Low Speed Laser Welding of Aluminium Alloys Using Single-Mode Fiber Lasers

Abstract

Laser welding of aluminium alloys is an important industrial technology and yet many challenges still lie ahead. Laser welding studies were reported almost within two years since the first laser was invented in 1960. However, practical metal seam welding was not feasible until the early 1970s when multi-kilowatt, continuous wave CO2 lasers were developed to allow for deep penetration keyhole welding (Duley, 1999). Unfortunately, the application for deep penetration welding of aluminium was limited due to its very high reflectivity at the relatively long wavelength (10.6 micron) of CO2 lasers. Flash-pumped Nd:YAG lasers with a 1.06 micron wavelength were not suitable due to their low power and extremely poor efficiency at the time. Since the 1980s, high power seam welding of carbon steels using multi-kilowatt CO2 lasers has become a regular industrial practice, in particular in the automotive industry. In the mid 1990s, diode pumped Nd:YAG lasers were developed that offered kilowatt power and high efficiency. As a result, aluminium laser welding became more feasible because the beam absorption of aluminium alloys at 1.06 micron is three times as much as it is at 10.6 micron. Nevertheless, the poor beam quality and high cost of diodepumped Nd:YAG lasers still hinders their acceptance in industry. In the early 2000s, with the arrival of single-mode and multi-mode high power fiber lasers at a 1.075 micron wavelength, along with excellent beam quality and low maintenance cost, the expectation was that the advantage of laser welding aluminium components could be better realized. The requirement of very high laser power for aluminium welding is not only due to its high reflectivity and high heat conductivity. Aluminium has been known to be one of the most challenging metals to weld successfully (Mandal, 2002). Other factors affecting the weld quality of aluminium alloys include different kinds of porosity formation, hot tearing, solidification cracking, oxide inclusions and loss of alloying elements. It has been found that weld porosities can be significantly suppressed at high welding speeds. In order to maintain a stable keyhole at high speeds, very high laser power is needed. What has been less explored is the reason why the welding process becomes less stable and prone to defects as the speed is reduced. There are many applications where high speed welding is not suitable. With the expansion of modern miniaturized consumer products, the weld path can be short and with intricate shapes. High welding speed may not be effective due to the short paths and constant accelerations and decelerations required to follow the path precisely. One such application is the fusion of fatigue cracks in aluminium parts, where a crack path is irregular. It also has been shown that welding at a lower processing speed can reduce the tendency of transverse solidification cracking. Finally, with the availability of better laser sources such as high power fiber lasers, it is important to expand laser welding of aluminium to wider processing conditions for various applications. This book chapter will discuss latest research results in extending laser welding of aluminium in the low speed range by investigating the welding instability phenomena. The following topics will be discussed: - Aluminium alloys and welding defects - Brief review of high speed laser welding of aluminium - High power fiber lasers and optical setups - Process modelling of laser welding of aluminium - Experimental process characterization of low speed welding - The instability and defects at low speed welding - Applications of fatigue crack repair in aluminium First, the properties of aluminium alloys and the cause of welding defects are discussed. Prior to discussing low speed welding, a brief review of high speed laser welding of aluminium is provided. The characteristics of high power fiber lasers and their optical setups for welding applications, such as focusing lens, assist gas, alignments, and damage prevention due to beam reflection by aluminium are then presented. The chapter then proceeds to present recent research results in low speed laser welding of aluminium, which includes theoretical process modelling, experimental process characterization, in-process monitoring of several critical signals, such as plasma radiation and beam reflection, as well as the causes and consequences of process instability at low speeds. The transition of process stability from medium welding to a low speed threshold and its mechanisms are explored. Finally, an application of low speed laser welding of aluminium for fatigue crack repair is given. A discussion on different applications and future development conclude the chapter.