Study On Picosecond Laser Ablation Of Cold Work Mold Steel On Parameter,Mechanism And Quality Control

With the continuous development of micro/nano science and technology,the manufacturing industry has begun to shift towards miniaturization and precision,expanding the application of micro/nano textures.Micro textures are made on the surface of the molds that can be transferred to the surface of the product to make the surface functional.The ablation efficiency and surface quality of the micro textures on the surface of the molds are of primary concern.Therefore,it is extremely important to explore efficient technology to fabricate micro textures on the surface of mold steel.In this dissertation,a picosecond laser is introduced to ablate Cr12MoV cold work mold steel.Due to the ultrashort pulse duration,the heat diffusion depth is suppressed or disappeared,resulting in small recast layer and heat-affected zone.High-speed scanning galvanometer is used to fabricate micro textures on Cr12MoV.Laser processing parameters,ablatioin mechanism and ablation quality control are studied.The main research work and results are as follows:The effects of the scanning speed,laser mean power,number of scans and ablation width on the ablation depth and quality were systematically studied at 1064 nm at 0.3MHz.The experimental results show that the ablation depth is basically a few?m,no matter the laser mean power or the number of scans of single-or multiple-trace is increased or the scanning speed is decreased.Higher laser mean power or more number of scans cannot increase the ablation depth,however,will decrease the ablation quality(Ra=3.18?m)and ablation efficiency(84.23?m~3/mJ),and eventually decrease the ablation depth.The suggested reason is that the minimum laser fluence(9.99 J/cm~2)of single pulse at 0.3 MHz is large which causes a strong plasma shielding effect in the laser ablation process.A large amount of laser energy is consumed which causes a thick recast layer and large heat-affected zone(HAZ).The effect of laser repetition rates on the ablation depth,surface roughness and efficiency of Cr12MoV cold work mold steel were studied at 1064 nm and 532 nm,respectively.The results show that the laser repetition rate has a great effect on the ablation depth,surface roughness and efficiency under the condition of constant laser energy(34.25 mJ@1064 nm and 7.47 mJ@532nm).The effect is mainly depending on the competition of the single pulse energy,the plasma shielding effect and the heat accumulation effect.In the optimal repetition rate range(10-20MHz),the single laser pulse energy is rather small and the heat accumulation effect is significant while the plasma shielding effect is reduced significantly.Consequently,high ablation depth(51?m@1064 nm and 44?m@532 nm)and high ablation efficiency(1000?m~3/mJ@1064nm and 2373?m~3/mJ@532 nm)with surface roughness<1?m can be achieved at 1064nm and 532 nm,respectively.The effects of the laser wavelengths on the ablation depth,surface roughness and efficiency were studied at low repetition rate(0.4 MHz)and high repetition rate(10MHz),respectively.The results show that the ablation depth,ablation efficiency and surface roughness of 532 nm are all higher than that of 1064 nm and 355 nm when the laser fluence is less than 25.1 J/cm~2 at the repetition rate of 0.4 MHz.When the laser fluence is higher than 25.1 J/cm~2,the ablation depth and efficiency of 532 nm decrease to zero rapidly,meanwhile,the surface roughness increases rapidly.However,the ablation depth and surface roughness at 1064nm and 355nm increase slowly while the ablation efficiency decreases slowly when the laser fluence is higher than 25.1 J/cm~2.At repetition rate of 10 MHz,the ablation efficiency and depth of 532 nm are 2.56 and 1.52 times of1064 nm,respectively.A small number of pores are formed on the ablation surface of532 nm while no pore is formed at 1064 nm.The surface roughness of 532 nm(Ra=0.49?m)is smaller than that of 1064 nm(Ra=0.72?m).The research on quality control of Cr12MoV cold work mold steel by picosecond laser indicates that both the laser processing parameters and scanning strategies of the picosecond laser affect the dimensional accuracy and quality of ablation texture on the Cr12MoV cold work mold steel.Different textures should choose the appropriate scanning strategy and the optimal picosecond laser process parameters to ensure the accuracy and quality.The flatness and uniformity of the ablation edge should be guaranteed to ensure the dimensional accuracy first and then the ablation efficiency should be considered when the scanning strategy is selected.The selection principle of processing parameters is to ensure the ablation quality first,then the ablation depth and efficiency.First,the scanning speed and laser mean power should be matched in zone III to ensure the ablation quality.The surface roughness is smaller than 1.42?m.Second,a maximum ablation efficiency of up to 900?m~3/mJ can be achieved by increasing the laser mean power in zone III.Finally,the maximum ablation depth of up to 100?m can be obtained by increasing the number of scans or reducing the hatched space appropriately.A variety of functional textures can be prepared on the surface of Cr12MoV cold work mold steel and H13A cemented carbide by using the optimal laser processing parameters and the optimized scanning strategies.The results show that high precision and high quality functional textures can be manufactured by using picosecond laser,overcoming the difficulties of fabricating textures on mold steel due to its high hardness,mechnical strength and good wear resistance.The ablation accuracy error is less than 5%and the surface roughness is less than 1?m.