Study Of Repairng P20 Steel's Milimeter-leverl Surface Defects By High-power CO₂ Laser

In the plastic mold's machining and service process, scratches, groove, press pits, cracks and other surface defects are easy to occur. These surface defects usually cause the failure of the mould. Surface technology is a new and effective way to extend die life and it has a significant economic and social benefits. The trend of the development of plastic mould is toward miniaturization and precision, Laser cladding is the corresponding surface repair technology. Nowadays the repair of P20 plastic mold is a manual process, usually using a small YAG laser. And its results depend on the materials of the wire and tools, on laser process parameters and on operator skills.In this paper, we focused on how to obtain cladding layer with good performance using a high power CO2 laser and at the same time improve the repairing efficiency. We started with the study of the cladding parameters: laser power, cladding speed, defocusing amount etc. and find out the optimum combination. Then we conducted the cladding repair experiments with preheated specimen in order to improve the performance of the cladding layer.We designed the P20 steel's groove size to simulate the actual surface defects and preplaced P20 wire in the groove fixing it by tape to avoid the instability caused by manual wire feeding, and achieved fast and accurate positioning of the repairing position through the CNC console.We changed laser power, cladding speed, defocusing amount separately and respectively to investigate each parameters'influence on the repairing process. It turned out the depth and width of the cladding layer was increased with increasing laser power or decreasing cladding speed. However experiments with too large P or too slow V would lead to defects such as cracks etc. in the cladding layer. The research of the defocus amount showed its change could acquire penetration welding mode or heat conduction welding mode of the laser. So we could get a hemispherical shape cross-section of the cladding layer instead of a pushpin one. The optimum parameters are: P=8KW, V=2.25m/min,Δf=10mm,Vflow=28L/min.Through the analysis of the microstructure we could know that the repairing zone was mainly consisted of martensite compared with granular bainite of the base material. And this explains the cladding layer'macro-hardness distribution. From the base material to the heat-affected zone and to the cladding layer, the hardness value increases first and then decreases; the hardness in the fine grain zone has a maximum value Hmax, which is due to its fine grain size.It turned out that preheating treatment could decrease the appearance of defects and the hardness of the cladding layer. And compared to the specimen without preheating the cladding zone's hardness of the specimens preheated under 60℃, 130℃, 200℃respectively decreased by 13.2% 15.4% 16.3% accordingly. At the same time Hmax value decreased by 13.7% 15.6% 14.8% respectively. Analysis show this change may due to the formation of precipitation in the cladding layer.