Advanced Materials Research Vols. 887-888

Abstract: The hot stamping and quenching process of U-shaped parts is simulated by ANSYS/ LS-DYNA software. The influence law of forming process and temperature field in quenching process of U-shaped parts is obtained through the simulation of forming and quenching process under different blank holder force, punch velocity and mould initial temperature. The results show that plate temperature changes with the punch velocity and mould initial temperature changes, while the effect of blank holder force on plate temperature can be neglected.

Abstract: A GMAW additive manufacturing process is performed with H08Mn2Si wire to fabricate a cube forming part. A optical microscope was used to observe the cross section aspect of forming part and the structure mechanism has been analysed. The results demonstrate that the forming part structure presents a characteristic that macrostructure is uniform and microstructure changes circularly. The circularly heating pattern of GMAW surfacing forming results in circular microstructure change and preheating of formed bead to subsequent bead and postheating of subsequent bead to formed bead are primary reason. Electromagnetic stirring effect of longitudinal dc magnetic field to molten pool can break dendritic crystal and refine crystal grain, which is in favor of enhancing forming part performance.

Abstract: Thixo-forming is an unconventional semi-solid forming process, by which complex-shaped products can be manufactured using a single forming operation. It can even be applied to difficult-to-form materials, including those which are impossible to process by conventional methods. Today, commercial semi-solid processing is used for low-melting materials, primarily aluminium and magnesium alloys. Due to its technological complexity, thixo-forming of high-melting alloys is still under development. For this reason, the present experimental programme was focused on the tool steel CPM REX 121 with a melting point above 1200 °C produced by powder metallurgy. The total content of alloying elements in this steel is 37.5 %. Owing to the high levels of alloying elements, namely Co, Mo, W, V and Cr, this material cannot be formed by conventional methods. The purpose of the present experiment was to explore its potential for forming in semi-solid state and to find suitable processing parameters. Experimental forming took place in a mini-thixoforming die, a tool specially-developed for this thixo-forming variant intended for producing very small parts. The resulting microstructures were examined by means of optical and electron microscopy. It was found that semi-solid processing leads to the development of microstructure with austenitic grains, martensite, chromium and V-W-Mo complex carbides and also a eutectic formed by partial melting of carbides.

Abstract: The deformation behavior of a Udimet720Li superalloy under hot compression tests was characterized in the temperature range of 1060~1160°C and strain rate range of 0.001~20s^-1. Processing maps were conducted at a series of strains to calculate the efficiency of hot working and to recognize the instability regions of the flow behavior. A Zener-Hollomon parameter is given to characterize the dependence of peak stress on temperature and strain rate. The efficiency of power dissipation of the Udimet720Li superalloy obtained in a strain range of 0.1~0.7 are essentially similar, which indicates that strain does not have a significant influence and the instability region shown in high strain and high strain rates at all temperatures. The regions for the full recrystallization can be divided by the dissolution beginning temperature of primary γ'which are the optimum hot working parameters.

Abstract: According to the detonation wave collision theory, initiation from two ends of charge, the liner will close to the middle to form high velocity MEFP. The software Autodyn to be used to study the penetration capacity of a single Annular MEFP was studied. With increase of axis curvature radius, the ratio of length of width is decrease, the penetration capacity declined. It is partitioned hole into two parts which is formed gourd-shaped in the typical penetration. For equal liner and unequal liner, although the velocity of unequal liner is higher, the penetration capacity is weaker.

Abstract: Acoustic Emission (AE) may be defined as a transient elastic stress wave generated by the rapid release of strain energy in local area of material. To overcome the limitation of some traditional techniques, the AE technique, which provides high sensitivity and responding speed, were developed in the present paper. AE signature is usually difficult to be extracted and characterized in grinding process of 1Cr18Ni9Ti coatings due to their high hardness, great ductility, inhomogeneous structure and irregular surface with lots of hard points and pores. In this paper, AE signal of stationary grinding status before wheel-workpiece contact was characterized first, then AE signal of the grinding process was analyzed using root-mean-square (RMS) and power spectrum method. Results showed that before the contact occurred, the grinding signal is stable, with low amplitude and frequency ranging all frequency channels and no peak signal. However, when contact occurred, the RMS and spectrum of AE signal increased obviously and the bandwidth varied exquisitely between 100 KHz and 300 KHz. The real contact time between wheel and workpiece was about 0.5 to 1 ms.

Abstract: Cutting force coefficients and cutter runout parameters are the key factors for accurate prediction of instantaneous milling forces. A new two-step identification method is presented to calibrate them in end milling. Based on analyzing effects of cutter runout on milling forces, a method of extracting nominal milling forces from measured milling forces is proposed. By calibrating average cutting force coefficients and corresponding average chip thickness, an approach to evaluate the instantaneous cutting force coefficients is proposed. Then, an iterative method is presented to identify cutter runout, and the procedure is also given in detail. Milling tests are performed to test the proposed method, and validity of the identification approach is proved by a good agreement between predicted results and experimental results.

Abstract: Workpiece temperature in orthogonal turn-milling compound machining was studied with experimental method in this paper. The orthogonal turn-milling process was simulated through engagement of a milling tool and cylindrical surface on a five-axis milling center. The cutting parameters were designed into an orthogonal parameter table of seven factors three levels based on factors having effects on workpiece temperature. Variance analysis of data achieved from this experiment was carried out and conclusion about the order of effects each factor has on workpiece temperature was drawn.

Abstract: Thermal energy sources have been applied for softening the difficult-to-machine material when it is combined with conventional machining processes. Cutting forces has been reduced during the process. To investigate the plastic deformation property of workpiece materials heated by thermal sources, and its influence to the cutting forces, the analytical model of orthogonal cutting is established. The impact of cutting speed and initial temperature of the shear banding to the cutting forces are taken account of, based on adiabatic shear banding model and Johnson-Cook material constitutive law. The shear banding average shear stress failure criteria has been proposed to decide the fracture between workpiece and chip. Simulation has been carried out and compared with experimental data of laser-heat assisted titanium alloy milling, showing good agreement.

Abstract: Wire electrical discharge machining (WEDM) is extensively used in the mold, instrument and manufacturing industries, and rough cutting operation in WEDM is treated as a challenging process because improvement of more than one machining performance measures viz. metal removal rate (MRR), roughness (Ra) are sought to obtain a precision work. In this paper, first of all, a set of Taguchi experiment (L₁₈ 2¹×3⁴) is carried out based on the Taguchi method. Secondly, two groups of ANOM are completed to obtain the influence trends of each parameters on material removal rate (MRR) and roughness (Ra), respectively. Eventually, three groups of best process parameters combination are acquired to meet high material removal rate (MRR) and low roughness (Ra) simultaneously, it can provide guiding significance to actual machining process.