Analytical model to determine the critical feed per edge for ductile-brittle transition in milling process of brittle materials

Brittle materials like glass are considered difficult-to-machine because of their high tendency towards brittle fracture during machining. The technological challenge in machining such brittle materials is to achieve material removal by plastic deformation rather than characteristic brittle fracture. In ductile mode machining, the material is removed predominantly by plastic deformation and any cracks produced due to possible fracture in the cutting zone are prevented from extending into the machined surface. This is achieved by selecting an appropriate cutting tool and suitable machining parameters. In ductile machining by milling process, fracture induced cracks are diverted away from final machined surface by selecting a suitable feed per edge less than a critical threshold value. Hence determination of critical feed per edge is of paramount importance to achieve ductile mode machining by milling process. This paper presents an analytical model based on fracture mechanics principles to predict the critical feed per edge in milling process of glass. The size and orientation of cracks originating from brittle fracture during machining have been quantified by using indentation test results and the critical value of feed per edge has been determined analytically as a function of intrinsic materials properties governing brittle fracture and plastic deformation. Furthermore, an equivalent tool included angle has been suggested for machining operation as against the indenter included angle to correlate the indentation and machining test results with improved degree of accuracy. Experimental results validated the proposed model fairly accurately. It has been established that if the longest cracks oriented in radial direction to the cutting edge trajectory are prevented from reaching the final machined surface by selecting a feed per edge less than or equal to a critical value, a crack-free machined surface can be achieved.     

Ductile and brittle material removal mechanisms in natural nacre—A model for novel implant materials

Nacre is a composite material found in the inner layer of sea shells. It consists of soft organic and hard inorganic components arranged in a complex hierarchical structure. Due to this arrangement, nacre exhibits outstanding mechanical properties (elastic modulus: 64.70 ± 3.50 GPa, hardness: 4.41 ± 0.45 GPa, density: 2.6 g/cm³). Therefore, nacreous implant materials have a high potential in many fields. In medical science, these materials might be used for bone replacement. This article provides an insight into the material removal mechanisms occurring in the scratching of natural nacre. Scratch tests are a simplification of the grinding process and are used to investigate the influence of single input parameters on the material removal. Different scratch tool geometries and varying processing parameters are applied, so that the material removal efficiency can be evaluated by analyzing the process forces. Additionally, the scratch geometry is examined by using a scanning electron microscope (SEM) and optical profilometer images as well as photomicrographs. The results of these examinations provide knowledge on the machinability of nacre and also on the machinability of new nacreous materials.     

A new one-phase material model for the numerical prediction of critical material flow conditions in thixoforging processes

Thixoforging allows one-step forming processes of near-net shape components having excellent mechanical properties. However, the high sensitivity of thixoforging regarding process conditions requires precise modelling and determination of process related parameters. At the same time, simple numerical design proves challenging because of the inaccuracy of existing one-phase material models regarding the shear thinning flow behaviour of semi solid metals. Consequently, this paper deals with the development of a new one-phase material model providing a more precise simulation of materials’ shear rate dependency. By using this model, simulations could be performed, which allowed the prediction of solidification and flow-related component defects.     

Analytical modeling of acoustic emission for monitoring of peripheral milling process

This paper presents the development of an analytical model relating the acoustic emission (AE) energy content to the cutting process parameters in peripheral milling and the experimental verification of the effects of the cutting parameters on the AE energy measurement to assess the applicability of AE sensing to the monitoring of peripheral milling. The model consists of an oblique cutting component and a rubbing component. It includes the contribution from both the AE attributed to plastic deformation in the primary shear zone and the AE resulting from interfaces between chips and the rake faces of the cutting teeth, between the workpiece and the cutter periphery as well as between the cutter flank and the machined workpiece surface. The adoption of an effective shear angle and a mean fraction angle in the model formulation provides insight into the fundamental mechanism of the cutting process responsible for the generation of the acoustic emission. A numerical example following the model development and a series of peripheral milling tests show that the true mean square voltage of the AE signal from peripheral milling increases with the primary cutting parameters (cutter rotating speed, feedrate, axial and radial depths of cut). The result of this work suggests that the energy measurement of the AE signal can be effectively used for the monitoring of peripheral milling processes.     

Aspects of material removal mechanism in plain waterjet milling on gamma titanium aluminide

Due to providing reduced mechanical and thermal damages to workpiece surfaces, waterjet machining that is one of the most promising non-conventional processing methods found its niche application in cutting/shaping of materials with low machinability indexes. It can be even a more attracting technology if plain waterjet (PWJ) milling is employed due to reduced running costs (absence of abrasives) and the elimination of surface contaminations (grit embedment). The paper reports for the first time PWJ milling of a notoriously difficult-to-cut material, gamma titanium aluminide (γ-TiAl). Trials of different jet paths with varied milling parameters (e.g. water-hammer pressures, stepovers, number of passes) were conducted for understanding the removal mechanism of γ-TiAl in plain waterjet milling. The findings showed that the threshold water-hammer pressure for eroding the target material and for achieving uniform erosion were in the vicinity of 800 MPa and greater than 1 GPa, respectively. In addition, different fracture modes were observed on γ-TiAl when PWJ milling: (i) plastic deformation and crack initiation; (ii) stress wave propagation; (iii) micropits due to joint of crack lines; (iv) intergranular cracking/fracture, triple split and interlamellar/translamellar fracture. The stages (i)–(iii) occurred at lower water-hammer pressures and number of passes while the subsequent stage (iv) was only observed at higher water-hammer pressures or number of passes. The knowledge accumulated when studying the material removal mechanism and surface morphology enabled successful generation of 3D PWJ milled features (e.g. shallow pocket). To evaluate the capability of PWJ milling process, the geometrical accuracy and surface quality of the pocket has been examined. Finally, the advantages and drawbacks of the PWJ milling process are discussed to allow the definition where the technology is economically viable.     

Modeling of material removal rate in electric discharge grinding process

The mechanism of material removal in electric discharge grinding (EDG) is very complex due to interdependence of mechanical and thermal energies responsible for material removal. Therefore, on the basis of conceived process physics for material removal, an attempt has been made to predict the material removal rate (MRR). The proposed mathematical model is based on the fundamental principles of material removal in electric discharge machining (EDM) and conventional grinding processes. The inter-dependence of the thermal and mechanical phenomena has been realized by scanning electron microscopy (SEM) characterization of the samples machined at different processing conditions. The key input process parameters like pulse on time, pulse current, gap voltage, duty cycle, pulse off time, frequency, depth of cut, wheel speed and table speed are co-related with MRR for three distinct idealized processing conditions. The constant showing the extent of interdependence of two phenomena were evaluated by experimental data. The obtained expressions of MRR have been validated for processing conditions other than those used for obtaining constants. It was found that the discharge energy plays prominent role in material removal. The percentage difference in experimental findings and theoretical predictions was found to be less than 3%.     

脉冲GMAW熔滴过渡动态过程的解析模型

在脉冲GMAW过程中，电流的突然转变会引起熔滴在焊丝末端的明显振荡，合理的利用向下振荡的动量可控制熔滴过渡。该文基于熔滴受力分析，建立了脉冲GMAW熔滴过渡的解析模型，并对熔滴的振荡和脱离过程进行分析，定量计算焊接工艺参数对熔滴过渡过程动态行为的影响，可用于指导脉冲GMAW主动控制模式下一脉一滴过渡工艺参数的优化设计。     

Analytical model for prediction of deformed shape in three-roll rolling process

In process design of bar rolling, accurate prediction of deformed shape of the rolled workpiece is important in determining the total number of passes. Thus, development of an approximate analytical model for predicting an accurate rolled shape with less computation time will be beneficial for practical purpose. In the present study, such a model for the three-roll system with multi-passes was developed by deriving spread and geometrical configuration formulae from the finite element (FE) analysis results of a seven-passes sequence rolling. In FE simulations, process parameters such as the initial workpiece geometry and the amount of roll draft and roll speed were varied. By comparing spread ratios of FE simulation results three representative geometry changes (round–curved hexagonal, curved hexagonal–hexagonal, and hexagonal–hexagonal) were identified. For each representative geometry change those two formulae were determined in the present investigation. The formulae determined were further applied to a three-roll process with four-passes sequence. The final workpiece geometry predicted based on the model developed was found to be in good agreement with experimental and FE simulation results. Thus, the analytical model developed in the current approach can be effectively used in the design of multi-pass three-roll rolling processes.     

Reliability of electrode wear compensation based on material removal per discharge in micro EDM milling

This paper investigates the reliability of workpiece material removal per discharge (MRD) estimation for application in electrode wear compensation based on workpiece material removal. An experimental investigation involving discharge counting and automatic on the machine measurement of removed material volume was carried out in a range of process parameters settings from fine finishing to roughing. MRD showed a decreasing trend with the progress of the machining operation, reaching stabilization after a number of machined layers. Using the information on MRD and discharge counting, a material removal simulation tool was developed and validated.     

Effects of cutting conditions on dynamic cutting factor and process damping in milling

This paper investigates how cutting conditions affect dynamic cutting factor and system process damping in a dynamic milling process. By considering variation of edge plowing force, a frequency domain method is presented to identify the dynamic cutting factor through measured vibration in a milling process, and cutting conditions most suitable for the identification experiments are also discussed. A series of experiments are carried out to investigate the effects of cutting conditions on the dynamic cutting factor. This factor is shown to be significantly affected by the cutting speed, but relatively independent of the feed per tooth and the radial depth of cut. An average process damping model is further constructed and shown to be effective in representing the time-varying damping function. The average process damping is shown to increase rapidly at lower cutting speed, but remain constant as the cutting speed beyond a critical value.     

Advanced cutting conditions for the milling of aeronautical alloys

This paper deals with possible improvement aspects on the chip cutting milling of two alloys that are used frequently in the aerospace industry, in particular the titanium alpha–beta-based alloy Ti6Al4V and the nickel alloy usually known as type 718. Both alloys are used widely in the manufacture of different turbo-engine parts, considering their excellent mechanic features, and their resistance to high temperatures. These alloys, however, are extremely difficult to be milled, due to different factors, which are analysed later in this paper. For of this reason, their milling, drilling, and turning are carried out at very low speeds and feeds. This paper studies the tool influence as to its geometry and coating, and as to the parameters of the process (i.e. cutting speed, tooth feed, and depth of radial cut), looking for an increase in the productivity of the milling process. The cutting conditions thus searched for are successful in increasing the efficiency in the milling of actual parts in this field.     

An expert troubleshooting system for the milling process

Common problems experienced in milling processes include forced and chatter vibrations, tolerance violations, chipping and premature wear of the tools. This paper presents an expert system which attempts to troubleshoot the source of milling problems by utilising dynamics data coupled with the opinion of the operator and acoustic Fourier spectrum data taken from the cutting process. The expert system utilises a fuzzy logic based process to interpret the signals and information, and recommends possible alterations to the process to achieve high-performance milling operations.Specific inference engines were developed to assess the chatter stability, variation in cutting force coefficient, tool run-out and forced vibration characteristics of the system. Lastly, a stability lobe plot interpretation engine to automate the lobe selection process and recommend new, chatter free cutting conditions, was also developed. The chatter stability inference engine was tested with real cutting data, through acoustic measurements taken from various cutting conditions on an aluminium milling process. The chatter inference engine successfully determined the stability of the system for each sampled cutting condition. The robustness of the troubleshooting system depends on the accuracy of acoustic and frequency response measurements.     

Heating and material removal process in hybrid laser-waterjet ablation of silicon substrates

A hybrid laser-waterjet micromachining technology has recently been developed for near damage-free micro-ablation. It uses a laser to heat and soften the target material and a waterjet to expel the laser-softened elemental material to decrease thermal damages and increase the material removal. A computational model for the hybrid laser-waterjet micro-grooving process for single crystalline silicon is presented in this paper using an enthalpy-based finite difference method. Laser heating and waterjet cooling and expelling with the temperature-dependent silicon properties are considered in the model to predict the temperature profiles of silicon and groove characteristics under different machining conditions. The simulation results show that the introduction of a high pressure waterjet enables to remove material at its soft-solid status much below its melting temperature, while the waterjet cooling effect can reduce the workpiece temperature during the laser non-pulse period and eliminate the effect of heat accumulation, so that the thermal damage induced by laser heating is minimized. The temperature field model is also used to predict the groove depth and profile, and it is found that the model can reasonably represent the machined groove characteristics when comparing to the experimental data.     

Modeling of material removal and surface roughness in abrasive flow machining process

Abrasive flow machining process provides a high level of surface finish and close tolerances with an economically acceptable rate of surface generation for a wide range of industrial components. This paper deals with the theoretical investigations into the mechanism of abrasive flow machining (AFM) process. A finite element model is developed for the flow of media during AFM and the same is used to evaluate the stresses and forces developed during the process. Theoretical analysis to estimate the material removal and surface roughness obtained during AFM is also proposed. The theoretical results are compared with the experimental data available in the literature, and they are found to agree well.     

A wafer-scale material removal rate profile model for copper chemical mechanical planarization

Chemical mechanical polishing (CMP) models based on the Preston equation, which states that the material removal rate (MRR) is proportional to the product of the pressure and relative velocity, have focused on representing the average MRR as a function of the pressure and relative velocity. In this study, we tried to establish a semi-empirical CMP model, which can provide the MRR profile. The model is based on a modified form of the Preston equation and involves the use of a spatial parameter (Ω). The relative velocity distribution, normal contact stress distribution, and chemical reaction rate distribution are considered for obtaining the MRR profile in the copper CMP process. The results of the modeling and experimental analysis performed in this study facilitate process optimization and provide information that can contribute to the development of a wafer-scale CMP simulator.     

Studies on the material removal rate of two-phase eutectic alloys by EDM process

Two types of permanent mould materials, spheroidal graphite (SG) cast iron and Al-Si alloy with various compositions, were selected to study the effect of heterogeneous second phase on material removal rate (MRR) by the electro-discharge machining (EDM) process. Fe-Si alloy and Al-1wt%Si alloy with a mainly single-phase structure were also prepared for comparison.

Experimental results indicated that the amount and morphology of second phase particles significantly influence the material removal rate (MMR). This is closely related to ridge density and discharge density during the EDM process.

The EDMed surface of the specimens had a continuously ridged appearance and the ridge density increases with higher amount of second phase. Worthy of notice is that the graphite particles are embedded in the troughs on the EDMed surface of SG cast iron, while the eutectic silicon particles are located on the ridge region in the case of Al-Si alloys.     

固结磨料抛光垫作用下的材料去除速率模型

在对研磨抛光过程作出适当简化的基础上，推导出了固结磨料研具研磨抛光工件的去除速率模型，并进行了数值模拟。结果表明：固结磨料研磨加工时的去除速率不仅与工件的材质有关，还与固结磨料研磨盘的结构与加工参数相关；去除速率与相对速度V成正比，与压力的3／2次方成正比，与磨料直径成反比，并随着凸起间距的增加而下降。