Surface hardening of AISI 4340 steel by laser linear oscillation scanning F. Qiu* 1 and V. Kujanpa¨a¨ 2 This study investigated the surface hardening of AISI 4340 steel by linear oscillation scanning with a fibre laser. Various frequencies and amplitudes of oscillation were used under laser power of 2020 and 3020 W. Microscopic evaluation was done, and the effect of oscillation frequency on the hardened depth was examined. Hardness profiles were measured along the centre of the irradiated track toward the feeding direction of the laser, across the irradiated width and into the depth below the irradiated surface. The homogeneity of hardness and hardened depth with different processing parameters was investigated. The hardness profiles were compared with the results obtained with conventional single track hardening. Keywords: Surface treatment, Hardening, 4340 steel, Oscillation scanning, Multitrack, Fibre laser Introduction Laser surface transformation hardening is a process of producing hard, wear resistant regions on the workpiece while retaining the base material unaffected, 1–3 which typically uses a defocused laser beam with the laser energy density in an order of magnitude of 10 3 –10 4 W cm 22 . 1,4 A focusing lens can be simply used to produce an out of focus laser beam profile, while the width of the irradiated track is limited by the laser spot size, and the laser energy distribution is mostly inhomogeneous. Multitrack hard- ening has been investigated for the purpose of large area treatment, but the decrease in hardness in the overlapp- ing zone due to tempering remains to be a problem. 5,6 Various types of special shaping optics have been de- veloped as an advanced solution to produce a desirable shape (e.g. rectangle) and size of the laser spot with relatively homogeneous energy distribution. 7–9 However, such optics are relatively expensive, and their flexibility in use is considered to be limited. Laser linear oscillation scanning (LLOS) provides an alternative method for generating laser irradiated track with customisable width, as described in Fig. 1. A raw laser beam is converted by the oscillation scanner to a linearly oscillating beam that scans the sample’s surface back and forth in the direction of the y axis. As the oscillating laser beam moves along the x axis, a laser irradiated track in a zigzag pattern is produced on the workpiece. This process is, in nature, a continuous multitrack surface irradiation in which the treated region consists of a number of overlapped laser irradiated tracks. Such studies as LLOS are thought to have good potential for practice, yet such studies have been rarely available so far. In recent years, diode pumped fibre laser systems have been developed and proved to be applicable in the surface treatment of steel. 10–13 Owing to the simplicity and integratability of fibre laser, an oscillation scanning head can be installed easily with an external controller unit connected. 14 Experimental Material and methods The tested material was AISI 4340 steel, which was a quenched and tempered low alloy steel composed of tempered martensite structures. 15 The composition of the material is 0?347C–0?331Si–0?007V–1?397Cr–0?697Mn– 1?355Ni–0?169Mo (wt-%). The initial hardness of the base material was 329 HV. The surface roughness of the samples, R a , was 2?5 mm. This study used a work cell consisting of a CNC XY work table, a YLR-5000-S multimode fibre laser system and an ILV DC scanner installed as the laser head. The fibre laser produced a laser beam with a wavelength of 1070–1080 nm and a maximum nominal output power of 5 kW. An output fibre core with the diameter of 200 mm was used. The DC scanner, which was primarily designed for laser welding applications, contained a parabolic scanner mirror with focal length of 250 mm and a controller unit connected to it. The distance off the focus was 60 mm, producing a laser spot of 2?1 mm in diameter. The oscillation speed was affected by the oscillation frequency and followed the sine waveform. The parameters are given as tests A1–A6 in Table 1. For comparison, a conventional single track scanning test was done, shown as tests B1 and B2 in Table 1. To produce laser power densities of 15136 and 22629 W cm 22 , 2020 and 3020 W laser power with 75 mm off focus distance were used respectively. The feeding speed was constantly 15 mm s 21 in the whole test. Modulation of laser power profile Figure 2a shows the laser energy distribution with oscillation frequency of 100 Hz and amplitude of 6?2 1 Laser Processing Laboratory, LUT Metal, Lappeenranta University of Technology, Tuotantokatu 2, 53850 Lappeenranta, Finland 2 VTT Technical Research Centre of Finland, Tuotantokatu 2, 53850 Lappeenranta, Finland *Corresponding author, email feng.qiu@lut.fi ß 2012 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 24 May 2012; accepted 9 June 2012 DOI 10.1179/1743294412Y.0000000034 Surface Engineering 2012 VOL 28 NO 8 569