A study on laser cleaning and pulsed gas tungsten arc welding of Ti–3Al–2.5V alloy tubes

Abstract

An alternate cleaning process to chemical process of Ti–3Al–2.5V tube surface by a pulsed fiber laser and welding of the laser-cleaned tubes with end fittings by Pulsed Gas Tungsten Arc Welding (GTAW-P) technique is reported in this paper. Results on surface morphology, hardness, chemical composition, metallography and X-ray radiography are presented for laser assisted cleaning and welding of Ti–3Al–2.5V alloy tubes. Welding of laser-cleaned samples show excellent weld quality.

Introduction

Titanium and its alloys are used mainly in two areas, namely, corrosion resistant and specific strength efficient structural applications. Thus aerospace and chemical industries are the main sectors to utilize the major portion of titanium alloy production. Different grades of titanium alloys have been manufactured and put to use in many applications. A detailed account of this is given by Boyer (1996). Although Ti–6Al–4V is the most common alloy used in high strength applications, an alloy, Ti–3Al–2.5V is primarily used to manufacture seamless tubing for aircraft hydraulic systems because of better cold forming properties than Ti–6Al–4V (Donachie, 2000). Although this alloy has good weldability by conventional welding processes, thorough cleaning before welding and maintaining a pure inert atmosphere during welding is absolutely necessary to obtain a good weld. The cleaning process serves to remove surface oxide films and other contaminants that forms or otherwise deposited on the surface of these components. Conventional cleaning is obtained by pickling the components in nitric acid–hydrofluoric acid bath. Acid pickling removes a thin layer from the tube surface along with contaminants present on the surface. The chemical cleaning process is a multi-step process. Material removal rate is very much dependent on composition of solution, bath temperature and immersion time in pickling. Chemicals need to be replaced after regular interval. Handling, storing and disposal of waste of these environmentally malign chemicals are also a cause of concern. The footprint of these cleaning systems is large. Lasers are recognized as an attractive cleaning tool in many fields. It is being used for the removal of particulate contaminants, oxide layers, oil, grease, etc., from metallic and dielectric surfaces. The particulate contaminants are removed by irradiating the contaminated substrate with an intense pulsed laser beam of suitable wavelength and duration. The absorption of the incident laser energy by the substrate and/or the particulates cause rapid rise in the local temperature leading to their sudden expansion. If the resulting force exceeds the binding force, the particulates can be dislodged. This method has been used successfully by Lee and Watkins (2000) for cleaning copper surface. Nilaya et al. (2006) and Roberts and Modise (2007) removed radioactive particulates from metallic surfaces by this technique using a pulsed CO₂ laser and Nd-YAG laser, respectively. Another way of removing particulate contaminants is by laser shock cleaning. Here an intense pulsed laser beam is focused above the substrate. The medium ionizes due to the high electric field at the focal spot of the laser beam and produces plasma. A shock wave is generated and propagates spherically in all directions. If the force generated by the shock wave is more than the adhesion force, the particle is removed. In this mechanism there is no direct interaction between the laser and the substrate. Lee et al. (2003) has used this method to clean Silicon wafer surface. Oxide layers are generally more adherent to the substrate than particulates. These layers and other contaminants like oil, grease, crust on stones, etc., are removed by laser ablation or spallation phenomenon. In laser ablation, a thin layer from the top of the substrate surface is vaporized when irradiated by an intense laser beam. Continuous as well as pulsed lasers are used for this purpose. In spallation, the unwanted layer is removed due to the photo-mechanical effect produced by the incident pulsed focused laser beam. The laser fluence (energy/unit area) required to achieve ablation is more than particulate removal. The laser parameters like wave length, pulse duration, fluence and material properties like thermal diffusivity, thermal conductivity and absorptivity of the laser photons determine different mechanisms responsible for material removal. Sentis et al. (2000), Ferguson et al. (1998) and Veiko et al. (2007) have utilized laser ablation to remove oxide layers and contaminants from metallic surfaces using excimer laser/YAG laser. Nevin et al. (2007) and Tornari et al. (2000) have utilized lasers for art conservation. There has been very little research on laser cleaning of Titanium alloys. Turner et al., 2005, Turner et al., 2006 have investigated cleaning of contaminated titanium alloy (Ti–6Al–4V) aerospace components of planer geometry using pulsed Nd-YAG laser and continuous CO₂ laser. They have shown that effective cleaning of these components can be obtained as preparation for electron beam welding. However, they have not observed any significant difference between welds made with laser-cleaned samples and welds made with un-cleaned samples. In their work, solid-state diffusion bonds made with laser-cleaned samples have produced un-acceptable welds. Titanium alloys can be joined using Gas Tungsten Arc Welding (GTAW), Gas Metal Arc Welding (GMAW), Electron Beam Welding (EBW), Laser Beam Welding (LBW), Plasma Arc Welding (PAW) and solid-state welding processes. The preferred method for welding thin walled titanium tubes are GTAW because of its easier applicability, flexibility, better economy and amenability of the welds to standard non-destructive testing methods. Numerous reports are available written on welding of Ti alloys. Recently Balasubramanian et al. (2008) reported the effect of microstructure on impact toughness of a GTA welded titanium alloy. Kishore babu et al. (2007) correlated the microstructure of GTA weldments of Ti–6Al–4V alloy with and without pulsing. Pulsing the weld current is beneficial to conventional steady current due to reduction in heat input. Pulsing also helps in grain refinement (Kishore babu et al., 2007) and thereby improving mechanical properties of the weld joint. Huiqiang et al. (2004) studied the microstructure evolution of electron beam welding of Ti–6Al–4V alloy.

In this paper we report the results of laser surface cleaning of Ti–3Al–2.5V tubes and welding of these laser-cleaned tubes with end fittings. A thin adhered surface oxide layer, with a thickness of several tens of nanometers along with other residual contaminants is removed from the surface (internal and external) of the as received tube using a pulsed fiber laser as a preparation for pulsed gas tungsten arc welding. The cleaned surface has been evaluated by optical microscopy, profilometry, SEM, EDX and X-ray photoelectron spectroscopy (XPS). The advantage of laser cleaning is its dry nature and ability to remove selectively a controlled surface layer without affecting the property of the bulk. The waste generated in laser cleaning process is negligible in comparison to wet cleaning processes. After cleaning the tubes have been welded with end fittings using GTAW-P technique in an orbital welding machine to evaluate the effectiveness of laser cleaning. Welds have been analyzed by X-ray radiography, metallography, SEM. Micro-hardness of the weldment has been measured.