Coating optimisation for high speed machining with advanced nanomechanical test methods

Advanced nanomechanical testing has been used to evaluate key factors influencing tool life (1) a plasticity index (PI, the plastic work done/total work done during indentation), at room and elevated temperature (2) hot hardness and (3) fatigue fracture resistance, and determine their relative importance in different cutting applications. The optimum combination of hardness and toughness/plasticity to minimise wear and extend the life of coated WC-Co cutting tools was found to vary with the severity and nature of the cutting conditions. For interrupted cutting the plasticity index is critical, with high values (i.e. not extremely high H/E) resulting in extended tool life. Elevated temperature nanoindentation showed decreasing hardness and increasing PI with temperature. In high-speed turning hot hardness is the dominant factor whilst for interrupted cutting high hot hardness should be combined with improved plasticity for longer tool life. A novel test technique nano-impact, was used to simulate the interrupted contact (and cyclic loading) conditions occurring in milling applications and evaluate the fatigue fracture resistance of coated tools. It was able to successfully rank coatings in terms of tool life in end milling and reproduce the evolution of tool wear in the cutting test. In elevated temperature nano-impact testing the probability and extent of fracture during the test decreased at elevated temperature, consistent with the higher PI. Results from the advanced nanomechanical tests can be used in combination to predict which coatings have longer life in severe cutting conditions.     

Compact machining module for laser chemical manufacturing

The size of machines for the manufacture of micro-components commonly stands in gross disproportion to the component size. The achievable accuracy is limited by inaccuracies of mechanical positioning elements and external disturbances. Micro-machining of metals can be realised in manifold ways by laser processes. In this paper, a compact module for laser chemical processing using continuous wave laser radiation is presented. For the laser-induced chemical machining the material removal is a result of thermochemical reactions between an etchant and the surface of a metallic workpiece at low laser power densities. Laser-induced chemical machining results in improved surface quality of the machined workpiece and it also avoids stress and strain of the material. Through the use of a micro-mirror array (DMD) for flexible beam shaping a 2-dimensional machining of the workpiece is possible. The laser chemical machining method now allow for the first time to connect the DMD technology with laser-based micro-machining for compaction of the machining module.     

Anodic dissolution of the copper-nickel alloy under transient conditions

Anodic dissolution of an electrolytic copper-nickel alloy in 1 M HCl was studied by nonequilibrium electrochemical methods. The dissolution of the alloy began with selective ionization of nickel. Then the rate of the anodic dissolution was limited by nonsteady-state bulk diffusion. Original Russian Text ? V.N. Tseluikin, 2008, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2008, Vol. 44, No. 5, pp. 556–558.     

Design of reconfigurable machining lines: A novel comprehensive optimisation method

We present a novel comprehensive optimization model for designing reconfigurable machining lines. Due to the proposed fine mathematical modelling, it is possible to optimize simultaneously the whole set of machines and machining modules as well as their cutting parameters, their configuration that will be used for processing of each part and part position at each machine. The experimental results show that the proposed optimization approach substantially outperforms the existing heuristic design method and therefore it can be used by the designers in order to reduce the total system cost and improve the efficiency of reconfigurable machining lines.     

Chip geometries during high-speed machining for orthogonal cutting conditions

The originality of this work consists in taking photographs of chips during the cutting process for a large range of speeds. Contrary to methods usually used such as the quick stop in which root chips are analyzed after an abrupt interruption of the cutting, the proposed process photographs the chip geometry during its elaboration. An original device reproducing perfectly orthogonal cutting conditions is used because it allows a good accessibility to the zone of machining and reduces considerably the vibrations found in conventional machining tests. A large range of cutting velocities is investigated (from 17 to 60 m/s) for a middle hard steel (French Standards XC18). The experimental measures of the root chip geometry, more specifically the tool-chip contact length and the shear angle, are obtained from an analysis of the pictures obtained with a numerical high-speed camera. These geometrical characteristics of chips are studied for various cutting speeds, at the three rake angles −5, 0, +5° and for different depths of cut reaching 0.65 mm.     

Picosecond laser machining of tungsten carbide

Hard materials such as tungsten carbide (WC) are extensively used in cutting tools in high-value manufacturing, and the machining of these materials with sufficient speed and quality is essential to exploit their full potential. Over the last two decades, short (nanosecond (ns)) and ultra-short (picosecond (ps); femtosecond (fs)) pulse laser machining has been evaluated by various researchers and proposed as an alternative to the current state-of-the-art machining techniques for advanced materials like WC, which include mechanical grinding and electrical discharge machining. However, most of the established/existing research on this topic is based on low power lasers, which may not be adopted in industrial production environments due to its low material removal rate. This paper presents the results of a fundamental study, on using a 300 W picosecond laser for the deep machining of tungsten carbide. The influence of various laser parameters on the geometric precision and quality (surface and sub-surface) of the ablated area was analysed, and the ablation mechanism is discussed in detail. Laser pulse frequency and scanning speed have minimal effect on ablation rate at high power levels. The surface roughness of the ablated area increases with the ablation depth. At optimal conditions, no significant thermal defects such as a recast layer, micro crack or heat affected zone were observed, even at a high average power of 300 W. The material removal rate (MRR) seems to be proportional to the average power of the laser, and a removal rate of around 45 mm³ per minute can be achieved at 300 W power level. Edge wall taper appears to be a significant issue that needs to be resolved to enable industrial exploitation of high power ultra-short pulse lasers.     

Prediction of machining chatter based on FEM simulation of chip formation under dynamic conditions

Machining chatter is an inherently nonlinear phenomenon that is affected by many parameters such as cutting conditions, tool geometry e.g., nose radius and clearance angle and frictional conditions at the tool/workpiece interface. Models for chatter prediction often ignore nonlinearities or introduce them through simple models for friction and geometry. In particular, the effect of chip–tool interaction on the occurrence of chatter is not investigated thoroughly. This paper presents a novel approach for prediction of chatter vibration and for investigation of the effects of various conditions on the onset of chatter. This approach uses finite element simulation to investigate the inter-relationship between the chatter vibration and the chip formation process. Simulation of chip formation is combined with dynamic analysis of machine tool to determine the interaction between the two phenomena. Mesh adaptation technique is used to move the tool inside the workpiece to form the chip, while a flexible tool is used to allow the tool to vibrate under variable loading conditions. By repeating the simulations under various widths of cut, it is shown that the onset of chatter can be detected, and the simulation is able to realistically predict various phenomena observed in actual machining process such as variation of shear angle and the increase of stability at lower speeds known as process damping. The stability map obtained from simulations is compared with experimental data attained through orthogonal cutting tests. Reasonable agreement observed between the two sets of results demonstrates the effectiveness of the simulation approach.     

Design of operating conditions for crackfree laser-assisted machining of mullite

The present study focuses on the evaluation of the laser-assisted machining (LAM) of pressureless sintered mullite ceramics. Due to mullite’s low thermal diffusivity and inferior tensile strength, a new method for applying laser power is devised to eliminate cracking and fracture of the workpiece during laser heating by controlling the development of temperature gradients. Numerical modeling of workpiece temperature along with experimental measurement by a pyrometer is performed to determine the temperature at the material removal zone during LAM and to analyze the temperature gradient information to delineate the occurrence of cracking. With a designed gradual heating method, LAM of mullite is successfully performed without inducing any surface or subsurface cracks. Under the designed heating cycle, LAM of mullite is shown to be successful with a relative long tool life of carbide tools and good surface integrity.     

Non-stationary signal analysis and transient machining process condition monitoring

Non-stationary machine condition monitoring is very important in modern automated manufacturing processes. In this research, an innovative non-stationary (transient) signal analysis approach has been developed for non-stationary machine condition monitoring. It is based on time–frequency distribution analysis and a singular value decomposition approach. The singular value decomposition method is used to extract features from the time–frequency distribution data. These features will serve as machine condition indices and can be easily incorporated for on-line machine condition monitoring and diagnosis. Satisfactory results have been obtained through simulation and experimental data. Experimental studies have demonstrated the effectiveness of the proposed method for transient machine and process condition monitoring.     

Water-assisted laser thermal shock machining of alumina

A combined experimental and computational approach was undertaken to investigate water-assisted laser cutting of 96% pure alumina specimens through controlled thermal shock fracture mechanism. A low-power CO₂ laser (<300 W) was used for localized heating and scribing of alumina samples followed by water quenching to induce thermal stress cracking. In order to elucidate the cutting mechanisms and identify the regime of processing conditions suitable for controlled fracture, laser cutting experiments were performed under two different environments: water-assisted laser cutting and dry laser cutting. Temperature profiles of the heat-affected zones were obtained using thermocouples and data acquisition system. Finite element analysis was applied to predict the temperature and thermal stress distributions developed during both water-assisted and dry laser cutting operations. Temperature histories of the samples recorded during cutting were compared with numerical model predictions to determine heat transfer parameters associated with wet and dry laser cutting of alumina samples. Both experimental data and numerical analysis indicate that water quenching makes a substantial difference in thermal stress distribution, which governs the ability to control the fracturing of alumina. This in turn, resulted in better control of cutting and higher feed rates than previously reported in laser machining of alumina.     

On-machine three-dimensional monitoring technique using machining laser

An on-machine three-dimensional monitoring system has been developed that uses machining YAG laser as a source of light for monitoring. In the monitoring stage, the output of the laser is decreased. The reflected light from a workpiece is taken out with a beam sampler on the way of the optical system, and observed by CMOS image sensor. This system can measure both Z-direction depth and the position of X–Y-direction. Because the same laser and the optical system are used, the system has no gap of an optical axis between the measurement and the machining, and also has no limitation of viewing angle. Every three-dimensional shape which can be machined by laser beam can be measured. In this paper, system configuration, monitoring technique and the experimental results are described.     

Wear behavior of adaptive nano-multilayered TiAlCrN/NbN coatings under dry high performance machining conditions

Application of quaternary nitride nano-multilayered coatings results in significant improvements in tool life as well as wear behavior of ball nose end mills under severe conditions of dry high speed machining of hardened H13 steel (HRC 55-57). Tool life of different nano-multilayered TiAlCrN-based coatings with addition of transitional metals based (of V and VI groups) nitride layers has been compared. Tool life of TiAlCrN/NbN coating was found to be higher than compared to the other nano-multilayered coatings. Investigation of surface structure characteristics of the TiAlCrN/NbN coating using Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS) and High Resolution Electron Energy Loss Spectroscopy (HREELS) has been performed. The properties of the coatings such as microhardness, modulus of elasticity, coefficient of friction and oxidation stability at elevated temperatures were also studied. Cutting forces at the tool/workpiece interface have been measured in-situ. Temperatures on the surface of cutting tools were evaluated. The features of friction and wear behavior as well as mechanisms of tribo-adaptation of TiAlCrN/NbN nano-multilayered coatings were outlined.     

On the mechanism of material removal in electrochemical spark machining of quartz under different polarity conditions

Electrochemical spark machining (ECSM) process has been successfully applied for cutting of quartz using a controlled feed and a wedge edged tool. Contrary to the common belief that only cathode works as a tool, both cathode and anode have been used as a tool, i.e. ECSM with reverse polarity (ECSMWRP) as well as ECSM with direct polarity (ECSWDP) have been used to machine quartz plates. In ECSMWRP, deep crater on the anode (as a tool) and work-piece interface is formed because of chemical reaction. Chemical analysis of electrolyte solution after the ECSM experiments, also agrees with the feasibility of dissolution of quartz into solution due to chemical reaction. Reverse polarity cuts quartz plate at a faster rate as compared to the direct polarity. But in reverse polarity overcut, tool wear and surface roughness are higher as compared to the direct polarity machining. Magnified view of the machined surface also shows a difference in the mode of material removal in ECSMWDP and ECSMWRP. The cutting is possible even if we make auxiliary electrode of small size. In conclusion, experiments have revealed that cutting can be performed simultaneously at both the electrodes (anode and cathode) during ECSM.     

Thermoelectric signal characteristics and average interfacial temperatures in the machining of metals under geometrically defined conditions

The Herbert/Gottwein dynamic thermocouple method of temperature measurement was applied in order to experimentally evaluate the average interfacial temperatures arising in the external cylindrical turning of aluminium, brass, mild steel and stainless steel using high speed steel cutting tools. A detailed investigation of the EMF signal generated was undertaken for purposes of explanation of the DC and AC components which arise and of the influence of the cutting tool condition on the EMF signal generated. The thermoelectric characteristics of each material in conjunction with HSS were determined by means of the furnace method of calibration. A critical appraisal of each phase associated with the dynamic thermocouple method of cutting temperature measurement is undertaken in this paper and interfacial temperatures under a wide range of machine setting parameters for each workpiece material are presented and discussed.     

Mechanisms and processing limits in laser thermochemical machining

Metallic microparts can be produced with high quality by laser thermochemical machining when the etching liquid is injected coaxially to the laser beam directly into the irradiated area. The basic mechanisms and limits of the process are described. It is shown that the reaction is temperature driven independent of the laser wavelength. A limiting factor is the diffusion of the anions identified by comparing experimentally determined and calculated diffusion coefficients for the reaction products. The machining quality with respect to aspect ratio, edge radius and roughness can be enhanced by increasing the velocity of the etching liquid.     

An optimisation procedure to determine the coefficients of the extended Taylor''s equation in machining

An optimum experimental design to determine the coefficients of the Extended Taylor's Equation in machining is proposed. The technique is based on the minimisation of the ratio between maximum and minimum singular values of the matrix of sensitivity of the tool life related to the machining parameter variations. This procedure generates the best set of cutting conditions to be used in tool life tests which results in a fast convergence of the coefficients and their confidence intervals. This technique was compared to the commonly used fractional factorial design when face milling AISI 1045 steel with cemented carbide cutting tools. The results showed a considerable reduction in the number of tests required to obtain a reliable equation when the optimum experimental procedure was used when compared to the factorial design.     

Comprehension of chip formation in laser assisted machining

Laser Assisted Machining (LAM) improves the machinability of materials by locally heating the workpiece just prior to cutting. Experimental investigations have confirmed that the cutting force can be decreased, by as much as 40%, for various materials. In order to understand the effect of the laser on chip formation and on the temperature fields in the different deformation zones, thermo-mechanical simulations were undertaken. A thermo-mechanical model for chip formation was also undertaken. Experimental tests for the orthogonal cutting of 42CrMo4 steel were used to validate the simulation. The temperature fields allow us to explain the reduction in the cutting force and the resulting residual stress fields in the workpiece.     

Thermal effects in laser assisted jet electrochemical machining

The purpose of the laser in laser assisted jet ECM (LAJECM) is to localise machining to specified areas so that precision and the productivity is improved. Temperature is a predominant determinant of this localisation effect and must be carefully monitored to avoid any heat affected zones or spark damage due to electrolyte boiling. This paper investigates the thermal effects in LAJECM on several alloys by temperature distribution modelling and experimental analysis. The laser influence on material micro removal has led to 25 μm deeper cavities with a reflective surface of roughness 20 nm R_a, without any detectable heat affected zone.