Effect of anodization on the thermal performance of naturally cooled heat sinks in power electronic devices made of die-cast aluminum alloy A380 and machined aluminum alloy 6061 was investigated experimentally and numerically. Various types of anodization were examined with different thickness of anodic aluminum oxide layer (AAO), pore size distributions, and surface coloring conditions. A customized natural convection and thermal radiation experimental chamber was built to measure the cooling capacity and heat sink temperatures. A 3D numerical model was also developed and validated against the collected data to provide more details into the contribution of the radiation heat transfer. The total emittance of the anodized samples was determined by a Fourier transform infrared reflectometer (FTIR) spectroscopy method. The results show a significant improvement in total hemispherical emissivity from 0.14 to 0.92 in anodized die-cast aluminum samples. This increase resulted in a considerable reduction in overall thermal resistance, up to 15%; where up to 41% of the total heat dissipation was contributed by thermal radiation. In spite of the rather distinguishable surface morphologies, the measurements suggested that thermal emissivity of the anodized die-cast Al A380 and Al alloy 6061 samples were in the same range.

To create functional metal parts by depositing molten metal droplets on top of each other, we have to obtain good metallurgical bonding between droplets. To investigate conditions under which such bonds are achieved, experiments were conducted in which vertical columns were formed by depositing molten aluminum alloy (A380) droplets on top of each other. A pneumatic droplet generator was used to create uniform, 0.8 mm diameter, molten aluminum droplets. The droplet generator was mounted on a stepper motor and moved constantly so as to maintain a fixed distance between the generator nozzle and the tip of the column being formed. The primary parameters varied in experiments were those found to have the strongest effect on bonding between droplets: substrate temperature ( 250 – 450 ° C ) and deposition rate (1–8 Hz). Droplet temperature was constant at 620 ° C . To achieve metallurgical bonding between droplets, the tip temperature of the column should be maintained slightly below the melting temperature of the alloy to ensure remelting under an impacting drop and good bonding. The temperature cannot exceed the melting point of the metal; otherwise the column tip melts down. The temperature at the bottom of a column was measured while droplets were being deposited. An analytical one-dimensional heat conduction model was developed to obtain the transient temperature profile of the column, assuming the column and the substrate to be a semi-infinite body exposed to a periodic heat flux. From the model, the droplet deposition frequency required to maintain the tip temperature at the melting point of the metal was calculated.

During the casting process of green sand mold, air gaps will form between the metal and sand mold. The air gaps will make it difficult to analyze the heat transfer at the mold/metal interface. Generally, an interfacial heat transfer coefficient is employed to evaluate the heat flux transferred across the air gaps. Though the interfacial heat transfer coefficient is highly important, its value is not easily obtained by using the direct experimental or theoretical method. With temperature-measured data, some inverse methods can be used to predict the coefficient. However, the latent heat released and undercooling during the solidification of the molten metal and the moisture of the green sand mold complicate the associated temperature calculations. To overcome this difficulty, a lump capacitance method is proposed in this study to calculate the interfacial heat transfer coefficient for the casting process in green sand mold. Thermalcouples are utilized to measure the temperatures of sand mold and metal. The geometry of casting is cylindrical and the castings are A356 alloy and Sn-20 wt. % Pb alloy. With the predicted interfacial coefficients, the temperature field of the metal was solved numerically. Based on the solidification time, the numerical results are in good agreement with the experimental ones. This verified the feasibility of the proposed method and it can be applied in the future study or design of a casting process.

A continuum mixture model of the direct chill casting process is compared to experimental results from industrial scale aluminum billets. The model, which includes the transport of free-floating solid particles, can simulate the effect of a grain refiner on macrosegregation and fluid flow. It is applied to an Al-6 wt% Cu alloy and the effect of grain refiner on macrosegregation, sump profile, and temperature fields are presented. Two 45 cm diameter billets were cast under production conditions with and without grain refiner. Temperature and composition measurements and sump profiles are compared to the numerical results. The comparison shows some agreement for the grain refined case. It is believed that an incorrect assumption about the actual grain structure prevents good agreement in the non-grain refined billet.

A fully implicit control-volume finite element method is used to analyze the phase change during DC casting of aluminum alloy. The mathematical model is based on the integral form of the enthalpy equation. A Eulerian-Lagrangian transformation, together with a deforming grid technique, is introduced to efficiently track the evolution of the physical domain. The temperature distribution predicted by the numerical model is in good agreement with that measured in previous experiments. [S0022-1481(00)00502-8]

The mechanical properties of age-hardenable aluminum alloy extrusions are critically dependent on the rate at which the part is cooled (quenched) after the forming operation. The present study continues the development of an intelligent spray quenching system, which selects the optimal nozzle configuration based on part geometry and composition such that the magnitude and uniformity of hardness (or yield strength) is maximized while residual stresses are minimized. The quenching of a complex-shaped part with multiple, overlapping sprays was successfully modeled using spray heat transfer correlations as boundary conditions within a finite element program. The hardness distribution of the heat-treated part was accurately predicted using the quench factor technique; that is, the metallurgical transformations that occur within the part were linked to the cooling history predicted by the finite element program. This study represents the first successful attempt at systematically predicting the mechanical properties of a quenched metallic part from knowledge of only the spray boundary conditions.

Experimental studies of the local mass transfer characteristics of annularly finned tubes in crossflow are presented. Variations due to boundary layer development, forward-edge separation, the tube wake, horseshoe vortices, and tip vortices are discussed. In addition, regularly located local maxima in mass transfer rates associated with the horseshoe vortex system are found, and conjecture as to their mechanism is offered. Inferring heat transfer behavior from the mass transfer results, we find that the true fin efficiency is always less than that obtained with an assumed constant convective heat transfer coefficient. The difference is 3–7 percent for high-conductivity materials such as aluminum alloys, and 9–17 percent for low-conductivity materials such as mild steels.

The total hemispherical emittance of an oxide film that formed on 6061-T6 aluminum alloy parts in the Tower Shielding Reactor-II at Oak Ridge National Laboratory was measured from 295 to 773 K using an emissometer and/or a calorimeter. The emittance of this film was critically needed for heat transfer calculations in a simulated loss-of-coolant accident of the reactor. X-ray diffraction analysis identified the film as boehmite (Al 2 O 3 ·H 2 O), which dehydrated to alumina (Al 2 O 3 ) upon heating above 473 K. The measured emittances for the alumina film are in excellent agreement with published values for anodized aluminum films and for bulk alumina. Published values of the emittance of boehmite could not be found for comparison, but evidence is presented that some anodization processes for aluminum yield boehmite and not alumina films.

Proper roll cooling has been identified as a critical factor in the problems of excessive roll spalling and poor thermal crowning in modern, high-speed rolling mills. In this paper, an analytical model has been developed to determine the temperature profiles of the roll and the strip. This model uses basic heat transfer theory and provides the capability of studying the influence of operating parameters on both the work-roll and workpiece temperatures. Examples on cold and hot rolling of aluminum alloys are given to demonstrate the feasibility and capability of the model developed. Previous work on thermal modeling of rolling processes is also briefly reviewed.

Changes in microstructure occur in as-received aluminum alloy (Al-2024-T351) when it is subjected to elevated temperatures (150–260°C). These changes, which are called precipitation hardening, in turn influence the thermal properties, making them time as well as temperature dependent. A computer-assisted transient experimental procedure has been developed to determine the values of thermal conductivity of as-received Al-2024-T351 under the influence of precipitation-hardening. Based on isothermal experimental data and related algebraic modeling of the thermal conductivity, a mathematical model in the form of two differential equations is proposed. Instantaneous values of volume fraction of precipitate and thermal conductivity can be predicted using this model. A method for the simultaneous numerical solution of the partial differential equation of conduction and the proposed differential equations of precipitation are also given. The influence of precipitation—hardening on temperature distribution and on values of thermal conductivity is shown graphically for several cases involving the Al-2024-T351 material.