article covers the fusion welding of Al-Li alloys, highlighting potential problems and the suggested methods to overcome them. Advice is given on successful joining practices with various fusion welding techniques TIG, MIG, laser, electron beam and resistance welding.

Welding

When welding all Al alloys, the first objective is usually to produce a sound joint. This involves addressing the issues of porosity and weld cracking.

Weld metal porosity

The problem of weld metal porosity is greater in lithium-bearing than in conventional aluminium alloys. This is fundamentally because of the reactivity of the lithium and has been associated with a surface skin on the alloy ^[6] which forms at high temperatures during hot working. ^[7] It has been suggested that lithium oxides, hydrides, nitrides and carbonates which exist in the surface layers are responsible for increased porosity, ^[8] but improved shielding reduces porosity. Absorption of water vapour by LiO ₂ formed during solution heat treatment or hot rolling can also lead to a porous weld metal, ^[9] while it is believed that Al-Li alloys contain higher levels of hydrogen than other aluminium alloys, this hydrogen being contained in surface layers. ^[10]

Hot cracking

Solidification and liquation cracking in aluminium alloys are related to the contents of alloying elements. In 8090 alloy, solidification cracking has been investigated with the Houldcroft test.* The material performed in a similar manner to the Al-Mg-Si alloy, 6082, in terms of sensitivity to solidification cracking. ^[6] This is significant because 6082 is recognised as an alloy which is prone to solidification cracking and is usually welded with non-matching filler materials to prevent cracking. Further work has shown ^[11] that the Al-Li alloys, 2090 and 8090, are more susceptible to solidification cracking than other commercial aluminium alloys such as 2219, 5083 and 6061. The 2090 alloy appears more resistant to cracking than the 8090 and 2091 grades. The cracking was associated with networks of eutectic at grain boundaries which showed little evidence of eutectic cracking healing, which has been observed in 2219 and some 5000 series alloys. ^[11] It was suggested that, if more eutectic were developed during final stages of solidification of the alloy by a change in composition, solidification cracking resistance could be improved. ^[11]

*The Houldcroft fishbone test involves deliberately changing the strain induced by welding during laying of a test bead. Houldcroft varied the restraint by cutting stress relieving slots into the plate, the restraint decreasing as the weldbead progresses.

Overcoming welding problems

Porosity

The reaction layer must be removed from the surfaces of components before welding. Removing 0.2mm from each surface to be incorporated into the weld has been suggested for 1.5 and 6.5mm sheet. ^[6] This is generally done by machining, since normal wire brushing does not remove enough material. ^[6] The following techniques for surface preparation of Al-Li alloys before welding were investigated by Fedozeev et al. ^[12]

(a) Degreasing with organic solvents followed by mechanical scraping;
(b) Pickling in a 5%NaOH solution followed by a rinse in HNO ₃ to remove reaction products;
(c) Pickling such that a passive film is produced;
(d) Pickling followed by mechanical scraping;
(e) Mechanically milling up to 0.5mm from the surface;
(f) Chemically milling up to 0.3mm in a 200 g/litre alkaline solution (a more alkaline solution than (b) above);
(g) Vacuum degassing followed by mechanical scraping.

It was found that welds made using TIG with filler and surface preparation techniques (a) and (d) contained numerous pores. They were located mostly at the fusion line, with very few pores at the centre of the weld, whereas, preparation techniques (e) - (g) sucessfully prevented pore formation. Furthermore, Fedoseev et al ^[12] also noted that, of the latter group, the chemical milling technique (f) is generally the most practical because it can be performed on a wide variety of shapes, e.g. wide sheets and complex extrusions, where mechanical scraping is difficult. They observed a marked drop in porosity after removing only a 0.05mm layer from the surface, and recommended removing 0.2-0.3mm from sheets of less than 2mm thickness to ensure that weld zone porosity will be virtually eliminated.

Inert gas back purging can also help to reduce porosity ^[6,13] and promote formation of clean and shiny penetration beads. The effect of arc energy on weld quality has also been studied and it was shown ^[12] that, as arc energy decreased, porosity decreased to a minimum but then increased slowly from this low level. This minimum apparently occurred because of competition between growth of gas bubbles and gas trapping in the weld. ^[12]

Hot cracking

Use of non-matching, crack resistant filler wires is conventional practice in aluminium welding. Three types of filler wire were used for 8090 Al-Li alloy by Gittos. ^[6] Al-Si wire (4043A), which is generally regarded as being a crack resistant filler, Al-Mg wire (5556A) and Al-Cu (ER 2319) filler. Using 5556A filler wire, crack-free welds were made with both the MIG and TIG processes. The use of Al-Cu (ER 2319) filler wire produced the higher as-welded strengths, but some sensitivity to cracking was observed. The ER 2319 is also a heat treatable filler wire so a further increase in weld metal strength is possible. Similar observations were made ^[14] when Al-Mg wires were used to weld the Al-Mg-Li alloy 01420, and all welds produced had a high resistance to solidification cracking. For 8090, the use of matching filler wires not only produced crack-free welds but gave rise to weld strengths comparable with welds made in the stronger conventional weldable Al-Zn-Mg alloys after an appropriate post-weld heat treatment (PWHT) involving solutionising, quenching and ageing at suitable temperatures. ^[9] For practical purposes use of wires based on the 5556A composition is advisable. ^[6]

Fusion welding techniques

A study has been made on 2090 Al-Li alloy ^[15] showing joint strengths for a variety of fusion welding processes ( Fig.l). As can be seen, the power beam techniques produced higher tensile strengths, which were attributed to smaller heat affected zones and finer weld metal microstructures.

Metal inert gas (MIG)

The use of different filler wires produces welds with different mechanical properties ( Table 2) and profiles, as seen in Fig.2. ^[6] The process parameters for these welds are shown in Table 2 and can be used for guidance. Some porosity was observed in all the welds (although recommended surface preparation techniques were used, i.e. machining surface layers away), but the observed levels of porosity were not exceptional for Al-alloy MIG welds.

Table 2 Transverse tensile tests on MIG welded 6mm thick 8090 Al-Li alloy plate [6]

Tungsten inert gas (TIG)

Welding conditions for TIG welds ( Fig.3) made in 1.5mm thick 8090 Al-Li alloy, using a variety of different filler metals, are given in Table 3. These parameters can be used for guidance. The various wires produced welds of varying strength, depending on the filler choice and whether or not post-weld heat treatment was applied. It was possible to produce crack-free autogenous TIG welds in 8090 alloy. Weldalite 049 ^TM alloy sheet has also been welded using TIG and ER 2319 and parent filler wire. ^[16]

Table 3 TIG welding details for 1.5mm thick 8090 Al-Li alloy sheet [6]

Tensile strengths of weldments can be about as high as those of the strongest of welded conventional Al alloys, e.g. 310MPa for 7039 and 275MPa for 2219. ^[16] For example, cross weld tensile strengths of over 340MPa could be attained in the as-welded condition.

Laser

Use of autogenous laser beam welding (LBW) to join Al-Li alloys has proved successful, ^[17] and indeed work has shown laser welding can be more successful for Al-Li alloys than other Al alloys. Nonetheless, porosity can be a problem. In laser welded 8090 Al-Li alloy, two types of pore were seen; ^[18] near-spherical pores containing hydrogen with a dendritic solidification pattern on their walls, and a second, more irregular type of pore, probably related to keyhole dynamics. ^[19]

No liquation and solidification cracking was observed in laser trials on thin 8090 Al-Li alloy sheet ( Fig.4). Reheat treating can produce strengths up to 90% of the parent metal without the use of filler wires. ^[17] Some microporosity may be observed, mainly at the weld edge, but no large pores are evident in the weld metal in Fig.4.

Electron beam

When varying beam conditions were used to increase heat input (such as a circular beam deflection), joint efficiencies were reduced for the as-welded condition. Non-vacuum EBW produced the lowest strengths in the as-welded condition. Metallographic observations revealed the presence of small tears at the fusion zone interface, these being intergranular and transverse to the weld orientation. This tearing was related to heat input; the higher the heat input, the more tearing. The melting of eutectic films on grain boundaries coupled with thermal stresses were thought to be the cause of the problem. ^[15]

Electron beam welds have been made on 12.5mm thick plates of 2090 and 2091 Al-Li alloys to study effects of welding conditions on the weldability and microstructure. ^[19] The results indicated that these alloys are susceptible to fusion zone (FZ) cracking. All the welds contained about 2-5% porosity in the FZ. However, this is not excessive for EB welding of aluminium alloys. Softening was observed in the HAZ of alloy 2090 Al-Li alloy but, after postweld ageing, the HAZ strength was restored to the same levels as the base metal. No such softened zone was observed in the HAZ of 2091 Al-Li alloy because of its ability to age naturally at room temperature after welding. The results of hot ductility tests indicated that these alloys are not susceptible to HAZ cracking. However, there was some evidence of liquation cracks in the HAZ extending from the FZ to the unaffected base metal. ^[19]

Resistance spot

Resistance welding has been applied to 8090 Al-Li alloy sheet and is seen as a fast and economical way of joining these materials. Using a single electrode pressure schedule, welds containing both porosity and cracking were produced ( Fig.5a). However, on adopting a dual pressure sequence ( Fig.5b), the cracks and porosity were eliminated giving welds of good quality and acceptable size. ^[17] The low thermal conductivity of these alloys allows use of lower welding currents which increases electrode life, ^[20] while use of lower welding currents translates to an energy saving of approximately 50% when compared with the energy used to spot weld conventional aluminium alloys. Seam welding of 8090 Al-Li alloy sheet has produced good quality welds for a constant welding force. ^[17]

Conclusions

* The Houldcroft fishbone test involves deliberately changing the strain induced by welding during laying of a test bead. Houldcroft varied the restraint by cutting stress relieving slots into the plate, the restraint decreasing as the weldbead progresses.

References