L.C. Lim

Near-mirror-finish phenomenon in EDM using powder-mixed dielectric

Abstract This paper presents a study of the near-mirror-finish phenomenon in electrical discharge machining (EDM) when fine powder is introduced into the dielectric fluid as a suspension at the tool–workpiece or inter-electrode gap during machining. For this study, the dielectric flushing system of a conventional die-sinking EDM machine was specially modified to inject and distribute the powder into the dielectric fluid, especially at the gap between the tool and the workpiece. Machining was performed on various types of steel with different types of powder suspensions at a peak current of around 1 A. Particular combinations of powder-mixed dielectric and workpiece have been found to produce mirror-finish or glossy machined surfaces. Close scrutiny of the mirror-finish surfaces reveals shallow overlapping re-solidified discs with smooth rims, unlike typical EDMed surfaces, which are typically covered with deep craters, pock marks and globules. The various factors affecting the generation of the mirror-like surfaces are discussed. The appropriate settings of electrode polarity and pulse parameters and the correct combination of workpiece material and powder characteristics have a significant influences on the mirror-finish condition. The use of negative electrode polarity (i.e. with the tool as the negative electrode, which is a condition normally used for finishing EDM) is necessary to achieve the mirror-finish condition. Other features of the powder-mixed dielectric EDM are shorter machining time, more uniform dispersion of the electrical discharges, and stable machining. Based on the results of the experimental investigation, the types of material composition, powder properties and machine setting in bringing about near-mirror conditions are discussed.

Laser cladding of nickel-based hardfacing alloys

Abstract The effects of powder feed rate (PFR) and translation speed (TS) of a laser beam were investigated on laser cladding with three nickel-based hardfacing alloys: Colmonoy 6, Colmonoy 88 and AI-1236. The width-to-height ratio of single clad passes and the extent of dilution were found to decrease with increasing PFR and decreasing TS. Microstructurally, Colmonoy 6 clad layers consisted of a nickel-based structure of primary γ-nickel dendrites and interdendritic eutectics of γ-nickel, nickel borides and nickel silicides, Colmonoy 88 of mixed carbides and AI-1236 of partially melted WC particles and mixed carbides embedded in similar nickel-based microstructures. Block-on-ring wear tests showed that the wear mechanism is largely abrasive in nature for all three alloys studied. The wear resistance was found to correlate better with the type and amount of the main hard phase present in the clad layer: AI-1236 clad layers were much superior to those of Colmonoy 6 and Colmonoy 88 clad layers owing to WC particles in the alloy.

Frequency response and scaling of hysteresis for ferroelectric Pr(Zr0.52Ti0.48)O3 thin films deposited by laser ablation

The ferroelectric hysteresis response against periodically varying electric field over frequency range of 10−2–105 Hz and amplitude range of 2–45 kV/cm for YBa2Cu3O7 (YBCO)/ Pb(Ti0.48Zr0.52)O3 (PZT)/YBCO thin film capacitors prepared by laser ablation is measured by utilizing the Sawyer–Tower circuit. Given amplitude Δ of the field, the hysteresis area 〈A〉 first grows and then decays as a function of frequency φ. At low and high ranges of frequency, 〈A〉 can be scaled as 〈A〉∝φ1/3Δ2/3 and 〈A〉∝φ−1/3Δ, respectively. It is established that the dynamic hysteresis at the high frequency range for a PZT thin film capacitor does not follow the theoretically predicted scaling law. An empirical scaling law 〈A〉∝φ1/3(Δ−Δ0)2/3/(1+bφ2/3Δ−1/3) with Δ0 the critical field and b a constant, is proposed to characterize the frequency and amplitude dependence of the hysteresis area over all the frequency range. In addition, the remnant polarization Pr and coercive field Ec as functions of φ are investigated, respectively, revea...

Microstructures of laser-clad nickel-based hardfacing alloys

Abstract Distinctive microstructures of laser-clad passes with three nickel-based hardfacing alloys were observed. The boride-hardened Colmonoy 6 consisted of a structure of γ-nickel dendrites and interdentritic eutectics of γ-nickel, nickel borides and nickel silicides; carbide-hardened Colmonoy 88 consisted of mixed carbides, while WC-strengthened AI-1236 consisted of partially dissolved WC particles embedded in a similar microstructure to Colmonoy 6. For a given laser beam intensity, the microstructures of the clad layers are significantly influenced by the powder feed rate (PFR) employed and, to a lesser extent, the translation speed (TS) of the laser beam. A cellular-to-dendritic transition for Colmonoy 6, a morphology change of the mixed carbide from coarse leaf-like to fine, equiaxed but angular shape for Colmonoy 88, and a lesser degree of dissolution of WC particles for AI-1236 occurred with increasing PFR. At sufficiently high TS, the nucleation and growth of the mixed carbide phases in the Colmonoy 88 layer can be suppressed, especially when the PFR is low. The effect of PFR and TS on the clad microstructure can be understood in terms of the specific energy input ( E S ) and the specific heat energy ( E h ) available, respectively.

Effects of flushing on electro-discharge machined surfaces

Abstract Although the influence of flushing on the efficiency and stability of machining conditions in EDM has been extensively investigated, including the effects of the flushing configuration on the wear of the tool and the profile of the workpiece, little has been reported concerning the effects of flushing on the integrity of the electro-discharge machined (EDMed) surfaces. This paper presents the effects of flushing rates on the types and distribution of recast layers in commercially pure iron, 0.5% C steel and AISI 01 tool steel after EDM. There exists an optimal dielectric flushing rate where cracks and average thickness of the recast layer are at a minimum for all three materials. The trends for the crack density and recast thickness are similar, being higher at flushing rates below and above a basically similar optimum rate. The distributions of the crack density at the sides, bottoms and corners of the machined cavities under different flushing conditions are presented. Based on the results obtained, the effects of the quenching property and debris removal ability of the dielectric flow conditions on the recast layers of the three types of EDMed specimens are discussed.

Towards a better understanding of the surface features of electro-discharge machined tool steels

Abstract The paper reports on an extensive experimentation with various etching reagents to reveal the types of micro-structure found on different tool steels under different machining conditions. The recast layer can broadly be classified into multi-layer, single layer with features and single-layer featureless types. Of greatest interest is the single-layer type. It was observed that for this type of recast, three sub-layers can be discerned. The top-most sub-layer is most prominent for AISI A2, D2 and D6 tool steels and consists largely of columnar grains projecting inwards from the surface-dielectric interface. The inner-most sub-layer consists mainly of cellular (for AISI 01 steel) or spike-like dendrites (for AISI A2, D6 and D2 steels). The intermediate sub-layer is most conspicuous in AISI 01 steel and is made up of complex interlocking dendrites. Preliminary results obtained from examining the effects of reducing the steel temperature gradient at the work-dielectric interface is also presented. The objective of this is to minimise the crack susceptibility of EDM surfaces.

Solidification phenomena in nickel base brazes containing boron and silicon

Abstract 1. 1. The intermetallic compounds formed in brazes of nickel base filler metals containing chromium, boron and silicon are nickel boride, nickel silicide and chromium boride. 2. 2. The intermetallic compounds are found to form as constituents of binary or ternary eutectics with γ-nickel. The binary eutectics are γ-nickel/nickel boride, γ-nickel/nickel silicide and γ-nickel/chromium boride. The ternary eutectics are γ-nickel/nickel boride /chromium boride and γ-nickel/nickel boride/nickel silicide. 3. 3. Upon cooling from the brazing temperature, the first formed solid is primary γ-nickel, which either nucleates within the melt as nodules or nodular dendrites or simply grows onto the partially melted filler metal particles. On further cooling, boron and silicon in the melt tend to exclude one another, solidifying into different types of eutectics the constituents of which depend on the composition of the filler metal. 4. 4. When only silicon is present, the remaining melt simply solidifies into an eutectic of γ-nickel and nickel silicide. 5. 5. For boron-containing filler metals, the first formed eutectic is usually the binary eutectic of γ-nickel and nickel boride; this in turn enriches the melt with chromium. In the absence of silicon, this chromium-enriched melt will eventually solidify into a ternary eutectic of γ-nickel, nickel boride and chromium boride. When silicon is present, the formation of the above ternary eutectic is suppressed and a binary eutectic of γ-nickel and chromium boride is formed instead. When this happens, the remaining melt will be enriched in silicon and the last portion of the melt solidifies into a ternary eutectic of γ-nickel, nickel boride and nickel silicide. 6. 6. For the filler metal containing nickel, boron and silicon but no chromium, after the formation of binary eutectic of γ-nickel and nickel boride, the melt will become gradually enriched with silicon and eventually solidify into the ternary eutectic of γ-nickel, nickel boride and nickel silicide.

Surface intergranular cracking in large strain fatigue

The formation of surface intergranular cracks has been investigated with a coarse-grained polycrystal of nickel, deformed in low-cycle fatigue at 573 K. The evolution of the cracks was followed as a function of fatigue life fractions, and the factors favoring their formation were identified. It was found that in air, surface intergranular cracking occurs early in fatigue life and is induced by the impinging slip traces at the interface. Grain boundaries other than coherent twin boundaries and those with Σ < 5 are susceptible to such cracking. Depending on the boundary plane orientation and on the geometry of the operative slip vectors relative to the specimen surface, the grain boundary cracks may or may not grow to any appreciable extent. Crack growth is accelerated if the boundary plane makes a large angle with the stress axis and if the differential out-of-surface component of the operative slip vectors in the adjoining grains is large. In vacuum, slip is dispersed and surface rumplings become effective in grain boundary crack nucleation. The evolution of surface intergranular cracks, however, is delayed as opposed to tests conducted in air. The results are interpreted in terms of the interaction between crystal dislocations and grain boundaries and on the state of stress at the grain boundaries.

Design and fabrication of a high performance multilayer piezoelectric actuator with bending deformation

A new bending mode multimorph actuator was designed and fabricated successfully by a multiple screen printing process. Unlike the conventional bimorph actuator in which the bend occurs in the thickness direction, the bend in the multimorph actuator occurs in the widthwise direction because of synchronistical deformation of each single monolithic layer in the multilayer structure. The theoretical analysis and experimental measurements were conducted to study the performance of this type of actuator, and a comparison was made with the conventional bimorph actuator. Larger displacement, higher resonance frequency, and much larger blocking force could be achieved with the multimorph actuator than with a bimorph actuator of similar dimensions. The multimorph actuator presented in this paper provides a valuable alternative for actuator applications beyond those available with the popular bimorph and longitudinal multilayer actuators.

Parametric studies of laser cladding processes

Abstract Laser cladding has gained increasing popularity both in industry and in academic research. This work describes an investigation of laser cladding of mild steel substrate with Colmonoy 88 nickel-based hardfacing powder. Emphasis is placed on the effect of the translation speed (TS) of the laser beam and the powder feed rate (PFR) on the physical microstructural qualities of the clad layer. The results on the profile of single clad pass, extent of dilution, surface hardness and the microstructure produced by different cladding conditions are presented and discussed, through which the optimum cladding parameters for maximum hardness of the clad material are identified.