Thermal gradient effects in direct resistance heated metal matrix composites

The purpose of the present study was to discuss the effects of local solidification time and thermal gradient on porosity content in long solidification range A201 aluminum alloy plate castings. With systematic change in the riser size, together with variation of thickness and length, were cast in different kinds of molds. The sand molds with end chill for the plate castings were made of 100% silica sand. The porosity content of A201 aluminum alloy was affected by the local solidification time and thermal gradient at same time in this study. The correlation between porosity content and any individual thermal parameter is found to be split into 2 bands, each being associated with one of the two thicknesses of plate castings. The higher the thermal gradient, the lower the porosity content was measured in the A201 aluminum alloy plate casting. And high thermal gradient with short solidification time will get lower porosity content in aluminum alloy castings

This paper presents a method of testing sheet materials at elevated temperatures using direct resistance heating. The problems associated with the use of direct resistance heating are discussed by referring to investigations on the heating characteristics of short, rectangular sheet blanks. Experimental results show that elevated-temperature tensile tests at considerably fast strain rates can be carried out using direct resistance heating. The major drawback is shown to be the non-uniformity of temperature of the test specimen at its ends due to heat transfer from the specimen to the unheated jaws. Ways of minimizing this non-uniformity are discussed and a suggestion is made for further reduction by indirect resistance heating of jaws using an independent power supply.

Selective Laser Sintering (SLS) and Selective Laser Melting (SLM) are parent Additive Manufacturing processes using a laser to solidify metallic, ceramic, polymer or composite powders. During the process, the object is built layer by layer. A laser source is responsible for the consolidation, by local heating. The light is deviated by a scanning head according to the instructions of an STL file. Then partial fusion of the particles takes place, followed by a solidification of the liquid created. Kinetics of these steps are very high and play an important role in the final microstructure (rearrangement of the particles, pore creation, residual stresses). In the case of polymers and composites the technique is now well understood and widely used, but for metals and ceramics it suffers from a lack of precision, surface roughness and poor mechanical properties. The goal of this Ph.D. work is to understand the effect of the thermal gradients on the consolidation process, using different laser parameters (power, pulse frequency, scan speed,…). In SLS, microscopic thermal gradients arise, due to the fact that the laser is pulsed and that only the exterior of the powder particles is molten. A thermal model to describe the interaction between a laser beam and a (spherical) grain is proposed. This model allows for the incorporation of the latent heat of fusion and for a realistic surrounding. The absorbed laser intensity is modeled by means of the Mie theory. Experiments where two particles of powder are isolated and illuminated by the laser are carried out in order to measure the interparticular necks and the volume of liquid formed for different repetition rates. The thermal model leads to good predictions of the particles final sintering state. Fluid flow models are investigated in order to determine the dynamic of the molten liquid. The main issue is to explain the capillary flow mechanisms leading from the molten material to the neck formation. Two models are derived to simulate the fluid flow between the particles. The first one simulates a capillary flow between two parallel plates. The second one deals with energetic considerations arising from Frenkel's principle. In both cases, the final output is the liquid life-time necessary to get the interparticular neck lengths experimentally observed. We point out that the two models predict the same liquid lifetime, although they are derived with different hypotheses. In SLM, continuous lasers are used and one has to deal with macroscopic thermal gradients, since the particles are completely molten. A very high laser power is used and the negative thermal effects (like thermal stresses or balling effects) are important. A solution to control them is to adjust the scanning strategy. Four scanning strategies are investigated for material with different thermal conductivities. The three-dimensional model used to describe the laser-matter interactions and the temperature evolution of the scanned powder bed allows for a finite latent heat (Stefan-problem) and for conductivity modifications due to the consolidation. We show that this finite element thermal model can be efficiently used to anticipate most of the problems (like cracks or balling) arising in practice. The benefit of avoiding thermal gradients is shown, in particular by EBSD analysis. Finally, applications of the SLS/SLM technique on different pieces built during this work are shown. A new way to build support structure is also proposed.

We have investigated the influence of thermal gradient effects in inhomogeneously heated MEMS/NEMS. The actuation perturbations caused by thermal gradients have been studied through static optothermal actuation experiments of a bi-material polymer based cantilever and supported by finite element modeling. As a result, bidirectional bending has been experimentally observed and interpreted as the competition between bimorph and thermal gradient effects. The competition has illustrated the importance of including the thermal gradient effect in the behavior analysis of bimorph driven MEMS/NEMS devices.

This paper presents the results of a study aimed at assessing the effects of temperature and thermal gradient on psychrometer readings, and at developing a calibration protocol that takes into account these effects. An intensive calibration programme was conducted at different temperatures (20–35°C) and water potential levels (0·0 to ∼7·0 MPa), for this purpose. The thermal gradient was expressed as the difference in temperature between wet and dry junctions in the thermocouple psychrometer sensor. The collected calibration data were used to build an artificial neural network psychrometer model. The developed model was able to simulate successfully the psychrometer output at different conditions, and was used to conduct a detailed parametric study to evaluate the influence of temperature and thermal gradient on thermocouple psychrometer measurements. The outcome of this study shows the importance of including the effects of temperature and thermal gradient in the standard calibration process. Moreover, it introduces a new, precise calibration protocol that takes advantage of the artificial neural network approach.

A simple and straightforward implicit numerical technique for phase change problems in cylindrical geometry is proposed. When implicit schemes are used the moving interface along with the convective or radiative boundary condition pose a problem because of the requirement to calculate the interface location and boundary temperature implicitly. Due to this difficulty some of the methods available in literature used approximations near the wall and the rest used iterative methods. The present technique isolates the nonlinearity associated with the moving interface as well as that of any nonlinear wall boundary condition and permits the simultaneous evaluation of the unknown interface location and the wall temperature at the new time level. Thereafter the temperatures at the other nodes are obtained without nodal iterations. Numerical results are obtained for outward solidification with both finite and infinite heat transfer rates at the wall. Good agreement is obtained between the present numerical results and the results available in literature for the limiting cases.

This chapter deals with some specific applications of induction and direct resistance heating, that have been investigated also by the authors of this book. When available, analytical solutions for these ‘special’ applications are provided. Section 4.1 is about the resistance heating of ferromagnetic workpieces of rectangular cross-section, Sect. 4.2 describes the direct resistance heating of bended conductors. The transverse flux heating is a technology that is applied for treating thin slabs and stripes with high electrical efficiency and is the topic of the Sect. 4.3. Planar coils, often named pancake inductors, are described in the Sect. 4.4, while the effects due to the finite length of inductor are discussed in Sect. 4.5. Induction hardening of workpieces of complex geometry is one of the most important applications of induction heating, single and dual frequency processes for contour hardening are presented in Sect. 4.6. The innovative process of through heating by rotating a nonferrous billet inside a DC field produced by superconductive coils is detailed in Sect. 4.7, while in Sect. 4.8 the induction heating is produced by rotation of permanent magnets. Section 4.9 deals with the induction heating of hollow workpieces heated by internal inductors and Sect. 4.10 describes the forces acting in induction heating applications.

Saturation and stability in internal resonance of a rotating blade under thermal gradient

The purpose of the article is to analyze the static and dynamic stability of an asymmetric sandwich beam with viscoelastic core lying on a variable Pasternak foundation under the action of a pulsating axial load subjected to one-dimensional thermal gradient under three different boundary conditions by the computational method. A set of Hill’s equation has been obtained by the application of Hamilton’s principle along with the generalized Galerkin’s method. The effects of thermal gradient, elastic foundation variation parameter, thickness ratio of two elastic layers, the ratio of modulus of the shear layer of Pasternak foundation to the young’s modulus of elastic layer, the ratio of the length of the beam to the thickness of the elastic layer, the ratio of in phase shear modulus of the viscoelastic core to the young’s modulus of the elastic layer, the ratio of thickness of Pasternak foundation to the length of the beam, coreloss factor on the non-dimensional static buckling loads and on the regions of parametric instability are studied.

Aluminum composite matrix materials are regarded as the most popular type of composite materials. Metal matrix composites made of aluminum have better mechanical and thermal properties, including a higher strength-to-weight ratio, tensile strength, hardness, and a low coefficient of thermal expansion. In various types of applications viz, automobile, aviation, the thermal characterization of aluminum metal matrix composites has increased. Thermal conductivity as a function of temperature, thermal diffusivity, and the thermal gradient is one of the essential thermal characteristics of aluminum metal matrix composites needed to understand the material’s behavior. The current work evaluated thermal conductivity as a product of thermal diffusivity, density, and specific heat for Al6061/Ni/Cr hybrid nano metal matrix composites from 50 °C to 300 °C. Al6061 based metal matrix composite reinforced with varying wt.% of Ni and Cr nanoparticles whereas fixed wt.% of graphene and Mg added to improve thermal conductivity, self-lubrication, and wettability. Thermal diffusivity, specific heat, and density were evaluated using laser flash apparatus (LFA 447), differential scanning calorimetry (DSC), and Archimedes principle, respectively. Results revealed that the thermal conductivity of fabricated composites increases with Ni, Cr, Mg, and graphene nanoparticles. With further expansion of reinforced particles of Ni and Cr, the thermal conductivity decreases. Finite element analysis (FEA) has been conducted to determine the thermal gradient and thermal flux using experimental values such as density, thermal conductivity, specific heat, and enthalpy at various temperature ranges to validate the experimental results.

In recent years, carbon fibre reinforced thermoplastics (CFRTP) are expected to be used as lightweight structural materials for mass-produced vehicles. CFRTP with thermoplastics as matrix allows us to weld them using melting of matrix by heating. We have been developing a direct resistance heating method, which uses carbon fibres as the resistance heating element. Carbon nanotube (CNT) is expected to be used as additive to FRP and we reported that the fibre/matrix interfacial shear strength was improved by grafting CNT on the surface of carbon fibres and tensile lap-shear strength was improved by using CNT grafted carbon fibre as the heating element for welding. For the practical use of CFRTP for structural parts, flexural strength is also necessary to be evaluated. In this study, flexural test was carried out to clarify the effect of CNT deposition time to the surface of carbon fibres on flexural strength of resistance welded CFRTP using CNT grafted carbon fibre as the heating element. The highest flexural strength was obtained when CNT10, for which CNT is grafted on the carbon fibres for deposition time of 10 min, was used for the heating element of resistance welding. In the case of CNT deposition time of 60 min, the lowest flexural strength was obtained because of the poor impregnation of the resin into the carbon fibre due to the excess CNT on the carbon fibres.

In flip chip package, the majority of heat is from the die side and a temperature gradient builds up from the chip to substrate through the solder joint and underfill. This will induces biased atomic flux from hot end to cold end in the solder. Even in electromigration test vehicle, the joule heating from the Al trace at chip side is also higher than substrate side. Any small temperature difference can generate a large temperature gradient given the small dimension of the solder bump. The combined effect of electromigration and theromigration may induce earlier failure. In the current work, the electromigration reliability of Pb-free solder bump with thick copper UBM at the chip side is studied. The effect of temperature gradient was considered and characterized. We compared the electromigration reliability of SnAgCu flip chip solder joints with and without Cu columns. The solder joint size is about 90 micron and the Cu column has a height of 60 mum. For the copper column solder joint itself, we introduced two types of copper column joint structures: (1) The solder contacts the Cu column at bottom and side walls and (2) Only the bottom surface of the copper column joints with solder. In the solder bump where electrons from chip side to board side, we saw much more IMC formation in the solder joint. The solder joint can even be totally converted to IMC. We propose that when the copper column was converted to IMC, there is a redistribution of current density. To discuss the electromgration reliability of an intermetallic compound joint, we take Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> as an example to calculate the ldquocritical productrdquo and found that the current density needed to cause electromigration damage for Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> IMC is at least one order of magnitude bigger than Sn-based solder.

Prediction of grain structure evolution during rapid solidification of high energy density beam induced re-melting

This paper makes finite element analysis of fire resistance under different connections for portal steel frames. Fire protection not apply on the secondary beam, there are two ways of girder-column's connection to calculate: beam connection, and pin connection. The results of the analysis indicate that the mechanical behavior of steel frames in fire conditions is dominated by the effects of thermal loading, thermal expansion and thermal gradient. The overall deformation of pin connection is larger than it of beam connection, and the stress suffered by beam structure is bigger than pin structure. When the overall beam steel frames' connection is taken by local pin connection, the stress of structure will reduce, helping to ensure strength and stability of steel structures. The deformation of the girder can be reduced when the beam connection is taken place by pin connection. The conclusion provides a reference for structural engineers on fire resistant design of steel frame.

Combining two or more different types of fibers within a polymer matrix is defined as hybrid composites. The requirements for an impressive hybrid composites include compatible weight ratio, good compressive strength, low-cost production and ease of fabrication. Moreover, hybrid composites provide a combination of remarkable mechanical properties including tensile modulus, compression, impact strength, flexural and hardness, which are comparable with the metal-based product materials. Recently, hybrid composites have been established due to their remarkable performance and efficiency. This study presents a facile analysis of polypropylene (PP) reinforced benzoyl treated kenaf and sugar palm fibers for handgrip prototype application. The materials were further analyzed for their materials molding modeling, and facile mechanical behavior including flexural strength, flexural modulus, Rockwell hardness (HRF), and also weight reduction percentage test. Hybrid composite T-SP7K3 shows higher flexural strength and flexural modulus compared to the ABS (Perodua Axia) at 339.5 MPa and 17.19 MPa, respectively. In addition, the HRF testing shows a higher value of hybrid composite (at 92.96 N/mm2) compared with the ABS handgrip with. Furthermore, the weight reduction percentage also recorded hybrid composite T-SP7K3 with the highest value at 22.7%.