Avances en los procesos de soldadura láser y refundido láser en las aleaciones de aluminio y titanio

Resumen

In order to decrease the environmental impact of metallic aircraft structures, different design considerations must be taken into account, including assembly aspects such as the employment of rivets. Riveting is nowadays the main joining process in aircraft manufacturing. It is now a mature technology which has been significantly automated. The reduction of riveting (and consequently the reduction of the fasteners and sealants) would lead to great environmental and manufacturing impacts, as it would reduce manufacturing costs and the final weight of the fabricated structures. Welding research is currently in progress to replace riveting as joining fabrication method. The key advantages of welding are the weight reduction and the speed of the joining process when compared to riveting. General drawbacks of welding in comparison with riveting are the risk of having material property degradation and microstructural changes due to the heating and cooling steps of the welding process. Among the different welding techniques currently available, laser beam welding (LBW) is a technique now being seriously considered, as it generates low distortion and acceptable mechanical properties. Laser welding is a promising process for assembling the thin-walled components found in typical aircraft panels. LBW can be carried out by means of two different regimes: keyhole and conduction. Keyhole regime involves the employment of higher density power and usually leads to narrower and deeper welding beads than the conduction regime. Keyhole mechanism can generate welding beads with high porosity due to gas entrapment occurring during the solidification of the weld pool. In contrast, conduction welding is a more stable process because the metal evaporation takes place at a lower level than at keyhole mode. Conduction regime is especially interesting for the aeronautic and automotive industries, since they involve the use of sheets with relatively low thicknesses and require minimum levels of defects in the welds. On the other hand, Laser Remelting (LR) treatments are being widely investigated nowadays to modify the superficial microstructure and therefore to improve mechanical and corrosion resistance properties. The main objective of the present PhD has been to analyse the technical feasibility of the employement of laser beam welding (LBW) and laser remelting (LR) processes under conduction regime in aluminium and titanium alloys. LBW and LR procedures employed in the literature to process aluminium and titanium alloys have been reviewed in Chapter 2. It includes a general classification of the different welding technologies, basic concepts of laser radiation, description of the laser equipment most commonly used in materials processing, the most influencing parameters affecting the laser processing of materials, and advantages and disadvantages of laser welding over other joining methods. Differences between the laser weldability of aluminium and titanium alloys have been described. The main experimental details of the reviewed papers are reported, including description of the laser equipments, welding modes, powers and processing rates, shielding gas, superficial pre-treatments of samples, etc. The microstructure, main defects, mechanical properties and corrosion behaviour of the weld beads are also reviewed. Current trends in the research fields of LBW and LR of aluminium and titanium alloys have been reported. The most interesting findings of the research review regarding LBW of both light alloys have been published in Chapter 8 of “Handbook of laser welding technologies” [1], included in Annex 1. From an experimental point of view, LBW and LR under conduction regime of aluminium and titanium alloys have been optimised, employing a High Power Diode Laser (HPDL). In addition, other welding equipments have been employed for comparative purposes. Chapter 3 describes the materials and equipments used in this PhD, as well as the experimental procedures followed. Alloys, pretreatments, laser equipment, shielding gas systems, configuration and size of samples processed are also detailed. Furthermore, the methodology followed to perform the different laser treatments is described. Details regarding metallographic characterization of samples are also reported. Finally, tests performed to determine the mechanical and corrosion resistance properties of the processed samples are detailed. Results and discussion of LBW of aluminium alloys under conduction regime have been included in Chapter 4. Firstly, the influence of the shielding gas system and of the different surface pretreatments on the quality of the weld beads has been analysed. Furthermore, a comparative study of the laser weldability under conduction regime of six different aluminium alloys have been carried out. Butt joints with higher penetration than those previously reported in the literature for this regime could be obtained, demonstrating the weldability of all these alloys with the employed methodology. Afterwards, the depths and widths of the beads were fitted to a simple mathematical equation proposed by the authors. Taking into account the weld penetration values and the susceptibility to solidification cracking, the weldability order of aluminium alloys was seen to be: 5083 > 7075 > 2017 = 2024 = 6082 > 1050. The magnesium content and, to a lower extent, the zinc and silicon amount were observed to improve the weldability of the aluminium alloys. The most important results obtained in this activity have been published in “Welding Journal” [2], paper included in Annex 2. Chapter 5 of this PhD reports the main results regarding LBW and LR treatments of titanium alloys. In its first section, results obtained using different protection systems in LBW of titanium alloys are summarised. On the other hand, the influence of different surface pretreatments on the microstructure and microhardness of titanium alloys welds have been analysed. Thus, results show that grinding, sandblasting, and chemical cleaning pre-treatments lead to welds with the highest depth values, presenting also high joint strengths. Treatments based on the application of dark coatings generate welds with lower penetration and worse mechanical properties. These results have been published in “Journal of Materials Engineering and Performance” [3], paper included in Annex 3 of the present PhD. Subsequently, a comparative analysis of the laser weldability under conduction regime of different titanium alloys (CpTi, Ti6Al4V and Ti5Al5V5Mo3Cr) is reported. In addition, LBW of Ti6Al4V is compared with other technologies, such as TOPTIG. On the other hand, butt welds of Ti5Al5V5Mo3Cr were performed with different techniques for comparative purposes, as TIG, EBW, LBW-conduction regime and LBW-keyhole regime. Part of the results obtained in this collaborative study has been published in “Procedia Engineering” [4], paper included in Annex 4. Finally, different LR treatments were applied to CpTi and Ti6Al4V to modify their properties. The influence of the applied laser fluence on microstructure, microhardness and corrosion resistance has been investigated. Results show that laser remelting treatments with appropriate fluences provoke microstructural changes leading to microhardness increase and similar or slightly higher corrosion resistance. Main results of this latter investigation have been published in “Corrosion Science” [5], paper included in Annex 5. The overall results obtained in the present PhD allows us to conclude that LBW under conduction regime provides high quality joints (in compliance with applicable aviation regulations) in aluminium and titanium alloys. Moreover, LR treatments are consolidated as a very interesting processing tool for localised superficial hardening of these alloys. References: [1] Handbook of laser welding technologies. Chapter 8:”Laser welding of light metal alloys: aluminium and titanium alloys” J.M. Sanchez-Amaya, M.R. Amaya Vázquez y F.J Botana, Edited by S. Katayama, Osaka University, Japan. Woodhead Publishing Series in Electronic and Optical Materials. (2013), Nº 41. pp.215-254. ISBN: 978-0-85709-264-9. [2] J.M. Sánchez-Amaya, Z. Boukha, M.R. Amaya-Vázquez, F.J. Botana. ”Weldability of Aluminium Alloys with High Power Diode Laser”. Welding Journal, 91, (2012), pp. 155-161. [3] J.M. Sánchez-Amaya, M.R. Amaya-Vázquez, L. González-Rovira, M. Botana-Galvin, F.J. Botana. “Influence of Surface Pre-treatments on Laser Welding of Ti6Al4V Alloy” Journal of Materials Engineering and Performance, 23(5), (2014), pp. 1568–1575. [4] T. Pasang, J.M. Sánchez Amaya, Y. Tao, M.R. Amaya-Vazquez, F.J. Botana, J.C. Sabol, W.Z. Misiolek, O. Kamiya. ”Comparison of Ti-5Al-5V-5Mo-3Cr welds performed by laser beam, electron beam and gas tungsten arc welding”. Procedia Engineering, 63, (2013), pp. 397-404. [5] M.R. Amaya-Vázquez, J.M. Sánchez-Amaya, Z.Boukha, and F.J. Botana. “Microstructure, Microhardness and Corrosion Resistance of Remelted TiG2 and Ti6Al4V by a High Power Diode Laser”, Corrosion Science, 56, (2012), pp. 36–48.