Considerations in Welding Titanium-Based Aerospace Alloys

The high strength to weight ratio, excellent corrosion resistance, and good creep and fatigue properties of titanium alloys allow diverse application in various fields including the medical and aerospace industries. Commercially pure (CP) Ti and Ti-6Al-4V (6% Al, 4% V) are among the most widely used of the titanium alloys.

In the aerospace industry, Ti-6Al-4V is used to manufacture a number parts such as turbine disks, compressor blades, air frame and space capsule structural components, rings for jet engines, pressure vessels, rocket engine cases, helicopter rotor hubs, and fasteners.

Many methods are currently being used to weld this alloy with Tungsten Inert Gas (TIG) and Electron Beam (EB) the most widespread. However, lasers can be an effective alternative for welding even complex shaped components made of these alloys. In fact, laser welding can offer a number of advantages over both EB and TIG, including:

• Laser welding involves fewer manufacturing steps with edge preparation and cleaning and joint fixturing being the most time-consuming operations. Proper cleaning is required for all processes to minimize porosity in the weld.
• Laser welding, which offers low distortion and good productivity, is potentially more flexible than TIG or EB for automated welding. For example, laser welding is not restricted by a requirement to evacuate the joint region. Furthermore, laser beams can be easily positioned, enabling a large range of component configurations to be joined using various welding positions.
• The high power density of the laser beam creates a narrow, deeply penetrating weld pool, allowing through-thickness welds to be made rapidly and accurately in a single pass without the presence of a vacuum.
• The low heat input creates a narrow heat affected zone (HAZ) with limited distortion and residual stresses, which reduces the need for reworking.
• The process is easily automated for high volume production.
• Filler material can also be used to compensate for poor fit-up and mismatch for a butt joint configuration. Apart from compensating for inadequate fit-up of the material to be welded, filler material improves the weld geometry, that is, eliminates top and bottom bead undercut.

In recent months, Prima Power Laserdyne has undertaken a number of Initiatives to develop laser and processing parameters which produce good quality welds that meet the stringent requirements of aerospace.

Weldability of Ti-6Al-4V is, in general, very good; one fundamental challenge in welding titanium alloys is the elimination of atmospheric contamination since titanium has a strong affinity to bond with oxygen and nitrogen. The color of the surface after welding gives a good indication of the degree of atmospheric contamination. Under perfect shielding conditions the weld will be bright and silver colored. Discoloration at the outer edges of the heat affected zone (HAZ) is not generally significant and may be ignored.

As contamination increases, the color changes from silver to a light straw color to dark straw, dark blue, light blue, grey and finally a powdery white (see pictures below). The most likely contaminants are oxygen and nitrogen, picked up from air entrained in the gas shield or from impure shield gas, and hydrogen from moisture or surface contamination. The oxides, nitrides and hydrides that form as a result of contamination increase the weld and HAZ hardness and lead to reduced fatigue life and toughness.

Color of the weld metal and heat affected zone adjacent to the weld indicates the level of oxidation (ppm = parts per million) of the weld as well as quality and strength of the weld. Oxidation of less than 20 ppm produces a silvery, shiny weld.

Before welding, it is important to deburr and thoroughly clean the edges of the joint as well as any filler metal that is used. If not properly cleaned, contaminants, such as hydrocarbons, can lead to porosity. If filler wire is used, the wire should be cleaned with a lint free cloth and an efficient degreasing agent immediately before use. Following cleaning, the wire should not be handled with bare hands but while wearing clean, grease-free gloves.

The majority of the development work at Prima Power Laserdyne over the past year has centered on experimenting with different gas shielding devices, including coaxial, side jet, welding shoe, etc. The results show that the best looking welds (bright and silvery) were produced with the weld shoe because it provides inert gas coverage over a relatively wide area of the weld in the area of melting and as the material is cooled.

Coaxial shielding produced light contamination (light and dark straw colors) which is normally acceptable whereas the side jet produces welds with heavy contamination (dark blue/powdery white) which is not acceptable. The welds were cross sectioned and X-rayed to inspect for cracks and porosity.

These efforts have led to the demonstration of high quality butt and lap joint welds in various thicknesses of Ti-6Al-4V as highlighted in micrographs shown below.

Butt joint, 1.4mm thick	Butt joint, 3.2mm thick
Crossections of Ti-6Al-4V alloy welded using 3kW average power from a CW fiber laser and Argon shield gas

Further work is in progress to produce the welds with filler metal to eliminate the slight undercut of the top and bottom bead especially with thicker sections.