低温液氮冷却下高速切削淬硬钢的切屑形成及刀具磨损

针对低温液氮冷却下淬硬钢高速车削过程中切屑形成及刀具磨损机理尚缺乏相关研究的问题，开展了液氮冷却下的淬硬钢高速切削研究，并与干切进行了对比。分析了切削力、切削温度、切屑特征以及刀具磨损特征，讨论了冷却润滑、切屑形成及刀具磨损机理。结果表明: 与干切相比，各组实验中低温液氮冷却切削的切削温度降低了6.9%~9.9%，因材料硬化使得切削力增大了10.1%~12.8%；低温液氮冷却下，切屑锯齿化程度相比干切明显增大，这与第一变形区应力应变更大、切屑形成存在从热塑性剪切失稳到周期性脆性断裂的过渡有关；低温液氮对切削界面的润滑降低了切削热，液氮强射流吹离了高热切屑，强传热换热能力可迅速置换切削区热量；与干切相比，低温液氮明显减小了黏结磨损，通过冷却增强刀体减缓了刀具微剥落，刀具寿命延长了28.6%~47.1%;低温液氮冷却下，黏结不再是主要磨损机理之一，磨粒磨损、冲击磨损为主要磨损机理，后刀面片状剥落和微崩刃为主要刀具破损形式。

Chip Formation and Tool Wear in High-speed Cutting of Hardened Steels under Cryogenic Liquid Nitrogen Cooling

In view of the lack of relevant research on chip formation and tool wear mechanism in high-speed cutting of hardened steel under cryogenic liquid nitrogen cooling, a high-speed cutting of hardened steels under liquid nitrogen cooling was studied and compared with dry cutting. The cutting force, cutting temperature, characteristics of chip and tool wear were analyzed, and the mechanism of cooling and lubrication, chip formation and tool wear were discussed. The results show that compared with dry cutting, the cutting temperature of cryogenic liquid nitrogen cooling cutting is reduced by 6.9%-9.9%, and the cutting force is increased by 10.1%-12.8% due to material hardening. Compared with dry cutting, chip serration increases obviously under cryogenic liquid nitrogen cooling, which is related to the larger stress and strain in the first deformation zone and the transition from thermoplastic shear instability to periodic brittle fracture in chip formation. Cryogenic liquid nitrogen reduces cutting heat generation in cutting interfaces, the strong jet of liquid nitrogen blows away the chips, and the strong heat transfer capacity quickly take away the heat in the cut areas. Compared with the dry cutting, cryogenic liquid nitrogen may significantly reduce the adhesive wear, alleviate the tool micro-spalling by cooling and strengthen the tool, and improve the tool life by 28.6%-47.1%. Under cryogenic liquid nitrogen cooling conditions, adhesion is no longer one of the main wear mechanism, abrasive wear and impact wear are the main wear mechanism, and flank flaking and micro collapse blade are the main tool damage forms.