Shiqi Fang

Pulse electrochemical machining (PECM) is an unconventional material removal process, widely applied to machine or shape difficult-to-machine materials. A new concept of reproducing three-dimensional (3D) microstructures has been developed and elaborated. Picosecond laser was utilized to produce concave or convex patterns on PECM cathodes, and pattern replication was carried out to PECM anodes. The laser achieved high machining precision and excellent surface integrity; both patterns were successfully replicated on the anodes. However, considerable deviations were found in the reproduced concave patterns on the anodes, possibly due to the rounding and widening effects.

Chip formation is considered one important indicator to evaluate machining processes. In particular, geometric features of chips may provide important insights for the assessment of machining stability and productivity. In fixed-abrasive machining processes, such as grinding and honing, chips are simultaneously produced by many of the geometrically undefined cutting edges. Despite being “undefined,” geometric features of abrasive grains can still be statistically characterized or described. Accordingly, it can be assumed that, under stable machining conditions, the geometric features of abrasive chips may also conform to certain statistical patterns. However, statistical characterization of abrasive chips can be very challenging due to their large quantity, irregular shapes, minuscule size, and sometimes tangled condition. In this study, an analysis method combining metallographic preparation and micro-computed tomography (micro-CT) has been developed to characterize abrasive chips produced by a honing stone. The results regarding the geometric characteristics of the obtained massive abrasive chips, i.e., their sizes and shapes, were presented and statistically described. It was found most chips had a size around 50 µm and the shape being a slightly elongated and curved cone. Most of the geometric features could meet a positive skewness distribution.

Emerging laser precision machining, particularly using pulsed lasers, enlightens the innovation and functionalization of cemented carbides. These backbone materials of the tooling industry are usually considered difficult to machine or shape using conventional mechanical approaches. The coating of cemented carbide tools, deemed to improve their mechanical and thermal properties, is a common supplementary surface treatment prior to their application. This work aims to study the synergic effects of nanosecond laser ablation and coating deposition on the surface, as well as the mechanical integrity of cemented carbides. In this regard, two plain WC–Co grades with different metallic binder content (10%_wtCo and 15%_wtCo) were first processed by a short-pulsed nanosecond laser. Subsequently, an AlTiN film was physically vapor-deposited on the laser-processed surfaces. The resulting surface integrity was assessed in terms of topographical, morphological, and microstructural changes. Mechanical integrity was evaluated in terms of indentation and sliding contact responses using Vickers hardness and scratch tests, respectively, the latter including frictional, penetrating, and sliding performances under selected surface processing conditions. In general, the nanosecond laser ablation proved to be beneficial for the mechanical integrity of coated cemented carbides in most studied cases, as it increased surface hardness, reduced penetration depth, and hindered damage during sliding. This was the case despite a slight increase in surface roughness, as well as minor morphological and microstructural changes at the coating–substrate interface, discerned.

Defined topographic features may be produced on the workpiece surface of difficult-to-machine materials through the pulse electrochemical machining (PECM). Due to the contactless feature of PECM, mechanical influences and their side effects become ignorable. In this study, a picosecond laser, which induces little thermal impact, was applied to micromachine the cathode of PECM with groove patterns, and replication of the patterns on the anode of PECM was carried out using an industrial PECM installation. The results of the case study show that the picosecond laser achieved high machining precision (< 6.5% to the target size) with excellent surface integrity on the cathode of PECM, and the groove patterns were successfully replicated on the anode of PECM, although deviations of certain geometric parameters were detected between the two PECM electrodes. These deviations may be resulted from the rounding and widening effects during the PECM processing.

Defined topographic features may be produced on the workpiece surface of difficult-to-machine materials through the pulse electrochemical machining (PECM). Due to the contactless feature of PECM, mechanical influences, and their side effects become ignorable. In this study, a picosecond laser, which induces little thermal impact, was applied to micromachine the cathode of PECM with groove patterns, and replication of the patterns on the anode of PECM was carried out using an industrial PECM installation. The results of the case study show that the picosecond laser achieved high machining precision (<6.5% to the target size) with excellent surface integrity on the cathode of PECM, and the groove patterns were successfully replicated on the anode of PECM, although deviations of certain geometric parameters were detected between the two PECM electrodes. These deviations may result from the rounding and widening effects during the PECM processing.

Cemented carbides, or hard metals, are ceramic–metal composites usually consisting of tungsten carbide particles bound by a cobalt-based alloy. They are the backbone materials for the tooling industry, as a direct consequence of the outstanding range of property combinations, depending on their effective microstructural assemblage, i.e., the physical dimensions and relative content of their constitutive phases. Hence, reliable microstructural characterization becomes key for hard metal grade selection and quality control. This work aimed to assess the practical two-dimensional characterization methods for the most important one- and two-phase properties of cemented carbides, i.e., the carbide grain size, phase fraction, carbide contiguity, and binder mean free path. Three different methods—point, line, and area analysis—were implemented to characterize four microstructurally distinct grades. The images were acquired by optical and scanning electron microscopy, with the latter through both secondary and backscattered electrons. Results were critically discussed by comparing the obtained values of properties and the different characterization methodology. Inspection technique combinations were finally ranked based on accuracy, accessibility, and operability considerations. The line method was used to analyze all the properties, the area method, for the one-phase properties, and the point method, for only the phase fraction. It was found that the combination of optical microscopy and the line analysis method was suitable for a direct inspection and rapid estimation for carbides above fine grain size. The most precise results were achieved using line analysis of the images obtained by the backscattered electrons of the scanning electron microscope.

Chip formation is an important indicator of machining processes. Statistical characterization of machining chips’ geometric features can offer crucial information for evaluating the stability and productivity of the machining processes. In abrasive machining processes, an abundance of small chips are produced by the vast number of abrasives exposed to the cutting surfaces. Geometric features of abrasives, such as shape, dimension, and distribution, may be hierarchically passed on to the chips. Similar to those of the abrasives, geometric features of the chips may also vary to a certain extent and conform to some statistical distribution. To verify these characteristics, a metallographic preparation method in connection with chips formed in abrasive machining processes is proposed in this study. Challenges in collecting and segmenting chips have been successfully overcome through several steps using ultrasonic bath cleaning and powder cold embedding methods. Finally, a considerable amount of chips was formed and uniformly embedded in a resin mold, ready for microscopic characterization.