레이저빔을 이용한 열간금형강의 표면 합금화 공정 및 합금화 트랙의 표면특성에 관한 연구

Abstract

The laser surface alloying (hereinafter referred to as LSA) is referred to as the laser surface alloying, or the laser alloying. These LSA process is one method of the material surface engineering which make use of the focused laser beam of high density energy to melt surface and underlying part of the metal substrate. The LSA process is very simple. First of all, the laser beam of high energy generates a weld pool on the workpiece surface. At that time metal powder adds in the weld pool through powder nozzle of optic head. This creates tracks that are welded with one another, which then make new alloyed structures on existing base parts or entire workpieces. Since the melting happen very shortly and only at the surface of metal substrate,, there is little distortion of the workpiece. In this study, the M2 powder was chose through the various pieces of reference book for the purpose of mechanical properties enhancement. The condition of the LSA process parameters were studied to improve the alloying effectiveness using a high power disk laser. The H13 hot work die steel plate and the Fe-based M2 powder were used as the substrate and the filler metal for the LSA process, respectively. The LSA process parameters are diameter of the laser beam, work distance to the contact tip, focal position of the laser beam, tilt angle of the optic head, laser power, travel speed of the optic head, powder feed rate, track pitch, carrier gas flow rate, shield gas flow rate, track pass, etc. The LSA experimental equipment consisted in laser generator, laser optic head, powder nozzle and powder supply device. The laser generator is a TruDisk 4001 model which is the 4kW class disk laser of Germany Trumpf. And the laser beam from the laser generator injects to the metal surface through the optic head with the powder nozzle, at this moment diameter of the laser beam is usually kept as 800um. Simultaneously the powder was supplied from the feeding device of rotary disk type to the powder nozzle continuously, at that time the powder is transmitted by carrier gas. Feasibility experiments for the LSA were carried out by changing the laser power. As the laser power is increased, the heat input and the melting pool on the metal surface are also increases, therefore thickness of the alloyed track was increased, but the surface roughness deviation was decreased. Through the optical microscope observation of the cross-section in the alloyed track, the effect of the main factors were predicted by the track pitch. As the track pitch is increased, so the reheated zone width of the track, the overlap width and the minimum thickness of the track were decreased, and the hardness was also decreased in the heat affected zone. In recent years, the numerical analysis of the press dies for the metal sheet have been made based on the finite element method. The primary objective of numerical analysis for the die cutting of the ultra high strength steel are to figure out the die stress, and predict the penetration depth of the maximum stress during the shearing process. The commercial finite element program named Forge was used to this end, and the Arbitrary Lagrangian and Eulerian method with adaptive meshing capabilities was applied to improve the reliability of numerical analysis. Through the penetration depth result of the maximum stress, it is possible to analyze to which region does the high strength of the metal sheet affect the die stress during the shearing process of the ultra high strength steel. Since the penetration depth of the maximum stress to internal vertical direction from the cutting edge surface is approximately 0.52mm, the minimum critical thickness for forming the alloyed track is 0.52mm or more, which is equal to the penetration depth of the maximum stress. In other words, It is analyzed that the LSA track depth needs to be at least 0.6mm. There are so many parameters for the LSA process. And the selection of the suitable conditions and factors is very significant for the reliability of the LSA applied alloying track, but these various factors is normally caused a lot of time and cost consumption. So if these factors are combined for obtaining the optimal condition such as improvement of the track depth and the hardness, there are outstanding combination choices. Also this study presents the design of experiments using the orthogonal array in order to reduce the countless tests looking for the optimal condition. And the design of experiments through this LSA study makes the effective optimal condition for reduction of time and cost by applying to the surface of dies and tools. Here, improvement of the track depth and the hardness are adopted as the output and choice criterion of the LSA optimizing. The LSA experiments were designed and proceeded using the L27 orthogonal array of the Taguchi method. Among the various kinds of the LSA process factors, the laser power, travel speed of the optic head, powder feed rate and track pitch were adopted to analyze the alloying efficiency. In the results of the analysis of variance, especially in case of the track depth, the laser power and travel speed of the optic head have been shown to make the most crucial effect on alloying quality, but the powder feed rate and the track pitch had little effect. On the other hand, in case of track hardness, all four factors had nearly no effects on the alloying efficiency. The signal to noise ratio of the optimal combination could be maximized in the experimental range such as 2400W of the laser power, 50mm/sec of the travel speed, 10g/min of the powder feed rate and 400um of the track pitch. Through this experimental analysis, SNR of the existing and optimal condition were 57.7dB and 66.0dB each other, and the gain from the these result was 8.3dB. With a optimal combination, the highest LSA efficiency of 14.4% could be obtained. In addition, it is very important to carry out verification experiments to verify the reproducibility of these gain. The reproducibility was verified by comparing the SNR of the existing and optimal condition predicted using the orthogonal array to the SNR calculated by the verification experiment. From the result of reproducibility, there is no sudden hardness variation in case of the transverse direction, but the hardness becomes lower as it close to the substrate in case of the longitudinal direction. In addition, from the EPMA mapping analysis of the cross-section in the alloyed track, it was observed that all the elements are evenly distributed inside. However, the closer to inside of the alloying track, the chemical composition of the key elements decreased such as molybdenum, vanadium and tungsten. While the microstructure of the alloying track is typical dendrite-structured martensite phase forming dendritic framework. Above all, as the chemical composition of the key elements increases, the dendrite structure became more finer and the hardness increased. And in case of the failure, the dendritic waveform was propagated along dendritic grain boundaries by the solidification cracking mechanism. Finally, to predict the peak depth and valley depth of laser surface alloying track, regression analysis was used to develop polynomial function models of laser power, travel speed, powder feed rate, and track pitch.