Trailing heat sink effects on residual stress and distortion of pulsed laser welded Hastelloy C-276 thin sheets

A three-dimensional finite element model (FEM) was established to reveal the thermal-mechanical behaviors of pulsed laser welding (PLW) with and without trailing heat sink. Experiments were carried out to measure the welding temperature histories, residual distortions and solidification profiles. The simulation results agree well with the corresponding experimental measurements. The peak values of the temperature and transient longitudinal tensile stresses valleys in the weld increase as the cooling intensity increases from 5000 to 15,000 W/(m² K), while those of the temperature and transient longitudinal compressive stresses near the weld decrease. The peak values of the longitudinal residual compressive stresses and plastic strains, and the maximum deflections in longitudinal and transverse direction decrease as the cooling intensity increases from 5000 to 15,000 W/(m² K). The magnitudes of the transverse shrinkage distortions increase as the cooling intensity increases from 5000 to 15,000 W/(m² K). The proper cooling intensity to reduce the residual stresses and distortions of the PLW with the trailing heat sink is detected at 10,000 W/(m² K). The trailing heat sink is technically feasible for actual pulsed laser restraint welding in Hastelloy C-276 thin sheet structures.     

Experimental determination of heat transfer coefficients during squeeze casting of aluminium

Heat transfer coefficients during squeeze cast of commercial aluminium were determined, by using the solidification temperature versus time curves obtained for varying applied pressures during squeeze casting process. The steel mould/cast aluminium metal interface temperatures versus times curve obtained through polynomial curves fitting and extrapolation was compared with the numerically obtained temperatures versus times curve. Interfacial heat transfer coefficients were determined experimentally from measured values of heating and cooling temperatures of steel mould and cast metal and compared with the numerically obtained values and found to be fairly close in values. The values of the heat transfer coefficients were found to increase with increase in applied pressures and to decrease with fall of solidifying temperatures corresponding to three distinct solidification stages namely, liquidus, liquidus–solidus and solidus stages. Below temperatures of 500 °C, the effect of the increase in heat transfer coefficients with applied pressure application becomes less significant and the drop in values of the heat transfer coefficients with solidification temperatures at any applied pressures remains fairly constant. The peak values of heat transfer coefficients obtained for as-cast (no pressure application) and squeeze cast (pressure application) of aluminium are 2975.14 and 3351.08 W/(m² K), respectively. Empirical equations, relating the interfacial heat transfer coefficients to the cast aluminium surface temperatures and applied pressures at three distinct temperature range intervals, were also derived and presented.     

Effect of Cr7C3 on the mechanical, thermal, and electrical properties of Cr2AlC

In this work, phase pure Cr₂AlC and impure Cr₂AlC with Cr₇C₃ have been fabricated to investigate the mechanical, thermal, and electrical properties. The thermal expansion coefficient is determined as 1.25 × 10⁻⁵ K⁻¹ in the temperature range of 25-1200 °C. The thermal conductivity of the Cr₂AlC is 15.73 W/m K when it is measured at 200 °C. With increasing temperature from 25 °C to 900 °C, the electrical conductivity of Cr₂AlC decreases from 1.8 × 10⁶ Ω⁻¹ m⁻¹ to 5.6 × 10⁵ Ω⁻¹ m⁻¹. For the impure phase of Cr₇C₃, it has a strengthening and embrittlement effect on the bulk Cr₂AlC. And the Cr₂AlC with Cr₇C₃ would result in a lower high-temperature thermal expansion coefficient, thermal conductivity, specific heat capacity and electrical conductivity.     

Investigation of interface reaction between TiAl alloys and mold materials

This paper describes the investment casting of TiAl alloys. The effects of mold mateiral and mold preheating temperature for the investment casting of TiAl on metal-mold interfacial reaction were investigated by means of optical micrography, hardness profiles and an electron probe microanalyzer. The mold materials examined were colloidal silica bonded ZrO₂, ZrSiO₄, Al₂O₃ and CaO stabilized ZrO₂. When compared with conventional titanium alloy, the high aluminum concentration of TiAl alloys helps to lower their reactivity in the molten state. The Al₂O₃ mold is a promising mold material for the investment casting of TiAl in terms of the thermal stability, formability and cost. Special attention need to be paid to thermal stability and mold preheating when developing the investment casting of TiAl alloys.     

Fatigue characteristic and pyroelectric properties of highly (1 1 1)-oriented Nb doped Pb(Zr0.2,Ti0.8)O3 thin films with Pb0.8La0.1Ca0.1Ti0.975O3 seed layer prepared by a sol-gel route

A series of highly (1 1 1)-oriented tetragonal Nb-doped Pb(Zr_0.2Ti_0.8)O₃ (PNZT) films with and without Pb_0.8La_0.1Ca_0.1Ti_0.975O₃ (PLCT) seed layer were deposited on the Pt(1 1 1)/Ti/SiO₂/Si substrates by sol-gel processing; it was found the pyroelectric properties and fatigue resistance characteristics of PNZT films could be improved by introducing PLCT seed layer. Because the rough surface structures of 5 nm-thick PLCT seed layer can offer nucleation sites to reduce activation energy for the crystallization and lead to the polarization response easily, a large pyroelectric coefficient (460 μC/(m² K)) and a high figure-of-merit (F_d = 161 μC/(m² K)) was obtained for PLCT/PNZT/PLCT structure film. It was also found that PLCT seed layer could act as a capacitive interface layer possibly compensating for the vacancy-type defects from PNZT film effectively, which results in enhanced fatigue resistance characteristics of PLCT/PNZT/PLCT structure film.     

Aluminum alloy pistons reinforced with SiC fabricated by centrifugal casting

Hypereutectic Al-Si alloy-based composite pistons reinforced with SiC particles locally at the head were fabricated by centrifugal casting. The effects of various technique parameters, i.e., the slurry temperature of the alloy, the mold temperature and the rotation speed of the mold, on the particle segregation were investigated, and the macromorphologies and microstructures of pistons were observed. The mechanical properties, such as hardness and wear resistance along the axis of the piston and the thermal expansion coefficient at the piston head, were measured. The results showed that, (1) centrifugal casting can be used as a new and effective method in manufacturing pistons, and reasonable parameters were 850 °C, 600 °C and 800 rpm for the slurry temperature of the alloy, the mold temperature and the rotation speed of the mold, respectively; (2) the hardness values of pistons fabricated by centrifugal casting gradually increased from the piston skirt to the piston head, and the average hardness value in piston heads was improved by 23.7HRB over that of pistons fabricated by gravity permanent mold casting; (3) the piston heads showed excellent wear resistance, and the wear rate of piston heads decreased by 70.3% over that of the piston fabricated by gravity permanent mold casting; and (4) the average linear expansion coefficient of the piston head was 15.3 × 10⁻⁶ K⁻¹ and decreased by 23.1% over that of pistons fabricated by gravity permanent mold casting.     

Thermoelectric properties of Cu1−xPtxFeO2 (0.0 ≤ x ≤ 0.05) delafossite-type transition oxide

The samples of Cu_1−xPt_xFeO₂ (0 ≤ x ≤ 0.05) delafossite were synthesized by solid state reaction method for studying thermoelectric properties. The properties of Seebeck coefficient, electrical conductivity and thermal conductivity were measured in the high temperature ranging from 300 to 960 K. The results of Seebeck coefficient, electrical conductivity and power factor were increased with increasing Pt substitution and temperature. The thermal conductivity was decreased from 5.8 to 3.5 W/mK with increasing the temperature from 300 to 960 K. An important results, the highest value of power factor and ZT is 2.0 × 10⁻⁴ W/mK² and 0.05, respectively, for x = 0.05 at 960 K.     

Synthesis, thermal stability and properties of [(Fe1−xCox)72Mo4B24]94Dy6 bulk metallic glasses

A series of [(Fe_1−xCo_x)₇₂Mo₄B₂₄]₉₄Dy₆ (x = 0.1, 0.2, 0.3, 0.4 and 0.5 at.%) bulk metallic glasses (BMGs) in rod geometries with critical diameter up to 3 mm were fabricated by copper mold casting method. This alloy system exhibited good thermal stability with high glass transition temperature (T_g) 860 K and crystallization temperature (T_x) 945 K. The addition of Co was found to be effective in adjusting the alloy composition deeper to eutectic, leading to lower liquidus temperature (T_l). The [(Fe_0.8Co_0.2)₇₂Mo₄B₂₄]₉₄Dy₆ alloy showed the largest supercooled liquid region (ΔT_x = T_x − T_g = 92 K), reduced glass transition temperature (T_rg = T_g/T_l = 0.622) and gamma parameter (γ = T_x/(T_g + T_l) = 0.424) among the present system. Maximum compressive fracture strength of 3540 MPa and micro-Vickers hardness of 1185 kg/mm² was achieved, resulting from the strong bonding structure among the alloy constituents. The alloy system possessed soft magnetic properties with high saturation magnetization of 56.61-61.78 A m²/kg and coercivity in the range of 222-264.2 A/m, which might be suitable for application in power electronics devices.     

Mechanical and thermal properties of Tb2(MoO4)3 crystals

The mechanical and thermal properties of single crystal Tb₂(MoO₄)₃ have been systematically studied. The result of microhardness measurement indicates that the crystal belongs to the soft materials category. The thermodynamic parameters obtained from DTA analysis were used for determining the type of liquid-solid interface during crystal growth. Negative thermal expansion along the c-axis was observed, and this behavior was attributed to the bent Tb-O-Mo bonds. The specific heat of the crystal was measured to be 0.122 cal g⁻¹ K⁻¹ at 293.15 K. The thermal conductivity of Tb₂(MoO₄)₃ at room temperature was found to be smaller than that of representative ferroelectric LiNbO₃.     

Analysis of the non-uniform thermal behavior in slab continuous casting mold based on the inverse finite-element model

A full finite-element model of a mold, including four copper plates, nickel layer and water slots of different depths, was developed to reveal the complex thermal behavior of molds in slab continuous casting. An inverse algorithm was applied to calculate the heat flux and combined with the temperatures measured using thermocouples that were buried in different positions of the mold. The temperature distributions of the four copper plates are not symmetric, reflecting the non-uniform nature of heat transfer and the necessity of building a full model. The maximum hot surface temperature of the mold occurs in the region 70–100 mm below the meniscus. At heights 100 mm below the meniscus, the average temperature of the deep water slot root is higher than that of the shallow water slot by approximately 10 °C. In the off-corner regions, the hot surface temperature differences between the narrow face and wide face near the corners are approximately 20–30 °C 100 mm below the meniscus. It will provide a helpful tool for further improving the casting parameters and operations.     

Electrical and thermal characterization of Sm doped ceria electrolytes synthesized by combustion technique

Nanocrystalline samarium doped ceria electrolyte [Ce_0.9Sm_0.1O_1.95] was synthesized by citrate gel combustion technique involving mixtures of cerium nitrate oxidizer (O) and citric acid fuel (F) taken in the ratio of O/F = 1. The as-combusted precursors were calcined at 700 °C/2 h to obtain fully crystalline ceria nano particles. It was further made into cylindrical pellets by compaction and sintered at 1200 °C with different soaking periods of 2, 4 and 6 h. The sintered ceria was characterized for the microstructures, electrical conductivity, thermal conductivity and thermal diffusivity properties. In addition, the combustion derived ceria powder was also analysed for the crystallinity, BET surface area, particle size and powder morphology. Sintered ceria samples attained nearly 98% of the theoretical density at 1200 °C/6 h. The sintered microstructures exhibit dense ceria grains of size less than 500 nm. The electrical conductivity measurements showed the conductivity value of the order of 10⁻² S cm⁻¹ at 600 °C with activation energy of 0.84 eV between the temperatures 100 and 650 °C for ceria samples sintered at 1200 °C for 6 h. The room temperature thermal diffusivity and thermal conductivity values were determined as 0.5 × 10⁻⁶ m² s⁻¹ and 1.2 W m⁻¹ K⁻¹, respectively.     

Thermoelectric properties of indium-filled InxRh4Sb12 skutterudites

This study reports the synthesis and characterization of polycrystalline indium-filled In_xRh₄Sb₁₂ (0 ≤ x ≤ 0.2) skutterudites. The structural response to indium filling was monitored by whole pattern fitting of the powder X-ray diffraction data. Indium occupation of the oversized void-sites was verified by its unusually large thermal displacement parameter. The indium solubility limit approached 0.15. The principal thermoelectric properties were measured from 300 to 600 K. All samples are semiconducting. Indium void-site occupation reduced the lattice thermal conductivity of In_0.15Rh₄Sb₁₂ 30% at 300 K; however, the effect was subverted at elevated temperatures due to a coincident increase in bipolar thermal diffusion. The high-temperature thermoelectric figure of merits (ZT's) are low compared to the isostructural indium-filled In_xCo₄Sb₁₂ skutterudites due to a striking sign change in the Seebeck coefficients at 400 K and relatively high thermal conductivities.     

Preparation of Ni0.5Zn0.5Fe2O4/SiO2 nanocomposites and their adsorption of bovine serum albumin

The magnetic nanocomposites of (1 − x)Ni_0.5Zn_0.5Fe₂O₄/xSiO₂ (x = 0-0.2) were synthesized by the citrate-gel process and their absorption behavior of bovine serum albumin (BSA) was investigated by UV spectroscopy at room temperature. The gel precursor and resultant nanocomposites were characterized by FTIR, XRD, TEM and BET techniques. The results show that the single ferrite phase of Ni_0.5Zn_0.5Fe₂O₄ is formed at 400 °C, with high saturation magnetization and small coercivity. A porous, amorphous silica layer is located at the ferrite nanograin boundaries, with the silica content increasing from 0 to 0.20, the average grain size of Ni_0.5Zn_0.5Fe₂O₄ calcined at 400 °C reduced from about 18-8 nm. Consequently, the specific surface area of the nanocomposites ascends clearly with the increase of silica content, which is largely contributed by the increase in the thickness of the porous silica layer. The Ni_0.5Zn_0.5Fe₂O₄/SiO₂ nanocomposites demonstrate a better adsorption capability than the bare Ni_0.5Zn_0.5Fe₂O₄ nanoparticles for BSA. With the increase of the silica content from 0 to 0.05 and the specific surface area from about 49-57 m²/g, the BSA adsorption capability of the Ni_0.5Zn_0.5Fe₂O₄/SiO₂ nanocomposites calcined at 400 °C improve dramatically from 22 to 49 mg/g. However, with a further increase of the silica content from 0.05 to 0.2, the specific surface area increase from about 57-120 m²/g, the BSA adsorption for the nanocomposites remains around 49 mg/g, owing to the pores in the porous silica layer which are too small to let the BSA protein molecules in.     

不同铸造方法对7075铝合金热导率及力学性能的影响

采用金属型铸造、液态挤压铸造和半固态挤压铸造方法制备了7075铝合金,研究了不同铸造工艺对7075铝合金热导率与力学性能的影响。结果表明,金属型铸造晶粒粗大,产生枝晶偏析降低塑韧性,抗拉强度及伸长率最小,分别为121 MPa和2.78%,但晶粒粗大使热量传导路径宽,对电子散射几率小,电子的平均自由程较长,热导率相对较高,达到了139.67W/(m·K);液态挤压铸造晶粒细化,抗拉强度和伸长率分别为239MPa和5.75%,但晶粒细小且枝晶臂较多,对电子散射程度大,热导率最低,为120.94W/(m·K);半固态挤压铸造的晶粒致密细小且圆整,抗拉强度及伸长率最高分别达到248MPa和7.46%,且热导率为126.07W/(m·K)。     

Moderate temperature and high-speed synthesis of α-Al2O3 films by laser chemical vapor deposition using Nd:YAG laser

Al₂O₃ films were prepared at deposition temperatures (T_dep) from 980 to 1230 K by laser chemical vapor deposition (LCVD) using the continuous wave of a Nd:YAG laser with laser power (P_L) up to 260 W. γ-Al₂O₃ films were obtained at T_dep < 1100 K, whereas α-Al₂O₃ films were obtained at T_dep > 1100 K. γ-Al₂O₃ films were morphologically characterized by a cone-like structure, while α-Al₂O₃ films had hexagonal faceted grains. The highest deposition rate (R_dep) of γ-Al₂O₃ film was 570 μm h^− 1, while that of α-Al₂O₃ film was 250 μm h^− 1. α-Al₂O₃ films in a single phase were obtained at 170 K lower in T_dep and 100 times higher in R_dep than those by conventional thermal CVD.     

Determination of heat transfer coefficients by extrapolation and numerical inverse methods in squeeze casting of magnesium alloy AM60

In this work, a different wall-thickness 5-step (with thicknesses as 3, 5, 8, 12, 20 mm) casting mold was designed, and squeeze casting of magnesium alloy AM60 was performed under an applied pressure 30, 60 and 90 MPa in a hydraulic press. The casting-die interfacial heat transfer coefficients (IHTC) in the 5-step casting were determined based on thermal histories throughout the die and inside the casting which were recorded by fine type-K thermocouples. With measured temperatures, heat flux and IHTCs were evaluated using the polynomial curve fitting method and numerical inverse method. For numerical inverse method, a solution algorithm was developed based on the function specification method to solve the inverse heat conduction equations. The IHTCs curves for five steps versus time were displayed. As the applied pressures increased, the IHTC peak value of each step was increased accordingly. It can be observed that the peak IHTC value decreased as the step became thinner. Furthermore, the accuracy of these curves was analyzed by the direct modeling calculation. The results indicated that heat flux and IHTCs determined by the inverse method were more accurately than those from the extrapolated fitting method.     

Photoluminescence analysis of Cd0.8Mn0.2Te single crystal grown by the vertical Bridgman method

In this paper, the photoluminescence (PL) spectra were detected to characterize Cd_0.8Mn_0.2Te crystals grown by the vertical Bridgman method. Several monocrystalline (1 1 1) face plates with dimension of 30 × 40 × 2 mm³ were sliced from the as-grown Cd_0.8Mn_0.2Te ingot, where the PL analysis was conducted. The PL spectra revealed the neutral acceptor bound exciton (A⁰, X) peaks and the donor-acceptor pair (DAP) peaks. The activation energies derived from the temperature-dependent PL quenching of the (A⁰, X) peak was estimated to be 5.3 meV and 45.8 meV, which suggested the binding energy of excitons to acceptors at lower temperatures and the Auger recombination at higher temperatures. A relationship for the bandgap energy of Cd_0.8Mn_0.2Te as a function of temperature ranging from 10 K to 300 K was obtained. The composition uniformity of the (1 1 1) face Cd_0.8Mn_0.2Te plates were analyzed with PL spectra, which shown that the variation of Mn concentration is within 0.012 mole fraction.     

Evolution of structural and optical properties in the course of thermal evolution of sol-gel derived cobalt-doped gahnite

Thermal evolution of sol-gel derived gahnite (ZnAl₂O₄) with 4, 8 and 12 at.% of Zn replaced with Co was studied by thermal analysis techniques (DTA/TGA), X-ray diffraction (XRD) and UV-vis diffuse reflectance spectroscopy (DRS). Zinc-cobalt spinel powders were produced by gel heat treatment at temperatures as low as 400 °C. Crystal structure was characterized using Rietveld refinement of X-ray diffraction patterns for the samples annealed at 800 °C, simultaneously with the analysis of diffraction line broadening. It was found out that the distribution of Co²⁺ ions in tetrahedral and octahedral sites of zinc cobalt aluminate crystal lattice, crystallite size and lattice strain depend on Co loading. The green color of samples thermally treated at T < 800 °C has been explained as a consequence of partial oxidation of Co²⁺ ions at lower temperatures and accommodation of Co³⁺ ions in octahedral sites. Thermal treatment at higher temperatures promote gradual change of color from green to blue, characteristic for tetrahedrally coordinated Co²⁺ ions. The spectra evolution could be interpreted as a progressive reduction of Co³⁺ to Co²⁺ ions at higher temperatures.     

Mullite-rich plasma electrolytic oxide coatings for thermal barrier applications

A study has been undertaken of the characteristics exhibited by mullite-rich plasma electrolytic oxide coatings grown on aluminium alloys by using silicate-rich electrolytes. It is found that they can be grown at a higher rate, and to a greater thickness, than alumina PEO coatings on aluminium. The thermal conductivity of these coatings has been measured using a steady-state method. It is shown to be of the order of 0.5 W m^− 1 K^− 1, which may be compared with ∼ 1.5 W m^− 1 K^− 1 for pure alumina PEO coatings and ∼ 10-15 W m^− 1 K^− 1 for dense polycrystalline mullite. Coupled with excellent substrate adhesion and good mechanical properties, this relatively low conductivity makes these coatings attractive for thermal barrier applications. Furthermore, they are shown to exhibit a relatively low global stiffness (∼ 40 GPa), which will reduce the magnitude of thermally-induced stresses and improve the resistance to spallation during temperature changes.     

Optical and thermal properties of P2O5-Na2O-CaO-Al2O3:CoO glasses doped with transition metals

The aim of this paper is to report the optical and thermal properties of V₂O₅ and CuO doped P₂O₅-Na₂O-CaO-Al₂O₃:CoO glasses so as to investigate their possible use in solar collection applications. The optical absorption spectra of the glasses at room temperature were in the spectral range of 200-1100 nm. The optical band gaps of the glass samples were determined for direct and indirect transitions. When transition metal ions doped to the base glass, the optical band gap decreased. Changes in the refractive indices vs. wavelength for all the specimens were also examined by spectroscopic ellipsometry. By measuring the heat capacities and thermal diffusion coefficients of the specimens at varying temperatures, their thermal conductivities were calculated to be in the 320-620 K temperature range. The obtained glasses seem to be promising materials and can be used in solar collector applications.