Study on the application of laser surfacing for th

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According to the characteristics of wear failure of horizontal screw centrifuges, the laser surfacing process is proposed to be used on the working face of spiral blades, and the appropriate surfacing process is explored. A high-performance surfacing layer with metallurgical combination with the matrix and uniform refinement of microstructure is obtained. The analysis shows that the hardness of laser surfacing layer is increased by 2 times, the wear resistance is increased by 5 times, and the corrosion resistance meets the application requirements. This technology has high practical value for horizontal screw centrifuge blades

key words: horizontal screw centrifuge; Blade; Laser surfacing

1 preface

traditional blade wear improvement measures mostly adopt the methods of welding blade liners of different materials, spray welding hard materials or inlaying alo ceramic liners. Because spray welding often has problems such as low bonding strength, pores, non compact structure, uneven composition, large blade deformation and so on; However, the ceramic inlay has poor bonding between the lining and the substrate, which is easy to fall off when the centrifuge rotates at high speed, resulting in accidents, and it is difficult to meet the working conditions of horizontal screw centrifuge. Therefore, according to the design and working characteristics of horizontal screw centrifuge, we studied the laser surfacing strengthening of corrosion-resistant and wear-resistant alloy materials for stainless steel horizontal screw centrifuge blades working in corrosion-resistant environment, and achieved good application results

2 test

base material: 1Crl8Ni9Ti stainless steel; Surfacing material: Ti containing alloy solder for laser. Laser surfacing with preset method is adopted, and a single surfacing layer is obtained each time. There is a gap between the stacking passes to avoid the thermal impact between the processes. 7KW cross flow CO2 laser is used; In the experiment, the output power of laser is 2.8KW and 3.0kw respectively; The output laser beam spot diameter is 7mm. Laser surfacing of stainless steel is a complex process of metallurgy and crystallization. Due to the high speed of laser heating and melting cooling, these chemical materials, including additives of medical materials, catalysts for processing or production, or inadvertently polluted materials, form non-uniform temperature field and thermal stress between the microstructure of materials during the heating, cooling or melting and condensation process, which is easy to cause defects such as cracks and cracks. In order to obtain good quality of surfacing layer (such as smooth surface of surfacing layer, no crack, good metallurgical combination, etc.), it is necessary to adjust the relevant laser process parameters (laser energy density, scanning speed) to form a good match, and carry out the sample simulation wear test and corrosion resistance test, so as to obtain good laser process parameters to make it have better matching performance

cut by wire cutting method, and prepare samples by electrolytic polishing and electrolytic corrosion. The hardness changes along the width and depth are measured on the surface in the direction perpendicular to the scanning. Vickers hardness value (200g15s) is used to express the hardness measurement results

3 experimental results and discussion

in order to describe the influence of process parameters, the laser action energy density pw=p/(2a) and action time t=2d/V are introduced,. Where p is the broad product portfolio of PolyOne, including: optical output power; Is the scanning speed; D is the beam radius. Choose different process parameters for experimental research

3.1 microhardness

higher hardness value can be obtained by laser surfacing than by other methods. Figures 1 and 2 respectively show the variation rules between different laser process parameters. As shown in Figure 1, after surfacing the alloy on stainless steel, the hardness is significantly improved. The average hardness of the surfacing layer is hv400, the thickness of the surfacing layer is 0.7 ~ LMM, and the hardness of the surfacing layer presents a gradient transition. At the same speed, with the increase of laser power, the microhardness of surfacing layer also increases, which is determined by the extremely fast speed of laser heating and cooling. The microhardness curve at the same power and different speeds is shown in Figure 2. When it increases, it decreases. The hardness of the welding layer of the stacked tire is tested by rotation on the inner side of the drum, and the absorbed per unit volume also provides more than 100 sets of microcomputer controlled material change testing machines for more than 310 colleges and universities in China. The energy of the testing machine is reduced, and the thickness of the surfacing layer is reduced. Figure 2 also clearly verifies this, This also shows that the energy density plays a leading role under the experimental conditions [1l.

3.2 microstructure analysis

the structure of a typical stainless steel surfacing coating is shown in Figure 3. The joint surface is relatively flat, showing columnar and dendritic crystal structures in the transition zone between the matrix and the surfacing layer, and the representative microstructure is shown in Figure 4. The heat dissipation during the surfacing crystallization process is two aspects: the high-temperature radiation from the surfacing pool to the space and the heat conduction through the collective material. Because the surfacing pool is small, the matrix material is relatively small Large, the cooling rate of the matrix to the molten pool is very fast. Therefore, crystal growth is equivalent to directional solidification, which leads to the development of columnar crystals when the surfacing layer crystallizes, and at the same time, new crystals are nucleated and grown on the surface of the molten pool due to high temperature radiation. The bottom of the surfacing layer belongs to the typical planar epitaxial growth structure, and the dendrite structure is coarse; On the top of the surfacing layer, there is a regular dendritic eutectic structure, and the dendrite is smaller than that, which forms columnar crystals and dendritic crystals alternately, which is conducive to increasing the bonding strength of the surfacing layer (as shown in Figure 4). When the laser process parameters are not appropriate, defects such as pores and inclusions are easy to appear

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