STUDY OF LASER WELDING TECHNIQUE FOR TITANIUM ALLOY SHEET

In order to achieve reliable welds with minimum distortion for the fabrication of components for the aerospace industry. Several techniques were explored using the CO, and the Nd:YAG pulsed laser. In the tests it was established that a satisfactory weld can be obtained using the CW laser. 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 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. KEYWORDSLaser, Titanium Alloy, pulse and continuous wave. INTRODUCTION The high strength, low weight and outstanding corrosion resistance possessed by titanium and titanium alloys have led to a wide and diversified range of successful applications in aerospace, chemical plant, power generation, oil and gas extraction, medical, sports, and other industries. Welding of titanium by various arc welding processes is widely practiced, and good service performance of welds is proven. Newer joining methods, such as laser welding, have been successfully adapted for titanium. Application of appropriate welding technology to the design, manufacture and application of titanium products is as important in design as the specification of the alloy. Titanium is a unique material as strong as steel but half its weight, with excellent corrosion resistance. Traditional applications are in the aerospace and chemical industries. More recently, specific alloys are finding use in the manufacture of implantable medical devices and sensors. Titanium is one of the most important non-ferrous metals and finds extensive application in the aerospace industry, because of its light weight (density 4.5 g/cm 3 ), excellent corrosion resistance, high strength level, attractive fracture behaviour and high melting point (1678oC). Since pure titanium exists in two allotropic forms: alpha alloy and beta alloy. Laser welding technology was considering for the fabrication of these components because its heat input is much lower than that of arc welding, the distortion. Shrinkage and residual stress can be minimized and the mechanical properties are also excellent, resulting from the contribution of martensite due to the rapid solidification titanium alloys can be used as replacements for aluminiumbased 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. Alternatively, as titanium is exhibit very low corrosion rates in human body fluids as demonstrated 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. MATERIAL TYPES There are basically four types of alloys distinguished by their microstructure: Titanium Commercially pure (98 to 99.5% Ti) or strengthened by small additions of oxygen, nitrogen, carbon and iron. The alloys are readily fusion weldable; Alpha alloys These are largely single-phase alloys containing up to 7% aluminium and a small amount (< 0.3%) of oxygen, nitrogen and carbon. The alloys are fusion welded in the annealed condition; Alpha-beta alloys These have a characteristic twophase microstructure formed by the addition of up to 6% aluminium and varying amounts of beta forming constituents vanadium, chromium and molybdenum. The alloys are readily welded in the annealed condition; Ni-Ti alloysAlloys which contain a large amount of the beta phase, stabilised by elements such as chromium, are not easily welded. Commonly used alloys are listed in Table 1 with the appropriate ASTM grade, the internationally recognised designation. In industry, the most widely welded titanium alloys are the commercially pure grades and variants of the 6%Al and 4%V alloy. Table: 1 Commonly used titanium alloys and the recommended filler material LASER WELDING PROCESS 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 resolidifies. 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 Shirguppikar et al., International Journal of Advanced Engineering Research and Studies E-ISSN2249–8974 Int. J. Adv. Engg. Res. Studies/III/II/Jan.-March,2014/20-22 keyhole mode welding as reported. 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, burnthrough, irregular weld shape, or damage of beam delivery optics. 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 or thin material processing. Seam welding is the most important pulsed laser application 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 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 Fig: 1 Shielding gas nozzle setups and oxidation effects Lima 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 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 0.1mm. In this study, the effect of pulsed laser seamwelding parameters for joining 3mm thick Ti6Al4V has been investigated using the Lumonics JK760TR pulsed Nd:YAG laser. Fig: 2 Characterization of welding cross-section. Lima 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 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 0.1mm. In this study, the effect of pulsed laser seamwelding parameters for joining 3mm thick Ti6Al4V has been investigated using the Lumonics JK760TR pulsed Nd:YAG laser. MATERIALS AND METHODS Fig: 3 Effect of peak power on penetration dept In this study, butt welding of a small square shape (30mm×30mm×3mm) Ti6Al4V titanium alloy plaques have been done using GSI lumonics JK760TR Series Laser (Class 4) system in a CNC cabin. The chemical composition in weight percentage of the titanium substrate is shown in Table 1. The JK760TR Series of laser is an Nd:YAG laser that has 0.3–50ms Pulse length and 500 Hz maximum repetition rate. The average power that can be obtained is 600Watt. And also JK760 TR series laser has a pulse shaping ability. Laser output power is delivered via a 600 μm radius fiber optic cable to the Shirguppikar et al., International Journal of Advanced Engineering Research and Studies E-ISSN2249–8974 Int. J. Adv. Engg. Res. Studies/III/II/Jan.-March,2014/20-22 focus head at the workstation for process. In the experiment, square shape pulse has been applied to all workpieces. The laser beam is focused on titanium plates using 160mm Plano convex lens. The minimum spot size on the plates has been 0.4mm. During welding application, the laser beam has been focused on 2mm under the surface of the plates to obtain enough power density for the cross-section. In our case the spot size on the plates is 0.65mm. The laser output parameters are varied in the experiment. There is always a cracking risk due to the rapid cooling of welded joint. To overcome this defect during welding, samples have been fixed on the ground using clamps. Titanium is reactive material at high temperature with ambient gases. For this reason during the welding application a shielding gas has been used to protect the melt pool and HAZ from oxidation until sufficient cooling has occurred. At this point shielding gas usage and nozzle set up are very important; formation of turbulence on the sample surfaces must be avoided. In experiments two different nozzle designs have been used. One of the nozzles has been arranged as array and formed by small sized gas exits and 5 bars helium applied during t