Materials Science Forum Vol. 723

Abstract: Magnesium (Mg) components can significantly reduce energy use due to their low densities compared to the majority alloys. Mg alloys are often machined to fit individual cases. However, process mechanics by high-speed dry cutting of Mg alloys are poorly understood. This study focuses on machining ability of biomedical magnesium-calcium (Mg-Ca) alloys. First, it presents a modeling approach of mechanical behavior of Mg-Ca0.8 (wt %) alloy under cutting regimes using the internal state variable (ISV) plasticity model. Then, the ISV plasticity model is implemented to simulate high speed dry cutting of Mg-Ca0.8 alloy by finite element method. Last, machining performance in the context of sustainability is discussed. Excellent surface finish can be achieved in the range of high cutting speeds. Continuous chip formation predicted by the finite element simulation is verified by high speed dry cutting of Mg-Ca0.8 using polycrystalline diamond (PCD) inserts. Chip ignition as the most hazardous aspect in machining Mg alloys does not occur for in high-speed dry cutting with sharp PCD tools. The predicted temperature distribution well explains the reason for the absence of chip ignition in high speed dry cutting of Mg-Ca0.8 alloy. A mechanism of built-up layer (BUL) formation is proposed.

Abstract: In high speed machining, temperature distribution in workpiece is the main factor which directly affects the surface integrity and dimensional accuracy of machined workpiece. In this paper, the machined workpiece temperature in high speed peripheral milling is analyzed through using moving heat source method and inverse method. Firstly, the workpiece to be machined is considered as a semi-infinite solid to model the transient surface temperature using arc-shaped moving heat source. Inverse method is then applied for the calculating of heat flux. Peripheral milling experiments of 1045 steel is performed with coated carbide insert The machined surface temperatures were measured during experiments. The measured results were found to be in agreement with the predicted ones by transient models for machined surface temperatures. These results confirm the conclusion that the transient workpiece temperature will decline when the cutting speed increases to a critical value.

Abstract: Titanium Matrix Composites (TMCs) parts usually requires high mechanical performance. The good workpiece surface quality and long tool life is the two target of finishing machining. In this paper, finishing turning tests are performed to investigate the workpiece surface roughness and tool life of machining TMCs with Polycrystalline Diamond (PCD) tools at the cutting speed range 60m/min to 120m/min. The results show that the workpiece surface roughness is obtained range Ra 0.44 to Ra 0.53m. tool life reaches about 10.6min, 7.9min, 7min and 5.5min at the cutting speed of 60m/min, 80m/min, 100m/min and 120m/min, respectively.

Abstract: Carbon fiber reinforced plastics (CFRP)/Ti super hybrid laminates are newly developed structural materials with excellent properties. But they are restricted in aircraft manufacturing because of their poor machining quality and short tool life. The machining quality and tool life are determined by machining ways, tool materials, drill point forms and drilling sequence. Spiral milling, drilling from Ti side, using the PCD tools and carbide drills with special point angle can improve the quality of hole and prolong tool life.

Abstract: The thermal conductivity of carbon fiber reinforced polymer(CFRP) and titanium alloy is lower which caused the increasing of drilling temperature and larger tool wear resistance. The low temperature air is aided for the drilling of laminated stack board of CFRP/Ti with double apex angles carbide drill at different feed rate. The drilling force and the quality of hole are analyzed. The results showed that the low temperature air can reduce the tool wear and the thrust force effectively.

Abstract: Based on the characteristics of austenitic stainless steel 1Cr18Ni9Ti, aiming to decrease surface roughness and milling force orthogonal face milling experiment with four factors and four levels was designed to optimize machining parameters with consideration of material removal rate. Three relatively excellent machining parameters were taken to further figure out the influences of cutting speed on tool life, and the tool wear mechanism was also analyzed. The results indicated that both surface roughness and milling force were greatly influenced by cutting speed and feed. The best machining parameters obtained in the experiments were v: 200m/min, f_z: 0.1mm/z, a_p: 0.9mm, a_e: 35mm. The main tool failure mode under this circumstance was tool chipping at the flank between main cutting edge and surface to be processed, besides, the wear of rake face was relatively slight.

Abstract: As the deformation of chip increases with cutting speed, the morphology of chip changes from continuous to serrated type. It is supposed that serrated chips generate due to adiabatic shear instability in the primary deformation zone. A new analytical model for predicting adiabatic shear critical condition in orthogonal cutting is proposed by considering cutting conditions and properties of workpiece material. It is found that the influence of shear strain on the onset of adiabatic shear could be neglected. The shear strain rate and temperature, however, play a leading role on the onset of adiabatic shear. At lower cutting speed the shear strain rate plays a dominant role while at higher cutting speed the situation is just the reverse. With the increase of cutting speed, the yield stress, material characteristic constant and uncut chip thickness will facilitate adiabatic shear instability, while the coefficient of strain rate hardening, coefficient of strain hardening, coefficient of thermal softening, thermal diffusivity and tool rake angle have negative effect on adiabatic shear instability.

Abstract: A series of milling experiments with and without cutting fluid, arranged by uniform design method, were carried out on rotor material. The influence of cutting fluid on cutting force and surface roughness was explored and compared for the two kinds of conditions. The associated model was established between cutting force & surface roughness and cutting parameters according to the linear multivariable regression method. The results show that the cutting force deceases with the increase of the cutting speed or with the decrease of the feed per tooth and the cutting depth. Cutting fluid has little effect on cutting force, and for surface roughness, the influence of cutting fluid is uncertain.

Abstract: An Al₂O₃-based micro-nano-composite ceramic cutting tool material reinforced with (W, Ti)C micro-particles and Al₂O₃ micro-nano-particles was fabricated by using hot-pressing technique, the composite was denoted as AWT. The cutting performance, failure modes and mechanisms of the AWT micro-nano-composite ceramic tool were investigated via intermittent turning of hardened AISI 1045 steel (44~48 HRC) in comparison with those of an Al₂O₃/(W, Ti)C micro-composite ceramic tool SG-4 and a cemented carbide tool YS8. Worn and fractured surfaces of the cutting tools were characterized by scanning electron microscopy (SEM). The results of intermittent turning revealed that shock resistance of the AWT ceramic tool was higher than that of the SG-4 and YS8 tools at all the tested cutting speeds. The excellent shock resistance of the AWT composite ceramic tool was attributed to its synergistic strengthening/toughening mechanisms induced by the (W, Ti)C micro-particles and Al₂O₃ nano-particles.