Yusuf Usta

Micro-manufacturing of micro-scale porous surface structures for enhanced heat transfer applications: an experimental process optimization study

Integrated and compact products necessitate the use of advanced thermal management systems with reduced footprint and cost as well as increased efficiency. Micro-scale, porous and modulated (i.e. channels, pyramids, etc) surfaces offer increased surface area for a given volume and lead to two-phase heat transfer conditions with efficiency enhancements up to 300%. Such surfaces made of copper powders were demonstrated to be quite effective by several researchers after they were produced in controlled lab environments. Similar surfaces made of high temperature resistant materials such as stainless steel, nickel and titanium can also be used in fuel processor, SOFC and PEM fuel cell applications as bipolar/interconnect plates. However, their fabrication under mass-production conditions for marketable and cost-effective products requires well-established process parameters. In this study, warm compaction of copper powders onto thin copper solid substrates was experimented with under different compaction pressure (15–50 MPa), temperature (350–500 °C) and surface geometry (flat, large and small channeled) parameters using a design of experiment (DOE) approach to determine the proper process conditions. Porosity and bonding strength of compacted samples were measured to characterize their feasibility for compact and/or micro-scale heat/mass transfer applications. Results showed that a minimum 350 °C temperature and 15 MPa pressure level is necessary to obtain sound porous and micro-channeled surface layers. It was also found that at higher pressure levels (50 MPa), fabrication of micro-scale surface structures is highly repeatable with enhanced bonding strength characteristics. DOE findings will be used to establish proper process conditions to produce such porous surfaces using a continuous roll compaction process in the future.

Pullout performance comparison of novel expandable pedicle screw with expandable poly-ether-ether-ketone shells and cement-augmented pedicle screws

Aim of this study is to assess the pullout performance of various pedicle screws in different test materials. Polyurethane foams (Grade 10 and Grade 40) produced in laboratory and bovine vertebrae were instrumented with normal, cannulated (cemented), novel expandable and normal (cemented) pedicle screws. Test samples were prepared according to the ASTM F543 standard testing protocols and surgical guidelines. To examine the screw placement and cement distribution, anteriosuperior and oblique radiographs were taken from each sample after insertion process was completed. Pullout tests were performed in an Instron 3369 testing device. Load versus displacement graphs were recorded and the ultimate pullout force was defined as the maximum load (pullout strength) sustained before failure of screw. Student’s t-test was performed on each group whether the differences between pullout strength of pedicle screws were significant or not. While normal pedicle screws have the lowest pullout strength in all test materials, normal pedicle screws cemented with polymethylmethacrylate exhibit significantly higher pullout performance than others. For all test materials, there is a significant improvement in pullout strength of normal screws by augmentation. While novel expandable pedicle screws with expandable poly-ether-ether-ketone shells exhibited lower pullout performance than normal screws cemented with polymethylmethacrylate, their pullout performances in all groups were higher than the ones of normal and cannulated pedicle screws. For all test materials, although cannulated pedicle screws exhibit higher pullout strength than normal pedicle screws, there are no significant differences between the two groups. The novel expandable pedicle screws with expandable poly-ether-ether-ketone shells may be used instead of normal and cannulated pedicle screws cemented with polymethylmethacrylate due to their good performances.

Experimental Investigations on Development of Rapid Die Wear Tests for Stamping of Advanced High Strength Steels

In a quest to achieve low-mass vehicles (i.e., higher fuel economy and lower emission), the automotive industry has been actively investigating the use of lightweight materials for a wide range of body panels and structural parts. Among the lightweight materials considered, Advanced and Ultra High Strength Steels (A/UHSS) hold promise as a prominent choice for the near future due to their relatively high formability and low cost compared to Aluminum and Magnesium alloys. However, due to their significantly higher strength than mild steel, in addition to the springback, blanking and joining issues, serious problems with the die wear are expected to arise during manufacturing. Although the die wear literature for the forming of conventional steels is prevalent, tribological issues of high strength steels have not been understood well yet. This study aims to develop a new, rapid and automated wear test for the die materials used in sheet metal forming operations of high strength steels (mainly DP and TRIP steels) and to investigate the wear, friction, and lubrication issues. With this test, the actual stamping conditions such as contact pressure, temperature, and sliding velocity can be represented well. Our preliminary tests on two different extreme contact conditions (soft-soft, hard-hard) indicate that this novel wear test method results in relatively reasonable wear rate estimations/measurements when compared to the results in the literature.Copyright

Laser cutting of kevlar and mild steel composite structure : End product quality assessment

In the present study laser cutting of composite structure, consisting of Kevlar laminate at the top and mild steel sheet at the bottom, is considered. The end product quality is assessed using the thermal cutting standards. To compare the end product quality of composite structure cuts, Kevlar laminate and mild steel sheet are cut using the same cutting parameters. The kerf widths for Kevlar laminate and mild steel sheet cuts are predicted from the analytical formulation based on the lump parameter analysis. It is found that the end product quality of composite structure cuts is lower than that corresponding to Kevlar laminate and mild steel sheet cuts.

Performance and surface alloying characteristics of Cu–Cr and Cu–Mo powder metal tool electrodes in electrical discharge machining

ABSTRACT The main objective of this study is to investigate the effect of Cu–Cr and Cu–Mo powder metal (PM) tool electrodes on electrical discharge machining (EDM) performance outputs. The EDM performance measures used in the study are material removal rate (MRR), tool electrode wear rate (EWR), average workpiece surface roughness (Ra), machined workpiece surface hardness, abrasive wear resistance, corrosion resistance, and workpiece alloyed layer depth and composition. The EDM performance of Cu–Cr and Cu–Mo PM electrodes produced at three different mixing ratios (15, 25, and 35 wt% Cr or Mo), compacting pressures (Pc = 600, 700, and 800 MPa), and sintering temperatures (Ts = 800, 850, and 900 °C) are compared with those machined with electrolytic Cu and Cu PM electrodes when machining SAE 1040 steel workpiece. Analyses revealed that tool materials were deposited as a layer over the work surface yielding high surface hardness, strong abrasion, and corrosion resistance. Moreover, the mixing ratio, Pc, and Ts affect the MRR, EWR, and Ra values.

Investigation of toggling effect on pullout performance of pedicle screws

Objective of this study is to assess the pullout performance of various pedicle screws in different test materials after toggling tests comparatively. Solid core, cannulated (cemented), novel expandable and solid-core (cemented) pedicle screws were instrumented to the polyurethane foams (Grade 10 and Grade 40) produced in laboratory and bovine vertebra. ASTM F543 standard was used for preparation process of samples. Toggling tests were carried out. After toggling test procedures, pullout tests were performed. Load versus displacement graph was recorded, and the ultimate pullout force was defined as the maximum load (pullout strength) sustained before failure of screw. Anteriosuperior and oblique radiographs were taken from each sample after instrumentation in order to examine screw placement and cement distribution. The pullout strength of pedicle screws decreased after toggling tests with respect to the initial condition. While the cemented solid-core pedicle screws had the highest pullout strength in all test materials, they had the highest strength differences. The cemented solid-core pedicle screws had decrement rates of 27% and 16% in Grade 10 and Grade 40, respectively. There are almost same decrement rate (between 5.5% and 6.5%) for all types of pedicle screws instrumented to the samples of bovine vertebra. The pullout strengths of novel expandable pedicle screws in both of early period and after toggling conditions were almost similar, in other words, the decrement rates of it were lower than other types. According to the data collected from this study, polymethylmethacrylate augmentation significantly decreases pullout strength following the toggling loads. Higher brittleness of cured polymethylmethacrylate has adverse effect on the pullout strength. Although augmentation is an important process for enhancing pullout strength in early period, it has some disadvantages for preserving stabilization in a long time. Expandable pedicle screw with polyetheretherketone shell may be good alternative to polymethylmethacrylate augmentation on both primer stabilization and long-term loading application with toggling.

Experimental Investigation of Laser-Drilled Holes Variations Depending on Laser Drilling Parameters

Parameters on a CO2 laser machine effects the variations on hole diameters. In this study, a CO2 laser machine is used for drilling processes. Processing parameters are selected between 1.2-4 mm workpiece thickness, 2500-4000 W laser output power, -4 to 2 laser focus setting, 8-14 bar assisted gas pressure and 500-1200 Hz laser frequency. After the drilling process, with the diameter measurements, pictures were taken with an optic microscope then the effect of the processing parameters on hole diameter variations were investigated.