Laser welding of Ti6Al4V titanium alloys

Abstract

The high strength to weight ratio and excellent corrosion resistance of titanium alloys allow diverse application in various fields including the medical and aerospace industry. Several techniques have been considered to achieve reliable welds with minimum distortion for the fabrication of components in these industries. Of these techniques, laser welding can provide a significant benefit for the welding of titanium alloys because of its precision and rapid processing capability. For pulse mode Nd:YAG laser welding, pulse shape, energy, duration, repetition rate and peak power are the most important parameters that influence directly or synergistically the quality of pulsed seam welds. In this study, experimental work involved examination of the welding parameters for joining a 3-mm thick titanium alloy using the Lumonics JK760TR Nd:YAG pulsed laser. It has been determined that the ratio between the pulse energy and pulse duration is the most important parameter in defining the penetration depth. Also it has been observed the variation of pulse duration at constant peak power has no influence on the penetration depth. Consequently, to increase the penetration depth during welding, the role of the laser parameters such as pulse energy and duration and peak power have been investigated to join 3 mm thick Ti6Al4V.

Introduction

The low density, excellent high temperature mechanical properties and good corrosion resistance of titanium and titanium alloys have led to a diversified range of successful applications for the demanding performance and reliability requirements of the medical, aerospace, automotive, petrochemical, nuclear and power generation industries as reported by Wang et al. (2003) and Casalino et al. (2005). When the operation temperature exceeds 130 °C, titanium alloys can be used as replacements for aluminium-based materials to achieve improved mechanical properties at elevated temperatures for applications such as the external shells of turbines, the power units for avionics and the landing gear structural components in Boeings 747 and 757 as mentioned (Lima, 2005). Alternatively, as titanium is exhibit very low corrosion rates in human body fluids as demonstrated (Choubey et al., 2005), other applications that are relevant to the medical industry include prosthetic devices such as artificial heart pumps, pacemaker cases, heart valve parts as well as load bearing bone such as for hip bone replacement.

Although the welding procedures and equipment used for austenitic stainless steel and aluminium alloys can be applied in order to join commercially pure titanium and most of titanium alloys, their increased reactivity with atmospheric elements at high temperatures necessitates additional precautions to shield the molten weld pool as determined (Key to metals, 2006). Nonetheless, laser welding has considerable flexibility for joining titanium alloys either autogenously or with the use of filler wire or powder. As laser welding permits the generation of a keyhole that effectively concentrates the energy input into a small area, there is good potential to join titanium alloys since the microstructural changes are confined to the weld region and a narrow heat affected zone, which has been reported to conserve the corrosion resistance and mechanical strength of the weldment (Liu et al., 2002).

To preserve the mechanical properties of titanium alloys during laser welding, gas shielding is of crucial importance to prevent embitterment of the weld region and the ensuing losses in ductility. Protection of the weld pool against atmospheric contaminations is performed by using shielding gas, which also has been reported to improve the coupling of the laser to the material (Wang et al., 2007). In order to enhance the coupling between the incident beam and the workpiece as well as prevent oxide formation on the weld surface, several laser welding nozzles have been developed. Conical nozzle has been designed by Gerevey et al. (2005) to stabilize the plasma plume that is used for obtaining good quality weld, ring nozzle has been designed to prevent contamination by the ambient air (Fabbro et al., 2004), and also a model has been employed by Ancona et al. (2006) describing the gas flow dynamics for coaxial conical nozzle. Examination of the influence of coaxial shielding gas or lateral assist gas on the laser welding process has indicated that the height of the side nozzle and current rate of gas flow can strongly influence weld seam characteristics. Specifically, Zhang et al. (2005a) have recently shown that, the largest weld bead width on the bottom surface is achieved when the joint angle of coaxial gas flow and side gas flow are at about 40°. To determine the effect of applying shielding protection with helium and argon gasses on titanium alloy using different nozzle designs, Caiazzo et al. (2004) have realized a study using CO₂ laser. As a result of this study, at constant power (1500 W), for each welding speed examined, a greater penetration depth has been obtained with helium gas than argon due to the lower ionization energy of argon that reduces energy transfer to the material. However, as well as the shielding gas used in laser welding protect the molten material from oxidation; it causes welding defects such as, porosity and cracks. The main reason of the porosity in welding process is the gas bubbles in the molten material that cannot escape before solidification. Doing the welding process in vacuum environment is an easy way to reduce the porosity problem. The effect of vacuum on weld penetration and porosity formation has been investigated on 304 stainless steel and A5083 Aluminium alloy by Katayama et al. (2001). The results have showed that vacuum welding has been effective in the prevention of porosity, no pores has been seen below 0.4 kPa and also penetration depth has increased and the fusion zone has become thinner with a decrease in pressure. Laser machining can replace mechanical removal methods in many industrial applications, particularly in the processing of difficult-to-machine materials such as hardened metals. Laser welding application of titanium alloys have been done by Akman et al. (2007), also Kacar et al. (2009) recently have shown that ceramics with 10 mm thicknesses can be drilled using high power laser. Among the high power density technologies, the electron beam and laser welding have showed a great capability in producing narrow and deep joints. The amount of heat used in laser welding is roughly comparable to that of conventional arc welding processes. As the heat is focused on a very small area, the weld pool is much smaller than in arc welding. As reported by Casalino et al. (2005), the welding speed is much higher, up to approximately the speed of conventional welding processes.

To understand laser welding phenomena of commercially available pure titanium, the molten pool behaviour has been synchronously observed by Kawahito et al. (2006). By taking into consideration the relationship between the in-process monitoring signals and the welding results, the feasibility of adaptive control of laser peak power and pulse duration has been examined for reducing the spattering or porosity. Of the three welding techniques (TIG, Plasma and laser) compared to titanium alloys by Zheng et al. (2001), it has been reported that laser welds possess the highest aspect ratio and narrowest weld bead.

Titanium alloys can be welded using a pulsed and continuous wave (cw) mode laser. In pulsed laser applications, a small molten pool is formed by each laser pulse and within a few milliseconds it re-solidifies. When the peak power is low or the spot size is increased, welding occurs in conduction mode and a shallow and smooth weld pool is produced. On the other hand, when the peak power is increased or the spot size is reduced, a much deeper weld pool is obtained that is characterized as penetration or keyhole mode welding as reported (The fabricator, 2003). In keyhole mode laser welding, two plasmas, one inside the keyhole and other above the workpiece surface, occur. The plasma produced by laser radiation affects the welding process and an excess in the plasma has some disadvantages such as blocking, reflecting or refocusing the laser beam that can result in insufficient penetration, burn-through, irregular weld shape, or damage of beam delivery optics. As mentioned by Tu et al. (2003) and Zhang et al. (2005b) inside the keyhole, two absorption mechanisms usually exist in laser deep penetration welding: the beam energy is absorbed by the material through either Fresnel absorption of the keyhole wall during multiple reflections of the beam on the wall or the inverse Bremsstrahlung absorption of the electrons of the plasma. Although in continuous type lasers it is easier to control the laser welding processes, it has disadvantages for thin material processing.

Seam welding is the most important pulsed laser application. Tzeng (2000a) describes the seam welding as a series of overlapping spot welds to form a fusion zone or seam. The formation and the quality of seam welds are the results of a combination of various pulsed laser processing parameters, such as the travel speed, the average laser power, the pulse energy, the pulse duration, the average peak power density and the spot area. As mentioned by Tzeng (2000b), this abundance gives control of the thermal input with a precision not previously available and also permits a wide range of experimental conditions to be applied. On the other hand controlling so many parameters increases the complexity of laser processing. Lima (2005) has recently shown that pulse shaping technique can be used to prevent cracking in welded TiN coated titanium alloy through an improvement in the transfer of nitrogen to the volume of the weld. The optimum laser parameters and filler wire diameter are investigated by Li et al. (1997) in the welding process. Also the gap between the joint interfaces has been varied to evaluate porosity formation and/or reduction in the titanium alloy. They have shown that, acceptable results can be obtained when the gap distance is 0.1 mm. In this study, the effect of pulsed laser seam-welding parameters for joining 3 mm thick Ti6Al4V has been investigated using the Lumonics JK760TR pulsed Nd:YAG laser.