International Journal of Applied Engineering Research ISSN 0973-4562 Volume 13, Number 8 (2018) pp. 5614-5617 © Research India Publications. http://www.ripublication.com 5614 Effects of Laser Parameters on Solidification Cracks Formation during LSM Treatment for AA6061 Zahraa Abdulsattar 1 , Waleed Al-Ashtari 2 1 Mechanical Engineering Department, College of Engineering, University of Baghdad, Baghdad, Iraq. 2 Mechanical Engineering Department, College of Engineering, University of Baghdad, Baghdad, Iraq. Abstract Laser surface melting (LSM) is a material surface modification process based on using high power laser beam as a heat source in order to enhance the mechanical properties of the material surface. In this article, the effect of pulsed laser power during LSM process on the formation of solidification cracks within AA-6061 and degree of hardening will be studied. Four level of power is selected (4.5, 3.5, 2.5 and 1.5) kW with maintaining the same magnitude of energy through increasing the pulse duration. It is concluded that if the power level reduced the solidification crack is eliminated. Furthermore, the melted region becomes smaller. It also has been noticed that the hardness of the sample is inversely proportional with laser pulse duration. Keyword: Laser surface melting, solidification crack, laser power, AA6061 INTRODUCTION Laser with high power has become intensively used as a tool for large number of manufacturing and surface modifications processes for its flexibility, its accuracy and it's considered to be easy automated energy source. The mechanical properties enhancement of the material surface can be obtained through changing either the microstructure or the chemical composition of surface. Laser can be used to achieve such surface modification. These modifications processes may applied in order to increase surface hardness, increase wear resistance, or enhance fatigue life through including residual stress (compressive) in the surface layers (Kannatey-Asibu, 2009). One of laser surfaces modification which has attracted interest in last years is laser surface melting (LSM). This process involves melting the surface layers and due to self-quenching rapid solidification occur, This laser treatment cause microstructure changes in the treated region as a result metal properties changed also such as hardness, wear resistance and corrosion resistance (WANG, 1983). This type of surface modification is suitable with aluminum alloys. Recently aluminum alloys is widely used in engineering applications due to its excellent strength to weight ratio. A large number of engineering applications requires special properties near the surface region such as hardness, wear resistance, corrosion resistance and enhancing fatigue properties, as mentioned before these properties can obtained from LSM for aluminum alloys. In the related literature, it can be found many research investigated the laser treatment of aluminum alloys. Sušnik et al., 2012, studied the effect of laser surface melting on Al-Si alloy and investigated the changes in micro-hardness and microstructure of the treated area. Laser surface melting was carried out using different energy value. A fine-grained microstructure is obtained after solidification. The fine-grained microstructure causes micro-hardness increment of Al-Si alloy up to 80%. It was concluded that the variation of mainly tensile residual stresses in melted layer depends on laser pulse duration i.e. the cooling rates. Reduction of residual stresses is obtained from Precipitation annealing after laser surface melting process. Tillová and Chalupová, 2014, used an Nd: YAG laser (BLS 720) to treat cast alloy AlZn10Si8Mg. Two levels of laser power were applied (50 and 80) W. microstructure changes in the treated surface of the AlZn10Si8Mg are acquired. Melting region is α_phase with fine columnar dendrites morphology without the presence of Si-particles and intermetallic phases. In the transition area, grain refinement of eutectic Si was obtained. Micro-hardness of the surface treated layer was increased from 94 HV0.01 to 140 HV0.01 after the laser treatment process. In the surface layer (laser power 80 W), cracks were observed due to uneven heat transfer. Nassar et al., 2015, used a pulsed Neodymium Yttrium Aluminum Garnet (Nd:YAG) laser with wavelength 532 nm and 5 min time of irradiation. They applied different powers ranging from 250 to 2100 mW with pulse width of 8 ns to treat the hypereutectic Al-18wt%Si. The microstructure and micro-hardness (HV) were studied after the laser treatment. The results showed that the laser-treated samples had positively affected eutectic silicon morphology according to the enhancement in the measured mechanical properties. On the other hand, increasing laser power led to the formation of precipitates which can be considered dendrite boundaries that divided the material in the same way that the grain boundaries do. It was concluded that increasing the laser power values led to the formation of precipitates and dislocations. This is contributed to the increased micro-hardness. Pakieła et al., 2016, studied the effect of laser surface treatment on the properties and structure of aluminum alloy ENAC-ALMg9 by using a high power diode. The treatment parameters were (1.8, 2.0 and 2.2)kW laser power and the scan rate of the laser beam was set 0.5 cm/s. argon was used In order to protect the liquid metal. To improve the wear and surface mechanical properties of the aluminum alloy, biphasic tungsten carbide WC/W2C was used. It was concluded that the highest properties of the treated surface were obtained at the lowest laser power of 1.8kW. The laser treatment with 1.8kW power a treated layer was achieved with a hardness of about 15HRF higher