Inclusion phases and the nucleation of acicular ferrite in submerged arc welds in high strength low alloy steels

Series of submerged arc welds of HSLA steel made with three different fluxes and metallic additions of Ti, Mo, and Cr have been examined to study the inclusions and their role in the nucleation of acicular ferrite. Inclusion phases and compositions have been analyzed by electron diffraction and X-ray microanalysis. These analyses have shown that the inclusions contained many different compounds, the proportions of each depending upon both the flux and metallic additions. Six inclusion phases have been identified: galaxite (Al₂O₃ ⋅ MnO), a titanium-rich compound (probably TiO), a copper sulfide, a manganese sulfide, a silica, and an aluminum-rich phase. No correlation was found between the amount of acicular ferrite in the weld metal and either average inclusion composition or individual inclusion phases. No epitaxial relationships between inclusions and adjacent ferrite grains could be identified. It has been concluded that inclusions nucleate acicular ferrite by acting as inert substrates according to the classical theory of heterogeneous nucleation. Because most inclusions are multi-phase and are touched by several ferrite grains, it has also been concluded that each inclusion can nucleate several ferrite grains, due to local regions of high surface energy on the inclusion.     

Surface-related phenomena in low amplitude fatigue of fcc metals

Perusal of the literature on fundamental aspects of the influence of the surface on fatigue in face centred cubic (fcc) metals shows that although understanding has moved forward, progress was hampered by such obstacles as lengthy semantic arguments, apparently conflicting observations, and certain aspects of the fatigue process itself. Most of the recent fundamental work has been carried out on single crystals, usually copper, fatigued in the low amplitude region; it is assumed that readers are familiar with this system. The following represents an outline of some of the authors' views on the role of the surface in fatigue; a wider account of their views on fatigue in fcc metals appears in a recent review article [1].     

高热导率低热膨胀系数Cu-ZrW_2O_8复合材料的制备与性能

以负热膨胀材料ZrW2O8与金属Cu为原料,分别采用常规烧结法和热压法制备具有高热导率低膨胀系数的新型Cu基复合材料Cu-ZrW2O8,研究ZrW2O8体积分数与烧结方法对该复合材料致密度、热导率及热膨胀系数的影响.结果表明:热压法制备的Cu-50%ZrW2O8复合材料的热导率达173.3 W/(m·K),致密度为91.6%,均明显高于常规烧结样品;热压样品的热膨胀系数为11.2×10-6K-1,稍高于常规烧结样品:在150～300℃温度范围内热处理后该样品的平均热膨胀系数降低到10.87×10 -6K-1,较纯Cu的平均热膨胀系数17×10-6K-1低很多,有望成为一种新型的电子封装材料.     

The influence of gas injection arc stabilization and electrode consumption in electric furnaces

Abstract

By means of relatively simple modifications a conventional arc furnace can be made to produce a long, stable plasma flame. This technique may be applied to scrap melting in the steel industry.

The results of laboratory experiments and plant trials, using à 3-ton production furnace, are discussed with respect to electrical characteristics, and potential refractory , electrode, and power savings

Résumé

Au moyen de modifications relativement simples, un four à arc conventionnel peut produire une flamme de plasma longue et stable.

Cette technique peut être appliquée à la fusion de la ferraille dans l'industrie de l'acier.

Les resultats d'expériences de laboratoire et d'essais en usine sur un four de production de 3 tonnes sont discutes par rapport aux caractéristiques électriques et du point de vue des réfractaires, des electrodes et de l'économie de puissance.     

Mechanism and optimization of oxide fluxes for deep penetration in gas tungsten arc welding

Five single oxide fluxes—Cu₂O, NiO, SiO₂, CaO, and Al₂O₃—were used to investigate the effect of active flux on the depth/width ratio in SUS304 stainless steel. The flux quantity, stability, and particlesize effect on the weld-pool shape and oxygen content in the weld after welding was studied systematically. The results showed that the weld depth/width ratio initially increased, followed by a decrease with the increasing flux quantity of the Cu₂O, NiO, and SiO₂ fluxes. The depth/width ratio is not sensitive to the CaO flux when the quantity is over 80×10⁻⁵ mol on the 5×0.1×50 mm slot. The Al₂O₃ flux has no effect on the penetration. The oxygen content dissolved in the weld plays an important role in altering the liquid-pool surface-tension gradient and the weld penetration. The effective range of oxygen in the weld is between 70 and 300 ppm. A too-high or too-low oxygen content in the weld pool does not increase the depth/width ratio. The decomposition of the flux significantly depends on the flux stability and the particle size. Cu₂O has a narrow effective flux-quantity range for the deep penetration, while the Al₂O₃ flux has no effect. The SiO₂ flux with a small particle size (0.8 or 4 μm) is a highly recommended active flux for deep penetration in actual gas tungsten arc welding (GTAW) applications.     

Cavity microstructure and kinetics during gas tungsten arc welding of helium-containing stainless steel

Helium was implanted in type 316 stainless steel, through tritium decay, to levels of 0. 18, 2. 5, 27, 105, and 256 atomic parts per million (appm). Bead-on-sheet welds were then made using the gas tungsten arc (GTA) process. Intergranular cracking occurred in the heat-affected zones (HAZs) of specimens with helium concentrations equal to or greater than 2.5 appm. No such cracking was observed in helium-free control specimens or in specimens containing the lowest helium concentration. In addition to the HAZ cracking, brittle, centerline cracking occurred in the fusion zone of specimens containing 105 and 256 appm helium. Transmission and scanning electron microscopy results indicated that both the HAZ cracking and centerline cracking in the fusion zone resulted from the stress-induced growth and coalescence of cavities initiated at helium bubbles on interfaces. For the HAZ case, the cavity growth rate is modeled and shown to predict the experimentally measured 1-second time lag between peak weld temperature and the onset of cracking.     

Mathematical modeling of the dynamic behavior of gas tungsten arc weld pools

The dynamic behavior of stationary fully penetrated gas tungsten arc weld pools was investigated through numerical simulation. The effects of arc pressure, electromagnetic force, and surface tension gradients on surface depression, convection, and temperature distribution were calculated. The top surfaces of fully penetrated pools were easily depressed since they were only supported by surface tension. Circulatory convection patterns were generated by electromagnetic forces and surface tension gradients and were significantly affected by the vertical velocity component produced by pool oscillation. The temperature distribution in and around the pool was influenced by pool convection. During pool formation and growth, the fully penetrated molten pool sagged dramatically when the bottom pool diameter approached the top diameter. The sagged pool oscillated with higher magnitude and lower frequency than partially penetrated or fully penetrated pools before sagging occurred. The dynamic behavior and the amount of material lost during melt-through were affected by the pool size and the magnitude of arc pressure.     

Evolution of microstructure and texture in Mg-Al-Zn alloys during electron-beam and gas tungsten arc welding

The evolution of microstructure and texture in the AZ-series Mg alloys subjected to electron-beam welding and gas tungsten arc welding are examined. Electron-beam welding is demonstrated to be a promising means of welding delicate Mg plates, bars, or tubes with a thickness of up to 50 mm; gas tungsten arc welding is limited to lower-end thin Mg sheets. The grains in the fusion zone (FZ) are nearly equiaxed in shape and ∼8 μm or less in size, due to the rapid cooling rate. The as-welded FZ microhardness and tensile strength are higher than the base metals due to the smaller grain size. The weld efficiency, defined as the postweld microhardness or tensile strength at the mid-FZ region divided by that of the unwelded base metal, is around 110 to 125 pct for electron-beam welding and 97 to 110 pct for gas tungsten arc welding. There are three main texture components present in the electron-beam-welded (EBW) FZ, i.e., (with TD// ), (with ND_∧ ∼15 deg), and (with WD_∧ ∼30 deg), where TD, ND, and WD are the transverse, normal, and welding directions, respectively. The crystal growth tends to align toward the most closed-packed direction, . The texture in gas tungsten arc welded (GTAW) specimens is more diverse and complicated than the EBW counterparts, due to the limited and shallow FZ and the lower cooling rate. The cooling rates calculated by the three-dimensional (3-D) and two-dimensional (2-D) heat-transfer models are considered to be the lower and upper bounds. The cooling rate increases with decreasing Al content, increasing weld speed, and increasing distance from the weld top surface. The influences of the FZ location, welding speed, and alloy content on the resulting texture components are rationalized and discussed.     

Fluid flow and weld penetration in stationary arc welds

Weld pool fluid flow can affect the penetration of the resultant weld significantly. In this work, the computer simulation of weld pool fluid flow and its effect on weld penetration was carried out. Steady-state, 2-dimensional heat and fluid flow in stationary arc welds were computed, with three driving forces for fluid flow being considered: the buoyancy force, the electromagnetic force, and the surface tension gradient at the weld pool surface. The computer model developed agreed well with available analytical solutions and was consistent with weld convection phenomena experimentally observed by previous investigators and the authors. The relative importance of the influence of the three driving forces on fluid flow and weld penetration was evaluated, and the role of surface active agents was discussed. The effects of the thermal expansion coefficient of the liquid metal, the current density distribution in the workpiece, and the surface tension temperature coefficient of the liquid metal on weld pool fluid flow were demonstrated. Meanwhile, a new approach to free boundary problems involving simultaneous heat and fluid flow was developed, and the effort of computation was reduced significantly.     

Fluid flow and weld penetration in stationary arc welds

Weld pool fluid flow can affect the penetration of the resultant weld significantly. In this work, the computer simulation of weld pool fluid flow and its effect on weld penetration was carried out. Steady-state, 2-dimensional heat and fluid flow in stationary arc welds were computed, with three driving forces for fluid flow being considered: the buoyancy force, the electromagnetic force, and the surface tension gradient at the weld pool surface. The computer model developed agreed well with available analytical solutions and was consistent with weld convection phenomena experimentally observed by previous investigators and the authors. The relative importance of the influence of the three driving forces on fluid flow and weld penetration was evaluated, and the role of surface active agents was discussed. The effects of the thermal expansion coefficient of the liquid metal, the current density distribution in the workpiece, and the surface tension temperature coefficient of the liquid metal on weld pool fluid flow were demonstrated. Meanwhile, a new approach to free boundary problems involving simultaneous heat and fluid flow was developed, and the effort of computation was reduced significantly.     

Aging characteristics of electron beam and gas tungsten arc fusion zones of Al-Cu-Li alloy 2090

A transmission electron microscopy (TEM) investigation of the electron beam (EB) and gas tungsten arc (GTA) fusion zones of 2090 indicates that in both the as-welded and aged conditions, the EB and GTA fusion zones lack the volume fraction and the homogeneity of strengthening precipitates found in the base metal. In the underaged and peak-aged conditions, the δ′ phase is the primary strengthener, the volume fraction ofT ₁ present being too low to be effective. TheT ₁, precipitates are found either in the vicinity of other inclusions or at the dendrite boundaries. As the strength increases with postweld aging, the elongation decreases to 1 pct. The presence of the boundary phases and Cu- and Cl-containing inclusions at the boundaries leads to poor elongation. The joint efficiencies of the peak-aged EB and GTA weldments (EBWs and GTAWs, respectively) are 75 and 55 pct at 293 K and 75 and 50 pct at 77 K, respectively. Both EBWs and GTAWs have relatively low elongations.     

纳米尺度孔隙内气体导热系数的分子动力学模拟

应用Lennard-Jone作用势,在300K和0.1MPa条件下,对边长20nm的立方体孔隙内氮气的导热系数进行了平衡分子动力学模拟.结果得出分子分速度和速率的分布与统计力学得到的Maxwell速度和速率分布曲线基本一致,并且分子的平均自由程受到孔隙壁的严格限制.通过Green-Kubo关系式计算得出了孔隙内氮气的导热系数,并与文献中的结果进行了比较,模拟结果接近于实验值,仅为同样条件下自由空间氮气的导热系数的1/3左右.     

铁铌精矿高温导热系数测定

在热平衡的基础上,根据一维稳态导热理论,测定了白云鄂博铁铌精矿在不同温度下的导热系数,回归得出了该材料在高温状态下导热系数λ与工作温度T之间的关系方程.     

高导热金属基复合材料的制备与研究进展

随着电子器件芯片功率的不断提高,对散热材料的热物理性能提出了更高的要求.将高导热、低膨胀的增强相和高导热的金属进行复合得到的金属基复合材料,能够兼顾高的热导率和可调控的热膨胀系数,是理想的散热材料.本文对以Si、SiCp、金刚石、鳞片石墨为增强相的铜基及铝基复合材料的研究进展进行了总结,并就金属基复合材料目前存在的问题...     

Laws of the DC arc in an inert gas during melting in furnaces with a tungsten electrode

The I?CV and voltage-baric characteristics of the dc arc glowing in argon between a tungsten cathode and a molten anode made of titanium, chromium, or manganese are studied at various gas pressures, arc currents, and arc lengths. The arc is probed to establish the relation between the voltage drop across the arc and its regions on the anode material and the melting conditions.     

高导热氮化铝陶瓷的粉末注射成形技术

氮化铝(AlN)陶瓷具有热导率高、热膨胀系数低、电阻率高等特性以及良好的力学性能,被认为是新一代高性能陶瓷基片和封装的首选材料,为了满足微电子技术发展对微型复杂形状高导热陶瓷零部件用量日益增加的需求,该文作者研究利用粉末注射成形技术制备高导热AlN陶瓷零部件.该技术以AlN粉末为原料,加入5%Y2O3为烧结助剂;选用蜡基粘结剂体系(PW+PP+SA),确定粉末装载量为62%(体积分数),注射温度为160～170℃,注射压力为90～100 MPa;采用溶剂脱脂+热脱脂工艺脱脂;在1850℃流动氮气氛中烧结.所制备出的AIN陶瓷热导率达232.4 W/(m·K).