| Hardened steels are widely used in mould manufacturing and high-end equipment manufacturing industries because of their high strength and good wear resistance.However,the high hardness and poor plasticity of hardened steels induce that the tools are subjected to extremely strong impact in the cutting of hardened steels and are easily damaged.In this dissertation,the cutting experiments and simulations were combined to study the propagation paths of tool fatigue cracks in high-speed intermittent cutting of hardened steels.The oblique-gradient ceramic tool materials were designed and fabricated based on the crack propagation paths.In addition,the mechanical properties and cutting performance of oblique-gtadient tool are systematically studied.An orthogonal experiment was carried out employing the ceramic tool in intermittent turning of 20CrMnTi hardened steel.The effect of cutting parameters on cutting forces was studied using Taguchi method and variance analysis.The results showed that the cutting depth had the greatest influence on cutting forces,and the proportion of influence is gradually increased as the damage of cutting tool.The influence of cutting speed on tool failure mechanisms was studied using fracture morphology.The results showed that the failure mechanisms of tool was fatigue fracture.At low speeds,the tool fatigue characteristics were fatigue striations,and the fatigue characteristics at high speed were fatigue beaches.The fatigue crack propagation path of the ceramic tool was analyzed.The results showed that the crack originated from the tool tip and then propagated along the direction of the maximum shear stress.With the damage of the tool,the direction of crack propagation was gradually parallel to the rake face,and eventually extended to the tool surface.High-speed milling H13 steel and SKD11 hardened steel experiments were carried out using coated cemented carbide tool.The variation of cutting forces with cutting parameters was studied,and the failure mechanisms of the tool were compared different workpiece.It was found that the failure modes of the tool was the flank wear for milling and analyzed for milling the H13 steel,and the fatigue fracture occurred at the rake face in milling the SKD11 hardened steel.Fatigue cracks originated at substrate of coated cemented carbide tool and then propagated to the rake face.Multiple-sources of fatigue were found on the tool fracture surface,and the river patterns were observed,which indicated that the crack modes were I-III compound crack.The orthogonal experiments were conducted to analyze the failure mechanisms during milling SKD11 hardened steel with coated cemented carbide tools.The effects of cutting speed on cutting forces were also investigated.The cutting simulation model was established and the stress state of cutting tool was analyzed in the cutting period.In addition,the extended finite element model was established to investigate the crack propagation path.It was found that the tool crack originated from the maximum tensile stress at the rake face,and propagation along the angle of 45 degrees under cyclic load of tension-compression.Based on the study of the crack propagation path in intermittent cutting hardened steel,the model of oblique-gradient ceramic tool was designed.The tool-workpiece and tool-chip contact zones of the tool were designed to have high hardness and wear resistance,and to increase the fracture toughness of the tool along the crack propagation path.The Al2O3-TiC oblique-gradient ceramic tool material TAA30 was fabricated.Through the gradient structure optimization,it can be concluded that the orientation angle a with 30 degrees,the five layers with thickness ratio e of 0.4,had the optimal mechanical propertie.The sintering process was also determined,that is,the sintering temperature was 1700℃,and the holding time was 15 minutes.The optimal comprehensive mechanical properties are a flexural strength of 761 MPa,the hardness of 21.22GPa,20.54GPa,19.90GPa,and the fracture toughness of 9.07MPa·m1/2,9.29MPa·m1/2,9.26MPa·m1/2 from the outer layer to core layer,respectively.The Al2O3-TiC-TiB2 gradient ceramic tool materials TT3 was fabricated by using hot-press sintering technology.The tool gradient composition,layer thickness ratio and sintering process were determined.It can be conlcued that when the layer thickness ratio was 0.2,the sintering temperature was 1700℃.,and the holding time was 20 min,the tool had the optimal mechanical properties.The hardness,fracture toughness and flexural strength were 23.59 GPa,7.45MPa·m1/2,and 764.11 MPa,respectively.The anti-crack propagation mechanisms of oblique-gradient ceramic tool materials were summarized,including:a.crack arrest effect;b.crack deflection effect;c.crack bridging effect.The cutting performance of self-made oblique-gradient ceramic tools of TAA30 and TT3 in the intermittent turning SKD11 hardened steel was investigated.The influence of depth of cut and cutting speed on cutting force,cutting temperature,chip morphology and tool failure mechanisms was analyzed.The results showed that the tool of TT3 had good cutting performance.The cutting force and cutting temperature were lower,the chip morphology was integral,and had a longer tool life compared with the commercial tool and the tool of TAA30.Due to the residual compressive stress of tool nose and the increasing toughness along the crack propagation path,tool failure patterns were breakage at the tool nose and peeling along the cutting edge.When the orientation angle was 15°,the tool of TT3A15 had the optimal cutting performance. |
PDF Full Text Request