Ti-6Al-4V is widely used in the aerospace industry to manufacture parts such as turbine disks, compressor blades, airframe and space capsule structural components, rings for jet engines, pressure vessels, rocket engine cases, helicopter rotor hubs, and fasteners.
This article summarizes results of a detailed study of laser butt welding of Ti-6Al-4V with filler wire prepared by the Prima Power Laserdyne Applications Engineering group.
About the alloy
Ti-6Al-4V is a two-phase alpha-beta (α-β) alloy. Aluminum serves as an alpha phase stabilizer, while vanadium is a beta phase stabilizer (Figure 1). Table 1 shows the chemical composition  of the alloy. The alloy is strengthened through various heat treatment cycles involving solution heat treatment (annealing), rapid quenching, and aging .
Table 1: Chemical composition of Ti-6Al-4V alloy (weight %).
Advantages of laser welding Ti-6Al-4V
Laser welding is one of several methods for joining Ti-6Al-4V. Tungsten Inert Gas (TIG) and Electron Beam (EB) are the most common techniques for welding these alloys.
Laser welding offers a number of advantages over both EB and TIG, including:
- Laser welding involves few manufacturing stages, with edge preparation and joint fixturing being the most time-consuming operations.
- The high beam power density creates a narrow, deeply penetrating weld pool, producing through-thickness welds rapidly and accurately in a single pass without the presence of vacuum.
- The low heat input creates a narrow heat affected zone (HAZ) with limited distortion and residual stresses, which reduces the need for reworking.
- Filler material compensates for poor fit-up and mismatch in the butt joint configuration. Apart from improving fit-up, filler material improves the weld geometry by eliminating top and bottom bead undercut.
Considerations for laser welding Ti-6Al-4V
Laser welding processes must take into account that thermal cycles generated during welding of this alloy can alter its microstructure. These changes in microstructure can lead to degradation of its mechanical properties .
Phase transformation in the fusion zone (FZ) or in the heat-affected zone (HAZ) is the basis for the low ductility of most alpha-beta alloy welds. However, welding with filler wire of matching composition improves ductility and toughness.
Weldability of Ti-6Al-4V alloy whether with or without (called autogenous) filler material is in general very good. However, the fundamental problem in welding of titanium alloys is the elimination of atmospheric contamination.
Surface discoloration gives a good indication of the degree of atmospheric contamination. Under perfect shielding conditions, the weld will be bright and silvery in appearance. As contamination increases, the color of the weld and surrounding area changes from the silvery, metallic appearance to tan to blue and, finally, to powdery white. With the increased contamination comes increased hardness of the weld FZ and HAZ. The brittle carbides, nitrides and oxides that increase hardness also reduce fatigue properties and toughness.
Process R&D for laser welding Ti-6Al-4V with filler wire
Prima Power Laserdyne has undertaken a number of projects to develop laser and processing parameters that yield high quality welds meeting the stringent requirements of the aerospace industry. An important part of the development work has centered on experimenting with different gas shielding devices, including coaxial, side jet, and welding shoe.
Several sets of laser and processing parameters, including welding speeds and wire speeds, were used to produce full penetration welds with different amounts of joint gap. Ti-6Al-4V titanium alloy plates were machined into welding specimens having the following dimensions: 50 mm wide, 100 mm long and 3.2 mm thick. The chemical composition of the base material and 1 mm diameter Ti-6Al-4V filler wire (ERTi-5; AWS A5.16 16-90) appears in Table 1.
Welding tests with filler wire were carried out using a Convergent Photonics CF3000 multimode fiber laser operating at a mean power of 3 kW. High purity argon at a flow rate of 40 l/min for the top bead and 20 l/min for the underbead was used for all tests.
The main processing and laser parameters for butt joints with different amounts of joint gap are listed in Table 2.
Table 2: Process parameters
|Power (kW)||Fiber size (µm)||Spot size (mm)||Joint gas (mm)||Welding speed (m/min)||Wire feed speed (m/min)||Focus position (mm)|
Welds were sectioned, polished, and etched in Kroll’s regent for 5-10 seconds. The FZ and HAZ of each sample were inspected for weld penetration, microstructure, including any micro cracking, and porosity.
Selected samples were x-rayed for porosity. Vickers microhardness was measured using a Struers Duramin A-300 hardness tester at a load of 500 g, a dwell period of 15 seconds, and an interval of 0.2 mm.
Observations during wire feed welding
Laser welding with the filler wire is a multiparameter process. Following are comments about parameters specific to welding with wire feed.
Welding/filler wire speed: The wire feed rate for a given air gap and plate thickness is an important parameter and depends on welding speed, cross sectional area of the gap between the joint face, and cross sectional area of the filler wire. Addition of filler wire generally results in a 10% to 20% decrease in welding speed for a given laser power to compensate for the laser energy required to melt the wire.
If the filler wire speed is too low, the heat generated from the laser beam can cause the wire to melt faster than it is fed into the laser beam. This can lead to areas along the weld in which no filler metal is deposited (Figure 2A).
Too high filler wire rate creates conditions in which there is excess metal added to the weld joint and/or in which the energy delivered to the welding area is not sufficient for stable and sustained wire melting.
In the former case, the volume of liquid metal at the end of the wire increases, flooding the area of the weld.
In the latter case, un-melted wire may enter the back area of the pool, pushing out the liquid metal, which solidifies to forms humps of weld metal and porosity at the root of the weld.
Excessive wire speed can also reduce the penetration depth, weld width, and top bead height (Figure 2B).
Laser beam-filler wire alignment: An exposed length of wire that is too short prevents the wire from melting at the start of the weld, resulting in the laser beam directly melting the material in the weld joint.
Conversely, an exposed length of wire that is too long causes the extended wire end to be pressed against the plate surface. At the beginning of the weld, the laser beam melts the wire through, dividing it into two parts. As a result, the weld includes some un-melted wire that is welded onto the surface and difficult to remove. In an extreme case, the welded-on wire end will collide with the gas shielding nozzle, disturbing or even eliminating the gas shielding.
Wire feed delivery angle: The optimum angle of the wire to the weld surface is typically 45 degrees, though an angle of the wire between 30 and 60 degrees from vertical may be used.
A 45-degree angle simplifies setting of the intersection of the wire and laser beam centerline. Angles less than 30 degrees lead to the wire intersecting a larger area of the laser beam, creating a tendency for melting and vaporization of the wire without incorporating it into the weld pool. Angles greater than 60 degrees make setup more difficult.
Focused spot size: The spot size should be close to that of the diameter of the filler wire. A laser spot size too small compared to the wire diameter leads to welds with porosity because the filler wire has not melted thoroughly.
Figure 3 shows the transverse sections of the laser welds with 0.2 mm and 0.4 mm joint gaps for 3.2 mm thick butt joints. All welds (Table 2) exhibited full penetration and no cracking. The shape of the top and under beads and width of the FZ were similar for all the gaps tested.
Figure 4 shows the microstructure of the FZ consisting mainly of martensite (α’) with a small amount of retained β. This structure indicates rapid solidification. The HAZ microstructure is a mixture of base metal and FZ, that is, transformed β grains and martensitic structure near the FZ boundary.
The two main defects associated with laser welding titanium based alloys are underfill of the top bead (Figure 5) and fusion zone porosity.
Underfill of the top bead is due to loss of material caused by evaporation, spatter, and flow of the molten material. The underfill defect reduces the cross sectional thickness of the weld, which may lead to reduced tensile strength, and creates a concave surface that can yield reduced fatigue strength of the weld.
Welds produced with filler wire displayed no underfill of the top bead. Weld geometry was very similar for all the joint gaps.
Porosity is another main defect in laser welding of titanium alloys. There are a number of causes for formation of porosity, including grease, oil, and dirt on the surface of the weld joint and filler wire. Improper gas shielding also contributes to formation of the porosity, with flow rate and the design and performance of the gas shielding device being most important.
The gas shielding shoe used for these tests provides inert gas coverage over a relatively wide area of the weld, shielding the area of melting as well as the welded material as it cools. Proper shielding of all welds was evidenced by their bright and silvery appearance.
All samples were x-rayed to inspect them for cracks and porosity. Welds produced with a 0.4 mm gap had a few micro pores, though all other welds were porosity free. Figure 6 shows x-ray images of two joints, including those with 0.2 mm and 0.4 mm gap.
Figure 7 shows the effect of joint gap on the microhardness of welded samples. As expected, the FAZ has the highest hardness because of its martensitic microstructure. The hardness of the HAZ is approximately 17% lower than that of the FZ because of reduced martensitic microstructure. Hardness is similar for all the joint gaps.
Butt joints in 3.2mm thick Ti-6Al-4V titanium based alloy were welded using a 3 kW multimode fiber laser with filler wire matching the parent metal.
The main conclusions from the present study are:
- Full penetration welds without any cracking were produced for all amounts of gap included in the project.
- The weld geometry in terms of FZ and HAZ as well as width of the FZ was similar for all the gaps tested.
- The microstructure of the FZ mainly consisted of martensite (α’).
- The HAZ microstructure was a mixture of base metal and FZ, that is, transformed β grains and martensitic structure near the FZ boundary.
- Addition of filler wire eliminated the top bead underfill.
- All the welds were pore free apart from weld with 0.4 mm gap.
- There was no significant difference in the hardness values observed for the range of joint gaps.
- ASTM B265; Grade 23 Titanium specification; 2015; Revision “15”.
- X. Cao, M. Jahazi; “Effect of Welding Speed on Butt Joint Quality of Ti-6Al-4V Alloy Welded Using a High-Power Nd:YAG Laser”, Opt. Laser Eng., 47(11) (2009), 1231–1241.
- L.W. Tsay, C.Y. Tsay; “The Effect of Microstructures on the Fatigue Crack Growth in Ti-6Al-4V Laser Welds”, Int. J. Fatigue, 19(10) (1997), 713–720.
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