Welding of Ti-6Al-4V with fibre delivered laser beams
Paul Hilton1 , Jonathan Blackburn2 , and Pak Chong3
1 TWI Ltd, Cambridge
2 Laser Processing Research Centre, Department of Mechanical, Aerospace and Civil Engineering, University of Manchester
3 Welding and Materials, Subsea7, Aberdeen
Paper presented at ICALEO 2007, Orlando, FL. USA, 29 Oct - 1 Nov. 2007. Paper #1607.
Titanium alloys offer higher strength to weight ratios than steels, better fatigue performance than aluminium alloys and better corrosion resistance than aluminium alloys and stainless steels, and are now increasingly used in applications in the aerospace, chemical plant, power generation, oil and gas, medical and sporting goods sectors. For aerospace applications in particular, weld quality is very important, the occurrence of porosity being of particular concern. There are many potential benefits arising from the use of lasers for welding titanium alloys, however, there is little published information which quantifies the levels of porosity formed during welding under different conditions. In addition, the qualitative information currently available would indicate it is easier to produce high quality welds in titanium alloys using CO 2 lasers, than by using one micron wavelength fibre delivered laser beams. This paper will present results using a Yb-fibre laser, for the autogenous welding of Ti-6Al-4V alloy at 3, 5 and 9.3mm thickness. The quality of the resulting welds will be discussed, particularly with respect to porosity levels obtained and the weld profiles achieved. The results are compared with respect to a stringent set of criteria typical of those used in the aerospace sector.
Introduction and scope of work
Today's aircraft are by nature very complex structures, employing components of intricate design and shape. In both aeroengine and airframe manufacture, depending on the application, these components are made from materials chosen for their particular properties, such as mechanical strength, operating temperature range and corrosion resistance. In this respect, of particular interest are titanium alloys and nickel based superalloys. In many applications, because of the high cost of these materials and the expense of manufacturing from bulk solid, it is necessary to fabricate the components, and often welding is the only method available which will join the materials with the required integrity. The disadvantages of using welding to fabricate parts, however, are that welding processes can compromise the properties of the material (and therefore the component) in the fused area and heat affected zones. This meansthat before a welding method can be accepted for production applications, it must be shown to meet an established set of weld quality criteria, and rigorous testing. For aeroengine applications, these criteria are known to be very stringent.
Notwithstanding the above disadvantages of welding and the associated NDE, many aeroengine components made from titanium or nickel based alloys are currently fabricated using either conventional inert gas arc welding, plasma welding or electron beam welding. Some titanium alloy airframe components are also electron beam or arc welded. A general problem with arc welding, however, is that the speeds available are low, and the problems with electron beam welding involve the use of high vacuum and difficulties in following exactly the required joint line. Friction stir welding and laser welding offer the possibility to remove some of the problems encountered in production when using arc and electron beam welding. However, friction stir welding of titanium and nickel alloys has not yet been demonstrated as a viable production process, primarily due to excessive tool wear and lack of weld performance data. As a non-contact process, laser welding does not suffer from tool wear, and due to the very flexible nature of delivery of the energy to the joint and the advantages in terms of high speeds offered by the keyhole variant of laser welding, employment of this welding technique has potential advantages in terms of overall cost reduction. For welds in titanium alloys used in the manufacture of aeroengine components, achieving acceptable weld quality is not straightforward. Of particular concern is porosity formation when laser welding in the deep penetration or keyhole mode, and achieving the required control of weld geometry. This paper relates to laser welding performed on 3 to 9.3mm thick Ti-6Al-4V, and attempts to quantify observed porosity levels and relate these results to weld geometry.
Ti-6Al-4V is an α- β, heat-treatable alloy, consisting of a two-phase microstructure formed by the addition of up to 6% aluminium and varying amounts of β forming constituents, in this case, vanadium. Factors believed to influence the weld quality include surface appearance, weld metal porosity and weld profile, with these factors being interrelated. Although titanium is weldable by fusion processes, including laser welding, it has a high affinity for oxygen in air above a temperature of 650°C. As a result, the weld area must be protected from the atmosphere during cooling, even after the molten pool has solidified (melting point 1677°C). Without this protection, surface oxidation/contamination can cause embrittlement of the weld.
Porosity is one of the most common defects in the welding of titanium alloys and is usually caused by shielding gas entrapment before weld solidification, as reported for laser welding by Penasa et al.  Since Ti-6Al-4V does not contain any elements with low vaporisation temperatures, hydrogen from the environment and contamination of the parent metal surface should be considered as causes of porosity as well as entrapment of the shielding gas. Shinoda et al.  , and Denney et al.  , used chemical pickling to clean the materials before welding, and reported better results regarding porosity than when using mechanical cleaning, but without quantification of the results. Coste et al.  and Shinoda et al.  , both compared CO 2 laser welds with Nd:YAG laser welds made in titanium, and reported that, generally, it was easier to produce less porosity at the CO 2 laser wavelength than at the Nd:YAG wavelength. No reasons for this were given. These papers, and the more recent paper by Mueller et al.  , however, provide some evidence that when welding titanium, the combination of weld speed and laser power used will have an effect on weld shape, which in turn has an effect on weld porosity, with pores in the lower half of a weld being more prevalent than at the top of the weld. It is unfortunate that none of the current published literature on laser welding Ti-6Al-4V fully explains the exact nature of the gas shielding and gas flows used in the experiments or quantifies the results obtained in terms of weld quality.
This paper will address some of these issues by presenting the results of welding 3, 5 and 9.3mm thick Ti-6Al-4V plates, both as bead on plate runs and butt welds, made using a 7kW Yb-fibre laser. The welds were made autogenously,thus no filler materials were used in the trials. Cross sections of the welds made are presented as a function of laser power and welding speed. The welds were also submitted to radiographic examination, and the distribution and sizeof the observed porosity in these welds were recorded.
Equipment and experimental procedures
The 7kW Yb-fibre laser was manufactured by IPG Photonics, and has been in use at TWI for well over 3 years. It has a beam parameter product of 18mm.mradians, and its beam is delivered to the process head via a 0.3mm diameter optical fibre. For this work the process head was assembled with optics producing a 0.6mm beam waist. In the work reported here, the beam waist was kept on the surface of the material being welded. The process head was manipulated over a stationary sample, using an articulated robot arm. The titanium used in this work was supplied as machined plates which were then chemically cleaned. However, the time between chemical cleaning and welding in these experiments was not controlled. Immediately before welding, however, the edges of the plates to be welded, and the top and bottom of the plates close to the weld line, were mechanically abraded (by hand) and then degreased with acetone.
For all the results reported here, the same shielding gas arrangement was used. This used a nozzle, co-axial to the laser beam, onto which, at its base, a rectangular gas shield was added. This was approximately ten times longer than it was wide (in the welding direction), and finished with a flexible 'skirt' which trailed on the surface of the sample. To this configuration, a copper tube was added to direct a flow of gas at the region of the laser beam/workpiece interaction point. In addition, a flow of gas to the weld underbead was also used. Technical grade argon, with a purity of 99.996% was the only gas used in the experiments. For all the results described here, the shielding gas flows were nearly identical for each material thickness evaluated, only slight changes being made to these values to 'optimise' conditions at each thickness. All the welds reported later, made using this gas shielding system, displayed bright, shiny and oxide free surfaces. All the welds were made in the flat (1G) position.
Due to the limited amount of titanium available, bead on plate runs were made and evaluated before butt welds were made. The experiments reported here took place over an elapsed time of about 12 months, with the Yb-fibre laser used for other work between titanium trials. For the 3 and 5mm thick material, the 'optimised' conditions were found to be reproducible, both in terms of weld profiles obtained (for a given laser power and weld speed) and in terms of the average amounts of porosity found. For example, in the 3mm thick material, the same quality of weld was reproduced, for the same welding procedure, at least 3 times.
The laser power was measured at the workpiece using an Ophir power meter. Travel speeds were taken from the robot display. Radiographic images of the welds were made in accordance with BSEN 1435:1997. This radiographic examination was able to detect pores of the order 0.1mm in diameter. Several months after the work was completed, each of the radiographic images of all the welds reported here was re-examined by a single person, for consistency, who counted the observed porosity in a 76mm central section of each weld. Pore size was established using a calibrated viewing optic with a magnification of x10. The porosity count was recorded as the number of pores per 76mm length of weld, in the size range from 0.1mm up to the maximum seen. From this data the 'cumulative length' of porosity was calculated. To obtain data on weld profile, image analysis software was used to measure the polished and etched weld cross sections.
Weld quality criteria
AWS D17.1:2001, Specification for Fusion Welding for Aerospace Applications, by the American Welding Society, is a current international standard that can be applied to laser beam welding of aerospace components in titanium based alloys.However, the standard was not developed specifically for welded primary structures, nor for laser beam welding, since it also provides general requirements for electric arc and other high energy beam welding processes. The standard is also not specific to titanium alloys. The European Standard EN ISO 13919-2:2001, Welding - Electron and laser beam welded joints - Guidance on quality levels for imperfections - Part 2: Aluminium and its weldable alloys, was developed specifically for electron and laser beam welding, although not for titanium based alloys. The weld profile geometry criteria stated in this standard are more stringent than AWS D17.1:2001, although the sub-surface porosity criteria are less demanding. For aeroengine applications however, the quality acceptance criteria are generally much more stringent than those cited by both AWS D17.1:2001 and EN ISO 13919-2:2001, and are often company specific. EN ISO 13919-1:1997, Welding - Electron and laser beam welded joints - Guidance on quality levels for imperfections - Part 1: Steel, is in parts, more stringent than both EN ISO 13919-2:2001 and AWS D17.1:2001, and although developed for steel,the weld quality criteria in this standard compare more closely to those internal standards used by the aerospace industry than both AWS D17.1:2001 and EN ISO 13919-2:2001.
In the current absence of a broadly accepted standard, developed specifically for the laser beam welding of titanium based alloys for primary aerospace applications, the quality of the welds produced in this work have been evaluated against a set of quality criteria formulated from several current international standards. Table 1 details both the weld profile criteria and the sub-surface porosity criteria used for comparison to the welds produced in this study, for the three material thicknesses used in this project of 3, 5 and 9.3mm.
Figure 1 depicts the weld profile terminology used in Table 1.
Fig.1. Schematic indicating the weld geometry terminology used
Table 1: Typical quality criteria for weld profile and sub-surface weld porosity, for aeroengine applications
Criteria Material Thickness (mm) Sub-surface porosity9.35.03.0 Maximum dimension for a single pore, mm18.104.22.168 Accumulated length in any 76mm of weld - maximum, mm22.214.171.124 Weld Profile9.35.03.0 Undercut (Ca), mm≤ 0.47≤ 0.25≤ 0.15 Excess weld metal (R), mm≤ 1.6≤ 0.95≤ 0.65 Excess penetration (r), mm≤ 1.6≤ 0.95≤ 0.65 Linear misalignment, mm≤ 0.50≤ 0.5≤ 0.30 Incompletely filled groove (Cr), mm≤ 0.50≤ 0.50≤ 0.30 Root concavity (cr), mm≤ 0.50≤ 0.50≤ 0.30 Shrinkage groove (ca), mm≤ 0.50≤ 0.50≤ 0.30 Face weld width (L), mm≤ 9.0≤ 5.0≤ 4.0 Minimum weld width (Io)
Upper limit (mm)
Lower limit (mm) 3.02.52.0 2.01.51.0 Root weld width (I)
Upper limit (mm)
Lower limit (mm) 9.05.04.0 2.01.51.0
3mm Thick Material
A series of bead on plate runs were made between powers of 2.9kW and 4.8kW at speeds which produced fully penetrating 'silver' coloured welds, with no surface breaking porosity. Selected representative cross sections of these welds can be seen in Figure 2, superimposed on a graph of laser power against welding speed. As can be seen, the profiles of these welds are quite similar, notwithstanding the range of power and speed used, with the welds made at the lowest laser power showing the narrowest minimum weld width, Io. The heat input range for the welds shown in Figure 2 was from 0.062kJ/mm to 0.054kJ/mm.
Fig.2. 3mm cross-sections arranged by laser power (kW) vs welding speed (m/min)
Figure 3 shows the cumulative porosity count, taken over 76mm of weld length, for the above welds and several more welds, made within the same power range, plotted as a function of pore size, at 0.1mm diameter increments. The maximum pore size seen was 0.3mm in diameter. No pores or other imperfections above this size were seen in any of the examined welds. The first group of three results, reading from left to right, are for a laser power of 2.9kW, at the three different welding speeds shown. The second group of results correspond to welds made at a power of 3.8kW, again at the three speeds shown. The next group of 15 welds were made at a power of 4.8kW, at speeds between 5.3m/min (left hand side) and 4.7m/min (right hand side). In all these 15 welds, the only porosity detected was less than or equal to 0.1mm in diameter. In addition, these 15 welds were not all made at the same time and represent three separate experiments, performed over a time interval of several months, indicating the reproducibility of the work. Following an analysis of the bead on plate runs, selected parameters were chosen to produce a series of butt welds in the 3mmthick material. The fourth grouping shown in Figure 3, containing six results, represents the porosity levels in butt welds made using 4.8kW of laser power and travel speeds from 5.2 (lhs) to 4.8m/min. Once again, very low levels of porosity were achieved, with no pores larger than 0.1mm seen. The last group of three bead on plate runs, were also made at 4.8kW but correspond to slight changes in the gas shielding system. Although not quantified in this paper, correct gas shielding must be applied to make welds with the minimum porosity levels shown in Figure 3.
Fig.3. Porosity count for 3mm thickness Ti-6Al-4V
When compared to the quality criteria for sub-surface porosity given in Table 1, all of the butt welds and all of the bead on plate runs made at a power of 4.8kW, plus three of the bead on plate runs made at 3.8kW, met the acceptance criteria.
The welds made at the lowest laser power of 2.9kW and all the welds made with a slight change to the gas shielding system would fail the sub-surface quality criteria, in terms of accumulated length of porosity (limit 1.6mm),primarily due to the increased numbers of pores of size 0.1mm in diameter seen in these welds. In Figure 3, welds which met the porosity criteria are identified with a 'P' and welds which met the geometric criteria are identified with a 'G'.
Not all of the welds shown in Figure 3 were sectioned and assessed against the weld geometry criteria shown in Table 1. However, at least 6 of the bead on plate runs met all of the weld profile criteria. For the butt welds, both extremes of welding speed used again produced weld profiles meeting all of the geometric criteria listed in Table 1. Figure 4 shows cross sections of both the bead on plate run and the butt weld, made at 4.8kW of power, at a speed of 5.2m/min. Both these welds met the porosity and geometry criteria listed in Table 1. It is also interesting to note that there were only very small differences in geometry between these two sections.
Fig.4. Bead on plate (left) and butt (right welds made at 4.8kW and 5.2m/min in the 3mm thick material
5mm Thick Material
A similar approach was taken for the 5mm thick titanium. In this series of experiments, laser powers between 3.8kW and 6.7kW, covering a speed range from 1.5 to 2.2m/min, were used to produce a series of bead on plate runs. The gas flow rates in the shielding system were changed slightly from those used for the 3mm material, but its configuration remained the same and the results reported below were all made at the same gas flow rates. Selected representative cross sections of these welds can be seen in Figure 5, superimposed on a graph of laser power against welding speed. An analysis of these weld profiles shows some interesting effects at laser powers above 6kW, where it has been possible to produce welds with smaller faceweld widths than root weld widths. For certain sets of parameters, these weld profiles also transferred to butt welds. All these fully penetrating bead on plate runs showed smooth, bright, top and underbeads, free from oxidation and surface breaking porosity. Figure 6 shows the porosity count for these bead on plate runs and a series of butt welds.
Fig.5. 5mm cross-sections arranged by laser power (kW) vs welding speed (m/min)
Fig.6. Porosity count for 5mm thickness Ti-6Al-4V
The first six groups of data in Figure 6 show (from left to right), bead on plate runs at 3.8kW and speeds from 1.5 to 2.0m/min, bead on plate runs at 4.4kW and speeds from 1.8 to 2.0m/min, bead on plate runs at 4.9kW at speeds from 2.0 to 2.2m/min, bead on plate runs at 5.4kW at speeds of 2.0 to 2.2m/min, bead on plate runs at 6.4kW at speeds from 2.0 to 2.2m/min and bead on plate runs at 6.7kW at speeds from 2.0 to 2.2m/min. It is clear that there is more small scale porosity in the welds made on 5mm thick material than seen in the welds made using 3mm thick material, and no clear trend as to how the power/speed combination affects the porosity is apparent. In fact, nine of these bead on plate welds met the acceptance criteria for cumulative length of porosity stated in Table 1 (limit 2.7mm) and these are indicated with a P in Fig 6.
Regarding weld profile, seven of the bead on plate runs met the weld geometry criteria for 5mm thick material shown in Table 1, and these are indicated with a G. All of these were at powers of 4.4kW and above. Only five of the bead on plate runs, (all made at either 4.4 or 4.9kW) met both the porosity and weld profile criteria. It is interesting to note that the two bead on plate runs made at 6.4kW, which met the porosity criteria, only failed the geometrical criteria in that the face weld width was wider than the root weld width.
Due to the lack of clear direction from the bead on plate trials, it was decided to make butt welds at four combinations of laser power and travel speed in the 5mm thick material. The results of the pore counting exercise for the butt welds can be found in the last group of eight sets of data presented in Figure 6. From left to right the conditions were, 3.8kW/1.7m/min, 2 off - 4.4kW/1.9m/min, 2 off - 4.9kW/2.1m/min and 3 off - 6.7kW/2.0m/min. In all these butt welds, no porosity greater than 0.3mm in diameter was seen but only the three welds made at the highest power (6.7kW) met the criteria for accumulated length of porosity, of 2.7mm. In addition, only one of the butt welds produced could meet all of the weld geometry criteria. The three welds meeting the porosity criteria failed the geometry criteria in that the face weld width was smaller than the root weld width. Figure 7 shows the similar cross sections of both a bead on plate run and a butt weld made at 4.4kW of power, at a speed of 1.9m/min, for the 5mm thick material.
Fig.7. Bead on plate (left) and butt (right) welds made at 4.4kW and 1.9m/min in the 5mm thick material
9.3mm Thick material
In order to penetrate the 9.3mm thick material, only two laser powers of 6.5 and 6.7kW were used to establish bead on plate welding conditions, at speeds from 0.95 to 1.1m/min. The left hand group of results in Figure 8 shows the porosity counts for the six bead on plate conditions used. Once again it should be noted that no porosity greater than 0.3mm in diameter was detected in any of the welds, but in the 9.3mm thick material,larger amounts of 0.2mm and 0.3mm porosity were observed. It is also worth noting that in the 9.3mm thick material, the number of pores of the order of 0.1mm in size, was generally less than for the 5mm thick material. Four of the bead on plate runs (marked with a P in Figure 8) passed the criteria for accumulated length of porosity. However, all the bead on plate welds which fully penetrated the material, managed to meet the weld profile criteria stated in Table 1.
Fig.8. Porosity count for 9.3mm Ti-6Al-4V
Based on the results of the bead on plate trials, five butt welds were produced, all at the highest laser power available of 6.7kW and at speeds of 0.90m/min, (2 off) 1.0m/min and (2 off) 1.05m/min (in order to obtain some idea of reproducibility). A single butt weld was also made at 6.5kW, with a speed of 1.0m/min. The porosity assessment for these welds can be seen in the grouping in the right hand side of Figure 8. The welds made at a power of 6.7kW and a welding speed of 1.0m/min, had very low porosity, in fact easily meeting the criteria for accumulated length stated in Table 1, but both of these welds failed the weld geometry criteria, with Io being too small. One of the other butt welds made at 6.7kW and a welding speed of 1.05m/min, failed both the weld geometry criteria (Io) and the porosity criteria. The remaining butt weld made at 6.7kW and 1.05m/min passed the sub-surface porosity criteria, but failed the weld geometry criteria (Io). The single weld made at 6.5kW met the weld geometry criteria and the sub-surface porosity criteria (although it contained significantly more porosity than the welds made at a power of 6.7kW and welding speed of 1.0m/min). Figure 9 shows cross sections of both a bead on plate weld and a butt weld made at 6.7kW of power, at a speed of 1.0m/min, for the 9.3mm thick material.
Fig.9. Bead on plate (left) and butt (right) welds made at 6.7kW and 1.0m/min in the 9.3mm thick material
This work has shown that it is possible to produce butt welds in Ti-6Al-4V, using fibre delivered laser beams, that, for a range of thicknesses, could be capable of meeting the extremely stringent weld quality criteria demanded by aeroengine manufacturers, particularly in terms of subsurface porosity and weld bead geometry. The work has also shown that as the material thickness increases, both these sets of quality criteria - especially those for porosity,become more difficult to meet. Figure 10 shows three sets of weld geometry criteria, superimposed on actual weld profiles, for the 3, 5 and 9.3mm thickness materials used. In this figure the relative sizes of the three welds are to the same scale. Fromthese figures it is clear that the area of most concern (with respect to the criteria in Table 1), is the minimum weld width and the acceptable range for this parameter. Also clear from this figure is that when using laser welding, the criteria for excess weld metal is easily met, as is that for excess penetration(at least for 5 and 3mm thick material). For the 9.3mm thick material, the molten volume forming the weld is quite large, resulting in quite significant excess penetration, in the case shown, quite close to the acceptable limit. It should be remembered that the 9.3mm thick material was welded at a speed of only 1m/min and it should be possible to increase this speed using a higher power laser. In these figures, even the criteria for shrinkage groove and undercut are easily met.
Fig.10. The geometric weld quality criteria outlined in Table 1 superimposed on weld macrosections for the three thicknesses of material used in this work. The images are in scale with respect to one another
An analysis of all the cross sections of welds made in this work indicated that if any porosity was evident, it occurred in the lower half of the weld. It is possible that one means by which this could be reduced further is by paying more attention to the underbead gas shielding arrangements. For all the results presented in this paper, it was always possible to meet the specification for maximum diameter of an isolated imperfection, even for the maximum thickness of 9.3mm. What was not met consistently, however, for the thicknesses above 3mm, was the criteria for cumulative length of imperfections. This reached a maximum of 4.7mm in the 5mm butt welded samples (2.7mm in Table 1) and a maximum of 6.4mm in the 9.3mm butt welds (4.9mm in Table 1). For the 3mm thick material the maximum cumulative length of porosity in any butt weld was only 0.4mm, i.e. 1.2mm below the specification.
Certainly for the 5 and 3mm thick material, and possibly for the 9.3mm thick material, there is some evidence that the weld profile is linked to porosity level. Of particular interest, are the low levels of porosity found in the welds in the 5mm thick material, made at high powers, which have a face weld width smaller than the root weld width. This could be due to unusual keyhole behaviour, producing a larger molten volume in the lower part of the weld, which in turn facilitates the escape of any trapped undershield gas. However, this phenomenon requires more work to fully understand what is happening. In addition, an analysis of the gas entrapped in the pores would be useful to determine its source.
This work has evaluated the suitability of utilising a 1 micron wavelength fibre delivered laser beam, for the butt laser welding of three different thicknesses (3mm, 5mm and 9.3mm) of Ti-6Al-4V. Analysis of the weld profiles and sub-surface porosity, via sectioning and radiography, has drawn the following conclusions:
- At the correct power, and with the correct shielding gas parameters, reproducible butt welds in 3mm thick Ti-6Al-4V were made which could achieve both sub-surface porosity and weld profile criteria consistent with those stated in current stringent aeroengine standards, at a variety of welding speeds.
- At the thickness of 5mm, it was possible to produce butt welds which met the comparison criteria for sub-surface porosity, but not the weld profile criteria. The only butt welds at this thickness which met the criteria for sub-surface porosity, had a face weld width that was smaller than the root weld width.
- At the thickness of 9.3mm, it was possible to produce a butt weld which met both the comparison criteria for the maximum pore size, the accumulated length of porosity and all of the weld profile criteria. At this thickness, other butt welds meeting the weld porosity comparison criteria, failed the weld geometry comparison criteria only in that the minimum weld width was too small.
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This work has been supported by both the Yorkshire Forward Regional Development Agency and the Objective 1 European Regional Development Fund for Yorkshire and Humber in the UK. The authors are also grateful to The Boeing Company and General Electric, for supporting this work, providing the necessary materials used and stimulating discussions of the results.