| In the forging process,the periodic hot and cold loads and mechanical loads will cause abnormal failures such as excessive wear and cracking on the surface layer of the forging die,which will not only increase the production cost and reduce the production efficiency of the enterprise,but also may threaten the personal safety of the employees.In this paper,with the goal of improving the life of hot forging die,the main factors affecting the life of die are determined through the failure analysis of 99MT1++B1 pre-forged piston skirt die of Anhui An Spring Machinery Co.Based on the theory of "deformation coordination" and "film-substrate integration",the ZrTiN/TiAl N nanomulti-layer gradient coating was designed,and the effects of arc source current and substrate bias on the performance of the prepared coating were systematically investigated,and the best preparation process parameters were determined by analyzing the microstructure,morphology,friction coefficient,hardness and adhesion strength of the coating.The surface crack formation,extension and damage mechanism of the prepared coating were studied through thermal fatigue experiments,and the preparation parameters and post-treatment process of the coating were optimized;finally,the effectiveness and economic applicability of the surface strengthening scheme were verified in the production.The specific research findings are as follows.(1)The main failure forms of 99MT1++B1 pre-forged piston skirt die are wear and cracking,accounting for 35% and 50% of the total number of failures respectively;die wear is the result of the coupling of various wear forms such as abrasive wear,adhesive wear,oxidative corrosion wear and hot and cold fatigue wear;die cracking is due to the redistribution of alloying elements such as Mn,Si and V on the surface layer of the die caused by the high temperature and high pressure working environment,resulting in the mold material loses its original strength and hardness,forming the source of cracking.(2)The surface defects of the coating prepared at the arc source current of ZrTitarget 160 A are the least;with the increase of the arc source current of ZrTitarget,the deposition rate of the coating shows a trend of increasing first and then decreasing,and the deposition rate reaches the maximum at the arc source current of 180A;the phase composition of the coating is mainly TiN(111),TiN(200)and ZrN(222),and the face-centered cubic structure is the main one.The hardness of the coating reaches the maximum value of 2700 HV at 170 A of the ZrTitarget arc source current,and the adhesion strength of the film base shows a decreasing trend in this range.(3)With the increase of substrate bias voltage,the number and size of large particles on the surface of the coating tend to decrease,the hardness of the coating increases continuously,and the adhesion strength of the film base tends to increase first and then decrease,and the optimum value of the adhesion strength of the film base is at the substrate bias voltage of-100 V.(4)After thermal fatigue experiments,large particles on the surface of the ZrTiN/TiAl N nano-gradient multilayer coating are oxidized and exfoliate,forming a source of cracks.Cyclic stress and oxide generation and spalling are the main mechanisms for the sprouting and expansion of thermal fatigue cracks in ZrTiN/TiAl N nano-gradient multilayer coatings.(5)It is verified by actual production that the composite treatment technology of "ion nitriding + ZrTiN/TiAl N nano-gradient multilayer coating" can improve the die life of hot forging piston skirt by more than 2 times and has good effect. |
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