Showing papers in "Journal of Materials Processing Technology in 1997"

Abstract: Although there have been great advances in the development of cutting tool materials which have significantly improved the machinability of a large number of metallic materials, including cast irons, steels and some high temperature alloys such as nickel-based alloys, no equivalent development has been made for cutting titanium alloys due primarily to their peculiar characteristics. This paper reviews the main problems associated with the machining of titanium as well as tool wear and the mechanisms responsible for tool failure. It was found that the straight tungsten carbide (WC/Co) cutting tools continue to maintain their superiority in almost all machining processes of titanium alloys, whilst CVD coated carbides and ceramics have not replaced cemented carbides due to their reactivity with titanium and their relatively low fracture toughness as well as the poor thermal conductivity of most ceramics. This paper also discusses special machining methods, such as rotary cutting and the use of ledge tools, which have shown some success in the machining of titanium alloys.

Abstract: Inconel 718, a high strength, thermal resistant Nickel-based alloy, is mainly used in the aircraft industries. Due to the extreme toughness and work hardening characteristics of the alloy, the problem of machining Inconel 718 is one of ever-increasing magnitude, This paper discusses the effect of cutting conditions on the machinability of Inconel 718. Flank wear of the inserts, workpiece surface roughness and cutting forces will act as the performance indicators for tool life while machining is carried out using a CNC lathe. Two types of coated cemented carbide inserts, grades EH20Z-UP (TiN coated by physical vapour deposition) and AC25 (TiN coated by chemical vapour deposition), were used. Various combinations of side cutting edge angles (SCEAs), cutting speeds and feedrates were tested at a constant depth of cut. Cutting results indicate that SCEA, together with cutting speed and feedrate, do play a significant role in determining the tool life of an insert when machining Inconel 718.

Abstract: Fiber Metal Laminates (FML) consist of thin, high strength aluminium alloy sheets alternately bonded to plies of fiber-reinforced epoxy adhesive. They provide an ideal combination of metals and composites that results in a material, which combines the best features of organic matrix composites and metals, without sharing their individual disadvantages. FML offer substantial weight savings relative to current metallic structures. Further, the number of parts required to build a component may be dramatically less than the number of parts needed to construct the same component of metal alloy. This can lead to labour savings, sometimes offsetting the higher price of the present materials. These features, together with superior fatigue behaviour, damage tolerant properties, inherent resistance to corrosion, good fire resistance for safety improvement, make FML very attractive candidate materials for future aircraft structures [1–3]. Later a new concept apply on this hybrid material: Fiber-Metal Laminates with Splice or Spliced Laminates. The development of spliced laminates has been a logical step after the identification of the favorable behavior of FML. Spliced laminates may provide a good solution obtaining substantially increased dimensions of spliced products. The splicing concept offers the same benefit (20 – 50% weight savings) as for a regular FML panel, but for much wider panels (>4 meters). This increased width capability can result in a significant reduction in manufacturing cost. These attributes make spliced laminates promising candidates for fuselage and lower wing materials for the next generation of Very Large Civil Transport (VLCT) aircraft and the Ultra High Capacity Aircraft (UHCA) for 600 to 800 passengers [4].

Abstract: Composites based on two aluminium alloys (A536 and 6061) reinforced with 10% or 20% volume fraction of SiC particles were produced by gravity casting and a novel two-step mixing method was applied successfully to improve the wettability and distribution of the particles. The SiC particles were observed to be located predominantly in the interdendritic regions, and a thermal lag model is proposed to explain the concentration of particles. It was found that the SiC particles acted as substrates for heterogeneous nucleation of Si crystals in one of the cast composites. This observation can also be explained by the thermal lag model proposed.

Abstract: The hydroforming technology has gained in importance over the last few years. Today the lightweight construction of automobiles is one of the main fields of application. This paper gives an overview of the fundamental principles of hydroforming processes and their variants. The correlations between the workpiece geometry and the design of tool and process and the forming result are exemplarily illustrated.

Abstract: This paper discusses the development of surface roughness prediction models for turning EN 24T steel (290 BHN) utilising response surface methodology. A factorial design technique has been used to study the effects of the main cutting parameters such as cutting speed, feed, and depth of cut, on surface roughness. The tests have been carried out using uncoated carbide inserts without any cutting fluid. A first-order prediction model within the speed range of 36–117 m min −1 and a second-order model covering the speed range of 28–150 m min −1 have been presented. The results reveal that response surface methodology combined with factorial design of experiments is a better alternative to the traditional one-variable-at-a-time approach for studying the effects of cutting variables on responses such as surface roughness and tool life. This significantly reduces the total number of experiments required.

Abstract: A suitable selection of machining parameters for the wire electrical discharge machining (WEDM) process relies heavily on the operators' technologies and experience because of their numerous and diverse range. Machining-parameters tables provided by the machine-tool builder can not meet the operators' requirements, since for an arbitrary desired roughness of the machining surface, they do not provide the optimal machining conditions. An approach to determine parameters setting is proposed. Based on the Taguchi quality design method and the analysis of variance, the significant factors affecting the machining performance such as metal removal rate, gap width, surface roughness, sparking frequency, average gap voltage and normal ratio (ratio of normal sparks to total sparks) are determined. By means of regression analysis, mathematical models relating the machining performance and various machining parameters are established. Based on the mathematical models developed, an objective function under the multi-constraint conditions is formulated. The optimization problem is solved by the feasible direction method, and the optimal machining parameters are obtained. Experimental results demonstrate that the machining models are appropriate and the derived machining parameters satisfy the real requirements in practice.

Abstract: This overview begins by considering the mature situation for polymer matrix composites (PMCs). After a further short section devoted to ceramic matrix composites (CMCs), the main text is devoted to the variety of routes available for processing metallic matrix composites (MMCs). These are divided into those where the main steps are performed in the solid state and those where the process route involves a stage where the matrix is molten. Some discussion is also given to the thermomechanical processing of MMCs and of their properties.

Abstract: Copyright (c) 1997 Elsevier Science S.A. All rights reserved. The term 'High Speed Machining' has been used for many years to describe end milling with small diameter tools at high rotational speeds, typically 10000-100000 rpm. The process was first applied in the aerospace industry for the machining of light alloys, notably aluminium. In recent years, however, the mould and die industry has begun to use the technology for the production of components, including those manufactured from hardened tool steels. This has only been made possible by advances in machine tools, cutting tools and CAD/CAM systems. Promising results have been demonstrated when milling a range of hardened tool steels using machining centres equipped with high speed spindles and cutting tools manufactured from cemented tungsten carbide, cermet, conventional ceramics and polycrystalline cubic boron nitride. The paper reviews HSM machinability work over the last decade and includes tool life, workpiece surface finish/dimensional accuracy and cost data. Sample components are illustrated and machining parameters correlated against workpiece hardness. © 1997 Elsevier Science S.A.

Abstract: Wire electrical discharge machining (WEDM) technology has been used widely in production, aerospace/aircraft, medical and virtually all areas of conductive material machining. Its complexity and stochastic nature have stimulated numerous attempts to model it accurately. This paper presents an attempt at modeling the process through Response Surface Methodology and Artificial Neural Networks. A response surface model based on a central composite rotatable experimental design, and a 4-16-3 size back-propagation neural network have been developed. The pulse-width, the time between two pulses, the wire mechanical tension and the injection set-point are selected as the factors (input parameters), whilst the cutting speed, the surface roughness and the surface waviness are the responses (output parameters). The two models are compared for goodness of fit. Verification experiments have been carried out to check the validity of the developed models. It is concluded that both models provide accurate results for the process.

Abstract: Simple ball- and roller-burnishing tools were used for the experimental work of the present study, these tools being quite similar in their design principles. The performance of the tools, together with the effects of the burnishing force and the number of burnishing tool passes on the surface roughness and surface hardness of commercially available aluminum and brass, were studied. The results show that improvements in the surface roughness and increases in the surface hardness were achieved by the application of both ball burnishing and roller burnishing with the non-ferrous metals under consideration.

Abstract: The effects of current setting and pulse-on time on the fatigue lives of AISI D6 tool steel specimens produced using three-dimensional cavity machining are investigated. Optical and scanning electron microscopy, microprobe analysis and microhardness tests have been used to study the characteristics of the machined and fractured specimens. The results have confirmed the deteriorating effect of electrodischarge machining (EDM) on fatigue lives. The discharge energy has been found to affect the fatigue lives as well as the thickness, the uniformity and the solidification cracking of the re-cast layer. Surface imperfections resulting from the EDM process lead to multiple fatigue-crack initiation and subsequent propagation into the substrate material. When sufficient cracks reach a critical length, fast fracture of the remaining cross-section takes place. The chemical compositions of the re-cast layers have been found to have slightly greater carbon content than that of the steel substrate and their hardness values have proven to be greater.

Abstract: To overcome some of the inherent problems that are associated with conventional processes, several in situ methods have been developed to obtain metal/ceramic composite materials. In this paper recently developed processes have been reviewed with a greater emphasis on directed melt oxidation and nitridation. These methods offer microstructures with varying metal/ceramic combinations. In some processes, metal matrix composites dispersed with controlled quantities of ceramic phase(s) are possible whereas in other ceramic composites with isolated/interconnected metallic dispersions are obtained. They could be varied by controlling the processing variables, i.e. temperature, time, reactant phases and filler materials. The properties of in situ processed materials primarily depend on the matrix and the volume fraction of the constituent phase. The mechanical properties, toughening mechanisms and potential applications are briefly reviewed.

Abstract: In the laser drilling process, the quality of the drilled holes is the main task. A method of studying the influence on the quality of the main process variables needs to be developed, which seeks to improve the quality and explains the drilling mechanism. In the present study, the effect of the laser parameters and the material properties on the hole quality when drilling is examined. A statistical approach, referred to as factorial design, is employed to test the significance level of the factors that affect the hole quality. Three materials, stainless steel, nickel and titanium, are considered. The experimental study yields tables of significance of each factor on the aspects that determine the quality of the holes. The hole geometry is evaluated by assigning marks for each geometric feature, the marking scheme being conducted relevant to the importance of the hole feature.

Abstract: This paper presents a study of the development of mathematical models for tool life in end milling steel (190 BHN) using high-speed steel slot drills under dry conditions The predictive models for tool life have been developed in terms of primary machining variables such as cutting speed, feed and axial depth of cut by response surface methodology A first-order equation covering a narrow range of the variables and a second-order equation covering a wide range of the variables are presented Response surface contours were constructed in speed-feed planes and used for determining the optimum cutting conditions for a required to life The adequacy of the predictive models was tested by analysis of variance and found to be adequate

Abstract: Distribution of particle reinforcements in cast composites is determined by the morphology of the solidification front. Interestingly, during solidification, the morphology of the interface is intrinsically affected by the presence of dispersed reinforcements. Thus the dispersoid distribution and length scale of matrix microstructure is a result of the interplay between these two. A proper combination of material and process parameters can be used to obtain composites with tailored microstructures. This requires the generation of a broad data base and optimization of the complete solidification process. The length scale of soldification microtructure has a large influence on the mechanical properties of the composites. This presentation addresses the concept of a particle distribution map which can help in predicting particle distribution under different solidification conditions Future research directions have also been indicated.

Abstract: Electrochemical discharge machining is a very recent technique in the field of non-conventional machining to machine electrically non-conducting materials using the electrochemical discharge (ECD) phenomenon. If a beyond-critical voltage is applied to an electrochemical cell, discharge initiates between one tool of the electrodes and the surrounding electrolyte, which is termed here ‘electrochemical discharge’. In the present work a simplified model is developed to predict the characteristics of the material removal rate for varying input parameters with the objective of finding the possibility of enhancing the capability of the process. The ECD phenomenon has been analysed as a switching process between the tool (one of the electrodes) and the electrolyte and it has been found that an extra control parameter can be obtained by introducing an additional inductance in the circuit. The theoretical and experimental results indicate that a substantial increase in the material removal rate can be achieved due to the additional inductance introduced.

Abstract: Precise predictions of springback after bending and bendability (the minimum bending radius) are the key to the design of the bending tool, to control of the bending process and to the assessing of the accuracy of part geometry. The effects of the normal anisotropic value R and the strain hardening exponent n on the pure bending of sheet metal have been studied. The highlight of this paper is that a simple approach, incorporating the normal anisotropic value R and the strain hardening exponent n, is developed to estimate springback, bendability and the maximum bending moment in pure bending. Comparison between predicted values and published experimental results has been made, a consistent agreement being achieved, reflecting the reliability of the present model. It is concluded that (i) the springback is almost proportional to the normal anisotropic value R, (ii) it decreases sharply with respect to smaller strain-hardening n-values or smaller thickness ratio t 2ϱ - values , and (iii) at large strain-hardening n-values or large thickness ratio t 2ϱ - values , the springback will concentrate to a small range. The minimum bending radius is proportional to the sheet thickness t, decreases with the normal anisotropic value R, and decreases sharply with the strain hardening exponent n to a small range. The maximum bending moment increases with the normal anisotropic value R; increases sharply with the sheet thickness t but decreases with the strain hardening exponent n.

Abstract: The relationship between the machinability and the brittleness of glass-ceramic materials is investigated. A brittleness index (B), given by the ratio of the hardness to the fracture toughness, is proposed as a parameter for estimating the machinability. This approach is confirmed by considering experimental data from the literature on turning operations of mica-containing glass-ceramics. It is shown that machinability parameters, such as the slope of the log-log plot of the specific cutting energy versus the cutting rate, or the specific cutting energy at low cutting rates, are in good agreement with the brittleness indices for seven different glass-ceramics. In order for a glass-ceramic to be machinable, it was found that the brittleness index of the material should be lower than B ≈ 4.3 μm−12.

Abstract: This paper studies the roles of two key factors in the mechanical alloying process, these factors being: activation energy, which is related to the formation of defects during the collision of powder particles; and crystalline size, which is related to the formation of nanometer crystalline during mechanical alloying. According to thermodynamic theory, the decrease in activation energy can result in an increase in diffusivity at constant temperature. Therefore, a decrease in activation energy is equivalent to an increase in temperature. High diffusivity can be obtained by creating a large number of defects through mechanical alloying. In addition, by creating nanometer size crystalline particles through the repeated fracturing and cold-welding of the powder particles, diffusion can take place easily through the grain boundaries. Consequently, elements which are difficult to diffuse may be alloyed using this technique.

Abstract: Metal matrix composite (MMC) materials have increasingly widened their use due to the merits of possessing high specific strength and modulus of elasticity while carrying good deformability and conductivity comparable to metals in addition to it's use structural and functional components for high-performance applications, such as aerospace vehicles and racing automobiles, MMC also has potential for molds. Especially for the large and mid-size molds in precision manufacturing, where handling is difficult due to the heavy weight, MMC can improve the productivity by saving the cost of loading, positioning and stocking. MMC is difficult to machine due to serious tool wear caused by the hard reinforcement. To exploit the potential industrial applications and investigate proper manufacturing processes, the machinability of electrical discharge machining (EDM) of MMC needs to be studied for reliable and economical production. The fundamental analysis starts from the material removal of MMC by a single spark. This paper presents the correlation between the major machining parameters, electrical current and on-time, and the crater size produced by a single spark for the representative material SiC/Al. The experimental results not only show the predicted proportionality based on heat conduction model, but are also compared with common steels regarding the material removal rate. Though the crater size of SiC/Al is larger than steel, the SiC particles can interfere the discharges. For effective EDM, large electrical current and short on-time are recommended. Based on the obtained knowledge, one can proceed to the study of machinability of MMC by EDM for optimal production cycle.

Abstract: In this paper, a feed-forward neural network is used to on-line monitor electrical discharge machining (EDM) processes. The relationships between tool-workpiece gap signals and various pulse types are established, based on the neural network through the back-propagation learning algorithm. As a result, the developed neural network can be used to monitor varying machining conditions in EDM. Experimental results have shown the effectiveness of this approach.

Abstract: The subject of improving the formability and fracture resistance of coated and laminated sheets, and sandwiches during forming and in service has received significant attention because these properties constitute an important design criterion for these composite materials. This paper reviews the mechanical and functional properties of these composite materials. A special emphasis is placed on the study of their forming and failure behaviour. Major failure mechanisms are identified in the laminated sheets during forming and in sandwiched sheets when subjected to static and dynamic loading. Shape fixability of coated and laminated sheets after forming are also discussed in terms of the surface friction and shear deformation in the core layer. Conclusions and generalisations which can be drawn from the literature and from the authors' previous experience are presented, with discussions of areas in which further research is required.

Abstract: This paper presents theoretical predictions of the effect of current, electrode polarity, electrode diameter and electrode extension on the melting rate, bead height, bead width and weld penetration, in submerged-arc welding.

Abstract: The traditional design methods of assembly tolerance allocation are usually based on engineers' experience, or the worst on worst tolerance analysis (WOW) method, or the root sum square tolerance analysis (RSS) method. However, the above-mentioned methods, whilst used frequently in the analysis of a single-dimensional chain, are not suitable for the analysis of geometrical tolerance and multi-dimensional chains. Also, the relationship between tolerance and manufacturing cost is not considered and a suitable tolerance allocation based on minimum manufacturing cost can not be obtained. Some research works have applied linear or non-linear programming methods to optimize the tolerance allocation of each part in an assembly. However, the convergence of the solution is not ensured. The purpose of this study is to provide an integrated approach, including tolerance design, manufacturing cost analysis and multiple chains consideration, using the Monte Carlo method to optimize the tolerance allocation with minimum cost. The Monte Carlo method, a statistical simulation method, was used to simulate the dimension variance of each part and each dimensional chain. The contribution percentage of each part on each dimensional chain was calculated. Tolerance cost was chosen as an object function and the tolerance allocation model as a constraint condition, the optimum tolerances of each part being obtained by the iteration method. Results of computer simulation for several examples were compared with published data for demonstrating the feasibility of the proposed method. It can be concluded that the tolerance-allocation model combined with a tolerance-cost relationship can provide a very practical and useful approach for design engineers.

Abstract: The present study confirms the indispensable stabilizing role of gap debris in precision electrodischarge machining (EDM). It is well known that failure to evacuate surfeited debris in a spark gap results in arcing that damages the tool electrode as well as the work. A machining process in pure kerosene is very unstable due to arcing. The latter cause for arcing is frequently observed, but is poorly understood. It is a well-known fact that the presence of minute particles in insulating liquids drastically lowers breakdown strength. A stable process without arcing depends on discharge transitivity rather than on the ease of breakdown. As a result, an easy breakdown process with the help of the foreign particles does not show the entire stabilizing contribution of debris. Our experimental and analytical investigations on general precision EDM reveal that discharge transitivity in gap space relies on the presence of a sedimentary debris layer on the work's surface. Actually, the inherent transitivity due to surface irregularity will disappear because of the very low surface roughness ( R a μ m). Spark movement should be attributed to effects from the mechanical impact of discharge pulses, the distribution of breakdown strength, and the distribution of electrical field. We can derive a process model regarding the tendency of spark movement to expound the functions of debris content and gap size. Pertinent new understanding will eventually give an explanation of the complicated role of gap debris. A process model will lead to a more relevant conception of gap size and its control than most currently accepted understanding.

Abstract: A mechanical model for ring rolling is established, the extremum parameters in ring rolling being derived based on the model, and predicted results compared with experimental results.

Abstract: Reported in the present paper are the requirements of the materials used for WEDM electrodes that will lead to the improvement of WEDM performance. Experiments have been conducted regarding the choice of suitable wire electrode materials and the influence of the properties of these materials on the machinability in WEDM, the experimental results are presented and discussed.

Abstract: An investigation has been made into the combined technology of ultrasonic and wire electrical discharge machining (W-EDM), in which the design of the vibrating device is described. The theory to describe the vibration modes of the wire under the action of ultrasonic has been established. Experimental results show that wire vibration induced by ultrasonic action has a significant effect on the overall performance of the W-EDM process. It was found that there exists an optimum relationship between the vibration amplitude of the wire and the discharge energy, by which the highest cutting rate and the best machined surface quality can be obtained. In addition, ultrasonic vibration reduced the residual tensile stress of the machined surface.

Abstract: This paper presents the processing of two advanced ceramics using Wire-Cut Electrical Discharge Machining (EDM) which is evolving as one of the promising method for the processing advanced ceramics. Two types of advanced ceramics, Sialon and Al2O3 TiC, were machined successfully by the Wire-Cut EDM process. The machining performance in terms of material removal rate and surface finish were compared under different cutting conditions. The extent of the surface damage resulting from this thermal machining process were evaluated further by the flexural strength data obtained from three-point and four-point-quarter bend test methods. Moreover, the variability of these flexural strength data were analysed by the Weibull statistical method, which gave a Weibull modulus m ranging from 4.1 to 8.8 for Sialon, and 9.7 to 13.6 for Al2O3 TiC respectively. The results showed that the Wire-Cut EDM process is a viable material processing method for the machining of advanced ceramics, but work have to be carried out to further study the ways and means of improving the surace finish and surface integrity of the machined ceramics such as to give a more reliable service life of components made of advanced ceramics.