Khaled S. Al-Athel

3.4 Residual Stresses in Thermal Spray Coating

Residual stresses generated in coatings during thermal spraying plays an important role on the service life and interfacial adhesion. Their magnitude and distributions are affected by both materials and processing parameters. In the introduction section of this chapter, the importance and sources of residual stresses are discussed. In the following sections, various experimental and numerical techniques used for the estimation of residual stresses in thermal spray coating have been discussed.

Modeling Residual Stress Development in Thermal Spray Coatings: Current Status and Way Forward

An overview of analytical and numerical methods for prediction of residual stresses in thermal spray coatings is presented. The various sources and mechanisms underlying residual stress development in thermal spray coatings are discussed, then the various difficulties associated with experimental residual stress measurement in thermal spray coatings are highlighted. The various analytical and numerical models used for prediction of residual stresses in thermal spray coatings are thoroughly discussed. While analytical models for prediction of postdeposition thermal mismatch stresses are fully developed, analytical quenching and peening stress models still require extensive development. Various schemes for prediction of residual stresses using the finite element method are identified. The results of the various numerical and analytical models are critically analyzed, and their accuracy and validity, when compared with experiments, are discussed. Issues regarding the accuracy and applicability of the models for predicting residual stresses in thermal spray coatings are highlighted, and several suggestions for future development of the models are given.

Thermal Behavior of Aluminum Alloy Metal Foam Heat Sinks: A Computational and Experimental Approach

Metal foams are structures of a cellular nature that contain a high percentage of porosity that can be produced in either a closed or open cell forms. The use of metal foams in engineering applications has increased significantly over the last decade due to their enhanced mechanical and thermal properties. An innovative approach for three-dimensional (3D) detailed finite element modeling of open cell metal foam has been taken to capture the versatile nature of metal foams’ geometry and predicting its thermal performance. The interior complex geometry of metal foams has limited studies to create computational model via common approaches. Therefore, not much computational work has been done in open cell metal foam applications. To overcome this difficulty, computed tomography (CT) scan has been used to extract the 3D structure surface model with extreme precision. Computer-Aided Design (CAD) software has been used for “stitching” and “healing” of the CT scan model before importing it to a finite element domain. The 3D computational model is used in a heat sink application and is calibrated against experimental results for the temperature distribution of one case. The validated and calibrated model is then used for simulating different metal foam heat sink cases to assess the thermal and mechanical behavior under different conditions.Copyright

Design and Performance Evaluation of Al 2 O 3 -SiC Composite for Direct-Bonded Copper Substrate

A computational material design approach is applied to propose a novel ceramic material for direct-bonded copper (DBC) substrate with enhanced thermal and structural performance. The material design inherently consists of many competing requirements that require careful decisions regarding key trade-offs in terms of material composition, inclusion size, shape, and distribution to achieve the target properties. The alumina-silicon (Al2O3-SiC) composite, as compared to commercial alumina, used in DBC is found to be the most suitable design among other candidates with improved thermal and structural properties. In order to study the performance characteristics and the effects of the new ceramic composite with improved properties in terms of structural behavior and fatigue life of the DBC substrate, the normal working and extreme thermal cycling conditions were simulated and analyzed using finite element method. The temperature, strain, and localized stress distribution within the substrate at a steady-state condition were analyzed, and the improved Coffin–Manson law was used to calculate the fatigue life of the substrate under extreme thermal cycling conditions. The proposed Al2O3-SiC composite is found to be more robust than the commercial alumina as DBC substrates considering the thermal–mechanical performance. The fatigue life cycle of the DBC substrate with the proposed material is predicted to be about two times longer than the commercial alumina DBC ceramic under transient thermal cycling test.

Effect of Fin Orientation and Forced Convection on the Performance of Metal Foam Fins using a μ-CT Scan Based 3D CFD Model

Metal foams are very attractive materials for thermal and electronic packaging applications due to their improved heat transfer capabilities. Their improved heat transfer effective properties are due to the relatively large contact area they possess because of their cell structure. This comes at the expense of the pressure drop. This work presents a methodology based on μ-CT scan to develop a realistic metal foam 3D model. The model is validated with experiments and used to study the behavior of the metal foam as a fin in terms of the temperature variation within the fin as well as the effect of the airflow velocity and fin orientation to the pressure drop. Results showed two major observations. First, this methodology could be used to identify a velocity value at which the fin orientation becomes obsolete and has no effect on the temperature variation. Second, the pressure drop alone could not be used to assess the fin, but also the fin orientation has to be taken into account to examine the total pressure drop.

Modeling and Placement of Thermopiezoelectro-Magnetic Materials

There has been a vast interest in the general coupled field analysis of thermopiezoelectro-magnetic materials under which smart piezoelectric, thermopiezoelectric and magnetostrictive materials can be studied. The smart materials are often bonded as thin films on host structures for the purpose of sensing and/or actuation. It is well-known that the placement of sensors and actuators is important in order to obtain the appropriate sensor input and to provide the adequate actuation power. This study aims at modeling the important phenomenon of thermopiezoelectro-magnetism suitable for beam and/or plate type-host structures. The thermopiezoelectro-magnetic materials are modeled using the finite element method and the resulting equations are used for decision making on the best placement of the smart actuators on various host structures.

Eulerian FEA With Volume of Solid (VOS) Application in Metal Forming

The adaptation of the volume of fluid method (VOF) to solid mechanics (VOS) is presented in this work with the focus on metal forming applications. The method is discussed for a general non-uniform mesh with Eulerian finite element formulation. The implementation of the VOS method in metal forming applications is presented by focusing on topics such as the contact between the tool and the workpiece, tracking of the free surface of the material flow and the connectivity of the free surface during the whole process. Improvement on the connectivity of the free surface and the representation of curves is achieved by considering the mechanics of different metal forming processes. Different applications are simulated and discussed to highlight the capability of the VOS method.Copyright