Polymeric materials are often used as structural binders for biomedical applications. The mechanical properties of the material strongly depend on the fabrication process. To this end, we illustrate a set of casting methods for the production of samples to be tested via destructive methods. The curing process of the artifact was controlled during fabrication, and the molds were also made of polymeric materials. The fabrication of molds is illustrated where particular emphasis is posed on the manufacturing and testing of silicone molds using off-the-shelf material. Cyanoacrylate (CA), Epoxy resin (EP) and Methacrylate ester monomers (MEMs) artifacts have been fabricated using said molds. Of the aforementioned resins, MEMs are a class of thermosetting biocompatible polymers in which fabrication is especially problematic because of the very narrow temperature window at which the monomers polymerize. This research analyzes the casting process of curable materials highlighting the setbacks of using plastic-based molds. Among the cast based manufacturing techniques, specific focus was given to the case where MEMs is made to polymerize in a silicone mold controlling the temperature of the environment. The thermal properties that the silicone-based molds require for the appropriate curing of the polymer are analyzed. It was found that due to the very high heat capacity of silicone, the regulation of the temperature within the mold is difficult often exciding the boiling point of the casted resin.

Pulsed laser ablation (PLA) under active liquid confinement, also known as chemical etching enhanced pulsed laser ablation (CE-PLA), has emerged as a novel laser processing methodology, which breaks the current major limitation in underwater PLA caused by the breakdown plasma and effectively improves the efficiencies of underwater PLA-based processes, such as laser-assisted nano-/micro-machining and laser shock processing. Despite of experimental efforts, little attention has been paid on CE-PLA process modeling. In this study, an extended two-temperature model is proposed to predict the temporal/spatial evolution of the electron-lattice temperature and the ablation rate in the CE-PLA process. The model is developed with considerations on the temperature-dependent electronic thermal properties and optical properties of the target material. The ablation rate is formulated by incorporating the mutual promotion between ablation and etching processes. The simulation results are validated by the experimental data of CE-PLA of zinc under the liquid confinement of hydrogen peroxide.

Powder bed metal additive manufacturing (AM) utilizes a high-energy heat source scanning at the surface of a powder layer in a pre-defined area to be melted and solidified to fabricate parts layer by layer. It is known that powder bed metal AM is primarily a thermal process and further, heat conduction is the dominant heat transfer mode in the process. Hence, understanding the powder bed thermal conductivity is crucial to process temperature predictions, because powder thermal conductivity could be substantially different from its solid counterpart. On the other hand, measuring the powder thermal conductivity is a challenging task. The objective of this study is to investigate the powder thermal conductivity using a method that combines a thermal diffusivity measurement technique and a numerical heat transfer model. In the experimental aspect, disk-shaped samples, with powder inside, made by a laser powder bed fusion (LPBF) system, are measured using a laser flash system to obtain the thermal diffusivity and the normalized temperature history during testing. In parallel, a finite element model is developed to simulate the transient heat transfer of the laser flash process. The numerical model was first validated using reference material testing. Then, the model is extended to incorporate powder enclosed in an LPBF sample with thermal properties to be determined using an inverse method to approximate the simulation results to the thermal data from the experiments. In order to include the powder particles’ contribution in the measurement, an improved model geometry, which improves the contact condition between powder particles and the sample solid shell, has been tested. A multi-point optimization inverse heat transfer method is used to calculate the powder thermal conductivity. From this study, the thermal conductivity of a nickel alloy 625 powder in powder bed conditions is estimated to be 1.01 W/m·K at 500 °C.

The particle-polymer composite can perform multiple functionalities according to particle property, local particle distribution, and alignment. This paper shows thermal management applications of in situ manipulations of particle dispersion patterns within a 3D printed polymeric composite architecture. A 3D printed particle-polymer composite with enhanced thermal conductive properties was developed. Composite structures containing 30-micron-sized aluminum particles embedded in the acrylate polymer were produced using a novel acoustic field assisted projection based Stereolithography process. Thermal properties of the pure polymer and prepared uniform composite with 2.75 wt% particle were characterized by using the transient hot bridge technique. To investigate the effect of material composition and particle distribution pattern on composite thermal behavior, heat sinks were designed and fabricated with the pure polymer, homogeneous composite with particles uniformly distributed in the polymer matrix, and composite with patterned particles for comparison. Infrared thermal imaging was performed on the 3D printed objects. The homogeneous composites displayed slight enhancement in thermal conductivity. A significant improvement of heat dissipation speed was observed for the patterned composite, due to a densely interconnected aluminum aggregate network. To further improve the thermal property of the patterned composite, varying layer thicknesses were tested. The developed patterned composites with superior performance compared to the inherent polymer material and homogeneous composites can be used for fabricating thermal management applications in electronic and fluidic devices.

Additive manufacturing (AM) processes allow for complex geometries to be developed in a cost- and time-efficient manner in small-scale productions. The unique functionality of AM offers an ideal collaboration between specific applications of human variability and thermal management. This research investigates the intersection of AM, human variability and thermal management in the development of a military helmet heat exchanger. A primary aim of this research was to establish the effectiveness of AM components in thermal applications based on material composition. Using additively manufactured heat pipe holders, the thermal properties of a passive evaporative cooler are tested for performance capability with various heat pipes over two environmental conditions. This study conducted a proof-of-concept design for a passive helmet heat exchanger, incorporating AM components as both the heat pipe holders and the cushioning material targeting internal head temperatures of ≤ 35°C. Copper heat pipes from 3 manufactures with three lengths were analytically simulated and experimentally tested for their effectiveness in the helmet design. A total of 12 heat pipes were tested with 2 heat pipes per holder in a lateral configuration inside a thermal environmental chamber. Two 25-hour tests in an environmental chamber were conducted evaluating temperature (25°C, 45°C) and relative humidity (25%, 50%) for the six types of heat pipes and compared against the analytical models of the helmet heat exchangers. Many of the heat pipes tested were good conduits for moving the heat from the head to the evaporative wicking material. All heat pipes had Coefficients of Performance under 3.5 when tested with the lateral system. Comparisons of the analytical and experimental models show the need for the design to incorporate a re-wetting reservoir. This work on a 2-dimensional system establishes the basis for design improvements and integration of the heat pipes and additively manufactured parts with a 3-dimensional helmet.

Joining of dissimilar metals and alloys has been envisioned since a long time with specific high end applications in various fields. One such combination is austenitic stainless steel grade SS304 and commercial grade titanium, which is very difficult to join under conventional fusion process due to extensive cracking and failure caused by mismatch in structural and thermal properties as well as formation of the extremely brittle and hard intermetallic compounds. One of the methods proposed in literature to control the formation of intermetallics is by fast cooling fusion process like laser beam welding. The present study has been done on laser welding of titanium and stainless steel AISI 304 to understand the interaction of these materials during laser welding at different laser power and welding speed which could yield different cooling rates. Two types of cracks were observed in the weld joint, namely longitudinal cracks and transverse cracks with respect to the weld direction. Longitudinal cracks could be completely eliminated at faster welding speeds, but transverse cracks were found little influenced by the welding speed. The thermal history, i.e. melt pool lifetime and cooling rate of the molten pool during laser welding was monitored and a relation between thermo-cycle with occurrence of cracks was established. It is inferred that the longitudinal cracks are mainly due to the formation of various brittle intermetallic phases of Fe and Ti, which could be minimized by providing relatively less melt pool lifetime at high welding speeds. The reason of the transverse cracks could be the generation of longitudinal stress in weld joint due to the large difference in the thermal expansion coefficient of steel and titanium. In order to mitigate the longitudinal stress laser welding was carried out with a novel experimental arrangement which ensured different cooling rates of these two metals during laser welding. With this the tendency of transverse cracks also could be minimized significantly.

This paper presented a novel 3D printing technique to fabricate graphene aerogel based on directional freezing. Thermal property of the graphene ink is one of key factors in this process which affects the material integrity and morphology as well as process efficiency and reliability. The major objective of this paper is to develop a heat transfer model to efficiently and reliably predict the temperature evolution of the printed materials and the waiting time between the layers for any input geometry. The simulation results show that the input geometry significantly affects the temperature evolution and waiting time. The proposed technique can not only improve the process efficiency and reliability, it can also serve as a flexible tool to predict and control the microstructure of the printed graphene aerogels.

Electrochemical discharge machining (ECDM) is a promising machining technology to process non-conducing and brittle materials, featuring high throughput and good accuracy in meso and micro scale machining of hard-to-machine materials. Currently ECDM has not yet attracted wide interest from the industry because of the low controllability and repeatability. There is a huge gap in process optimization to make ECDM viable in industry. A good process model is essential to achieve an improved and optimized process. The fundamental of ECDM is the discharging activity, which triggers various mechanisms to remove material. Therefore characterization of sparks from the aspects of electrical and thermal properties is the premise of process modeling. In this paper, experimental investigation and modeling of discharging activity was presented. The spark releasing process was studied in terms of discharge energy, intensity distribution, and material removal. Conic tool electrodes were fabricated to achieve more consistent discharging. The material removal mechanism was revealed by analytical derivation and simulated with numerical methods.

Residual stresses generated from finish machining have a significant impact on the fatigue life of mechanical components by controlling crack initiation and propagation processes in their near subsurface. As governing variables, tool geometry, tool wear, machining parameter, work material property, and lubrication conditions have been widely studied to determine their effects on residual stress pattern in machined surface and subsurface. Among those parameters, tool flank wear was seldom fully investigated although tool flank wear, as well as machining speed, has been identified as the most important contributor to residual stress. Especially, tool flank wear becomes more significant due to the poor work thermal property during the high speed machining of titanium Ti-6Al-4V alloy. This study aims to investigate the combined role of tool flank wear and machining speed in developing residual stress in the machining of titanium alloy using finite element method. A microstructure sensitive material model based on Self Consistent Method (SCM) is adopted to incorporate the phase state and its transformations during machining cycle. Critical flank wear land and corresponding machining speeds are identified, beyond which compressive residual stresses are transferred into tensile residual stresses. High machining speeds demonstrate a distinct influence on residual stresses by means of promoting tool flank wear rate. The numerical simulation results are validated by empirical data provided in previous research.

Reinforcement with nano-sized particles offers a promising potential to significantly enhance the mechanical, electrical, and thermal properties of a metal matrix composite (MMC). One of the challenges of synthesizing nanocomposites, however, has been the dispersion and control of the nano-reinforcement materials. In this study, a laminate nanocomposite has been synthesized by incorporating ultrasonic spray deposition technique. An ultrasonic spray deposition system was used to deposit nano-particles on substrate foils, which were consolidated to synthesize a laminate composite. Aluminum 6061 (Al6061) alloy foils were used as the matrix material. Nano-silicon carbide (SiC) particles were used as the reinforcement phase (deposited layer). The sprayed foils were stacked together to form the composite. The composite was then consolidated by hot compaction in the semi-solid regime of the Al6061. A three point bend test was carried out to evaluate the mechanical properties. In addition, the suspension and spraying parameters that control the deposited microstructure was studied to help control the final properties of the deposited structure. The yield and ultimate flexural strength of the SiC sprayed Al6061 laminate composite showed an increase (32% and 15%, respectively) compared with that of the unsprayed sample (reference sample) processed at the same condition.

To better predict the temperature distribution in the tool and chip, a modified theoretical model by considering material thermal properties as temperature dependent is developed to quantitatively describe the temperature elevation due to the shear and friction at the tool-chip interface. Work’s thermal properties of thermal conductivity and specific heat are modified and considered as functions of temperature. The semi-infinite method is utilized in the model, in which the back of the chip and the shear band are assumed as adiabatic. Temperature distribution in the tool and chip is then determined simultaneously by shear and friction. An imaginary heat source is set up to be plane-symmetric with respect to each original heat source in this approach. The effects of original heat source and imaginary heat source are superimposed to calculate the final temperature elevation in the tool and chip. To determine the ratio of total heat transferred into the chip and the tool, it is assumed that the temperatures in the tool and in the chip are in balance along the tool-chip interface in the stable cutting state. The model is experimentally validated with peak temperature data from previous literature. Results indicate that the model-experiment deviation is less than 10% when thermal properties are considered temperature dependent, and it is more accurate than that by considering the thermal properties as constants. The patterns of temperature distribution in the tool and chip are further analyzed by the model.

Microvia formation technology using lasers has become the dominant method for drilling microvia called blind via-holes (BVHs) in printed wiring boards (PWBs). Laser direct drilling (LDD), drilling directly outer copper foil by laser, has attracted attention as a novel method. In particular, when copper and resin with different processing thresholds are drilled at the same time, an overhang defect occurs on the drilled hole. However, the overhang generation mechanism has not been clarified. Therefore, we investigated it by detailed observation of the drilled-hole section. Moreover, the overhang length was estimated using the finite element method (FEM). Influences of surface treatment of outer copper foil and thermal properties of the build-up layer were evaluated experimentally and analytically. Consequently, an experiment with a prototype PWB with silica filler added in the build-up layer was carried out. Using the prototype PWBs, the overhang was reduced as shown in FEM analysis results.

Dry turning experiments on Ti-6Al-4V were conducted using two grades (finer and coarser) of carbides and polycrystalline diamond (PCD) inserts to study tool wear. Despite of minor compositional difference between two carbide grades, both grades contain 6% Co. Crater wear and flank wear were measured using Confocal Laser Scanning Microscopy (CLSM). Three dimensional rake surface topographies were reconstructed from the CLSM data and wear profiles were extracted. Finite Element Analysis (FEA) was conducted to study the effects of cutting conditions and thermal properties on rake face temperature. Flank wear on the carbide tools indicated that the inserts with the finer grain size exhibited smaller flank wear than the insert of the coarser grain size. This was attributed to reduced abrasive wear in the finer grained inserts as a result of a higher hardness. The carbide grade with a coarser grain size had an enhanced ability to resist crater wear, likely from lower rake face temperatures and the differences in the compositions. It is known that coarser grain carbides have a higher thermal conductivity resulting from increased grain contiguity. FEA was used to study the temperature difference between the two grain-sizes and the effect of thermal conductivity on temperature gradients. Tool wear of the PCD inserts was also studied. The PCD tools showed significant adhesive wear at the 200sfm cutting speed, transitioning to crater wear at 400sfm. With a high thermal conductivity, it is possible that rake face temperatures were low enough to alter the wear mechanism. FEA supports this hypothesis, as the maximum rake face temperature for the PCD inserts were only around 900°C at 200sfm.

Carbon nanotubes (CNTs) have been widely investigated as a reinforcement material to improve the mechanical, electrical and thermal properties of composite materials. Various routes have been employed to fabricate aluminum-carbon nanotube (Al-CNT) composites in the past few years. However, uniform distribution of CNTs in the metal matrix is still challenging. In this paper, a novel semi-solid powder processing (SPP) was used to incorporate CNT uniformly into the Al6061-CNT composite. Al6061-CNT powders mechanically alloyed for different durations were also examined to understand how the CNTs were dispersed in the Al6061 powders. As-received CNT cluster balls were crushed into dense thin CNT layers during mechanical alloying. As mechanical alloying time increased, CNTs were dispersed in the Al6061 particles. Well-densified microstructures with severely deformed grains were observed in the Al6061-CNT composite.

Ultra fine fibers, with a linear density below 1 denier, are high value products for textile, industrial and biomedical applications. Traditionally ultrafine fibers are produced by extrusion followed by mechanical drawing. In this study a customized fiber extrusion line was designed and set up with the inclusion of a jet nozzle in the spinneret pack. With only cold air attenuation, we were able to obtain drawing ratios exceeding 2500 and produce subdenier continuous filaments. We have conducted parametric studies to investigate the fiber attenuation mechanism and determine the optimal process window for making continuous microfibers. Different characterization methods were carried out to examine the resulting fiber properties, including microstructures, thermal properties, mechanical properties, orientation and crystallinity. Theoretical analysis of cold air drawing is discussed in this paper.

Cautery is a process to coagulate tissues and seal blood vessels using the heat. In this study, finite element modeling (FEM) was performed to analyze temperature distribution in biological tissue subject to cautery electrosurgical technique. FEM can provide detailed insight into the heat transfer in biological tissue to reduce the collateral thermal damage and improve the safety of cautery surgical procedure. A coupled thermal-electric FEM module was applied with temperature-dependent electrical and thermal properties for the tissue. Tissue temperature was measured at different locations during the electrosurgical experiments and compared to FEM results with good agreement. The temperature-dependent electrical conductivity has demonstrated to be critical. In comparison, the temperature-dependent thermal conductivity does not impact heat transfer as much as the electrical conductivity. FEM results show that the thermal effects can be varied with the electrode geometry that focuses the current density at the midline of the instrument profile.