Dave Kim

A study on the drilling of composite and titanium stacks

Abstract An experimental study on drilling of graphite/bismaleimide (Gr/Bi) titanium (Ti) stacks was conducted by using different cutter materials with a standard geometry to understand and characterize the process. The tool materials used were high-speed steel (HSS), high-speed cobalt (HSS-Co), and carbide. It was observed that at the interface of Gr/Bi–Ti, high temperatures induced material damage near and around the hole region. Dissimilar mechanical and thermal properties affected the tool life and allowed for increased matrix degradation and burr formation in Ti, regardless of the cutting tool material. As a result, fewer holes were produced when high spindle speeds and slow feeds were used. It was also found that carbide drills outperformed all other tools in terms of tool life, minimal surface damage, and heat induced damage on both workpiece materials.

Drilling process optimization for graphite/bismaleimide-titanium alloy stacks

Abstract In this study, the drilling process of graphite/bismaleimide-titanium alloy (Gr/Bi–Ti) stacks was optimized in terms of machined hole quality and machining cost. The drilling experiments were conducted by using two different cutter materials, HSS-Co and carbide. Drilled hole quality parameters include surface texture, titanium burrs, hole diameter, cylindricity, and roundness deviation. Machining cost was estimated through drill wear experimentations. A multiple objective linear program was used to optimize drilling feed and speed not only to maximize each hole quality parameter to the greatest extent possible but also to minimize machining cost. Optimum process conditions for achieving desired hole quality and process cost were found to be a combination of low feed and low speed when using carbide drills, and high feed and low speed in drilling with HSS-Co drills. The sensitivity of weighting factors on the feasible process conditions depends on the optimal point position and distribution of each objective.

Electrical Discharge Machining of Functionally Graded 15–35 Vol% SiCp/Al Composites

A functionally Graded 15–35 volume% silicon carbide particulate (SiCp) reinforced Al359 metal matrix composite (SiCp/Al MMC) was drilled by electrical discharge machining (EDM) to assess the machinability and workpiece quality. The machining conditions were identified for both the machining performance and workpiece quality of the EDM process, including some aspects of material removal mechanisms, material removal rate (MRR), electrode tool wear, and subsequent drilled hole quality including surface texture and roundness by using surface profilometry, coordinate measuring machine (CMM), and scanning electron microscopy (SEM). It was observed that the material removal rate increases with increasing peak current and pulse-on-time up to the optimal points and drops drastically thereafter. Higher peak current and/or pulse-on-time result in both the greater tool wear and the larger average diameter error. As the percentage of the SiC particles increases, MRR was increased and electrode wear was found to be decreased. At the EDM machined subsurface layer, the fragmented and melted SiC particles were observed under the SEM and EDX-ray examination.

Study on the Drilling of Titanium/Graphite Hybrid Composites

Titanium/graphite hybrid composites (TiGr) are a potentially enabling technology which satisfies the low structural weight fraction and long operational lifetime required for the high-speed civil transport. TiGr composites are made of thermoplastic polymer matrix composite plies with titanium foils as the outer plies. The two materials are assembled by bonding the polymer matrix composite plies and titanium foils to form a hybrid composite laminate. Both experimental and analytical work has been performed to characterize major hole quality parameters and cutting mechanisms encountered in drilling of TiGr composites. The effects of consolidation processing, such as induction heating press and autoclave processes, on drilling characteristics of TiGr composites were examined. The hole quality parameters and hole exit damage was investigated and discussed.

Influence of Consolidation Process on the Drilling Performance and Machinability of PIXA-M and PEEK Thermoplastic Composites

In this paper, the drilling performance of two composite systems, namely graphite-PIXA-M (PIXA-M) and graphite-PEEK (PEEK) induction heat-pressed thermoplastic composites, are compared with their counterpart from autoclave processing. Machinability is evaluated in terms of drilling forces and hole quality. Hole quality characteristics are evaluated in terms of geometric accuracy and surface finish. The experimental observations discuss unique chip characteristics during drilling both autoclaved and induction heat-pressed thermoplastic composites. The mechanism of material removal in drilling of thermoplastic composites is investigated through the further calculation of the specific cutting energy. The induction-processed composite material produced equivalent or better holes than the autoclave-processed composites although it required larger cutting force magnitudes and protruded fibers on the bottom ply are produced. The induction heat press may be a viable alternative for the processing of thermoplastic composites in terms of drilling performance.

Frequency analysis and characterization in orthogonal cutting of glass fiber reinforced composites

Abstract This study discusses frequency analysis based on frequency spectrum and autoregressive (AR) time series model, and process characterization in orthogonal cutting of fiber–matrix composite materials. A sparsely distributed idealized composite material, namely a glass fiber reinforced polyester (GFRP) was used as the workpiece. The analysis method employs a force sensor and the signals from the sensor are processed using either the fast Fourier transform (FFT) technique or AR time series model. Signal distortion measure based on discrimination information is also introduced. The experimental correlations between the different chip formation mechanisms and power spectrum or AR model coefficients are then established. In particular, only those features that are most sensitive to different types of cutting mechanisms are selected by feature extraction method in AR modeling. Selected features are used to characterize the chip formation. Discrimination information measure proves to be useful in signal analysis when any characteristic of the cutting process is apparent in the form of spectral peaks. Effects of fiber orientation, cutting parameters and tool geometry on the cutting mechanisms and surface quality are also discussed.

Densification and Properties of 420 Stainless Steel Produced by Three-Dimensional Printing With Addition of Si3N4 Powder

This study is aimed to investigate the effect of adding Si 3 N 4 on the three-dimensional printing (3DP) processed 420 stainless steel (SS). The final density, dimensional changes, and mechanical properties have been studied for the samples prepared under a series of sintering conditions. The contents of Si 3 N 4 powder ranging between 0 wt % and 15 wt % and the sintering temperature were varied in the experiments in order to understand the dependence of densification kinetics and changes in properties on these process parameters. The experiments provide the evidence that the addition of Si 3 N 4 can improve the densification kinetics of 420 SS significantly. The 420 SS samples produced by 3DP with 12.5 wt % Si 3 N 4 sintered at 1225°C for 6 h yielded 95% relative density, 190 GPa Youngs modulus, and 500 Knoop microhardness without any major sample shape distortion. The results are promising in that the 3DP process can be used to yield the prototype almost equivalent to a real part with a full mechanical capacity.

Machinability of titanium alloy (Ti’6Al’4V) by abrasive waterjets:

Abstract Titanium alloy is known as one of the difficult-to-machine materials using conventional machining processes, although it has superior formability. In the present study, a widely used aircraft structural titanium alloy (Ti—6A1—4V) was machined with an abrasive waterjet (AWJ) to investigate its machinability under varying cutting conditions. Machinability was evaluated in terms of kerf geometry, cut surface quality and microstructural integrity. Quality of the machined surface and microstructure features were examined using surface profilometry, scanning electron microscopy and energy dispersive X-ray (EDX) analysis. The surface roughness was ranged several micrometres near the jet entrance region and was observed to increase at the jet exit of the workpiece. Scanning electron microscopy (SEM) analysis of the surface microstructure revealed that the mechanism of material removal was a combination of scooping induced ductile shear and ploughing actions of the abrasive particles. EDX analysis showed garnet particle embedment in titanium throughout the cutting depth, and the particle size was estimated to be several tens of micrometres.

The Effects of Post-Weld Cold Working Processes on the Fatigue Strength of Low Carbon Steel Resistance Spot Welds

The investigation on the use of a post-weld cold working process to improve fatigue strength of low carbon steel resistance spot welds is presented. The cold working process generates uniform, and consistent large zones of compressive residual stresses in resistance spot-welded low carbon steel structures using a specially designed indentation device. The effect of the indentation process parameters on the mechanical properties of the resistance spot weld was investigated. Comparisons of the mechanical properties and qualitative results between the as-resistance spot-welded specimens and the post-weld cold worked resistance spot-welded specimens have beet made in this investigation. Fatigue testing was also conducted to evaluate the effect of post-weld cold working process on the fatigue characteristics of resistance spot welds. Results showed that a significant improvement in the fatigue strength has been achieved through the post-weld cold working process.

Usage of PCD tool in drilling of titanium/graphite hybrid composite laminate

Titanium/graphite (TiGr) hybrid laminates are made of thin sheets of titanium adhesively bonded with core thermoplastic polymer matrix composite (PMC) plies. The material system is a potentially enabling technology, which satisfies the low structural weight fraction and long operational lifetime required for high speed civilian transportation systems. In this paper, a polycrystalline diamond (PCD) drill is used to drill the TiGr hybrid composites. The drilling experiments have been performed to investigate and report on the drilling forces and drilled hole quality. Hole dimensional errors, burr generations, and hole defects were investigated. Drilling feed is the most important process parameter in hole size errors, which seems to be due to the high elasticity of thermoplastic material system. Exit burr heights were increasing with increasing both the speed and feed. Exit damage such as titanium foil cap formation and titanium foil tearing does not occur during drilling by the PCD tools.