Weld shape comparison with iron oxide flux and Ar–O2 shielding gas in gas tungsten arc welding

Abstract

The influence of iron oxide flux and O₂–Ar mixed shielding gas on weld shape and penetration in gas tungsten arc welding is investigated by bead-on-plate welding on SUS 304 stainless with low oxygen and low sulphur contents. The oxygen content in the weld metal is measured using a HORIBA EMGA-520 oxygen/nitrogen analyzer. The results show that both the iron oxide flux and the O₂–Ar mixed shielding gas can significantly modify the weld shape from shallow wide to deep narrow. A large weld depth/width ratio around of 0.5 is obtained when the oxygen content in the shielding gas is in the range of 3000–6000 vol. ppm. Oxygen over a certain critical value, i.e. 70 wt. ppm, in the weld pool alters the temperature coefficient of the surface tension on the pool surface, and hence changes the Marangoni convection. A thick oxide layer on the weld pool surface is generated when the oxygen content in the shielding gas is over 6000 vol. ppm, which becomes a barrier for the oxygen conveyance to the liquid pool and prevents the liquid pool from freely moving, and therefore, decreases the intensity of the Marangoni convection on the pool surface.     

Influence of pre- and post-weld heating on weldability of modified 9Cr–1Mo(V–Nb) steel pipe under shielded metal arc and tungsten inert gas welding processes

Abstract

Welding of modified 9Cr–1Mo(V–Nb) steel pipes has been carried out via shielded metal arc (SMA) and tungsten inert gas (TIG) welding processes. The weld joints have been produced using different preheating temperatures, followed by post-weld heat treatment (PWHT) at various temperatures. The microstructures of the weld and of the heat affected zone (HAZ) of the weld joints have been studied under the optical microscope and correlated with the preheating and PWHT. The average hardness of the weld and different regions of the HAZ, and tensile properties of the weld joints have also been studied and correlated with the preheating and PWHT. The tensile properties of the SMA and TIG weld joints produced using preheating and PWHT at various temperatures are compared and correlated with their microstructures. It is noted that a comparatively high preheating temperature of the order of 573 K is beneficial, and PWHT is necessary to reduce the susceptibility to cold cracking of weld joints of the present steel. The PWHT at 1123 K enhances ductility to fracture, but decreases the tensile strength of the base material, causing fracture of both the SMA and TIG weld joints from this region close to the HAZ. The tensile properties of SMA welds are found to be superior to those of the TIG welds, especially for PWHT at temperatures up to 1023 K.     

Development of torch position control and welding condition control technology for all‐position,multi‐layer GTA welding. Development of fully automatic GTA welding system for pipes (2nd Report)

In pulsed TIG welding, the current varies between two well-defined energy levels in a given frequency, thus being necessary to regulate a set of variables consisting of the peak current, peak time, background current, background time and the welding speed. Yet despite being a widespread technique, in practice, these welding variables are often regulated arbitrarily. This can lead to inefficient use of the pulsed current regarding the end result of the weld and aspects of productivity. This paper aims to present a roadmap developed in order to meet the practical need to establish criteria to assist in the determination of pulsed TIG welding variables, taking as its premise the desired width of the weld bead, overlap between the weld points comprising the weld bead and the welding speed. Finally, one application of this roadmap is presented in bead on plate welding of stainless steel plates with 1.2-mm thickness.     

Comparison between hexatriacontane and stearic acid behaviours under late Ar―O2 post-discharge

The transformations undergone in a late Ar―O₂ afterglow by the hexatriacontane, a long chain alkane, and the stearic acid, a C₁₈ alkane skeleton with an acid function, are compared. The diffusion of molecular oxygen is found to be the limiting step in the case of the HTC. When the SA is treated, this process is fast, likely because of the high diffusion coefficient of O₂ in the SA than in the HTC. Desorption of OH groups produced by the abstraction of one hydrogen from the alkane skeleton by an oxygen atom is proposed as the limiting step. The fragmentation process stands in the core of the material and creates by-products that are responsible for the appearance of bubbles whose mobility and coarsening depend on the viscosity of the treated material. Finally, by resorting to pulse mode, both the HTC and the SA can be etched whereas they are functionalized in the continuous mode where non-linear behaviours are observed.     

CO2 and Nd–YAG laser beam welding of 5754–O aluminium alloy for automotive applications

Abstract

Federal regulations have recently been enacted to reduce significantly the atmospheric pollution caused by motor vehicles. This has compelled automotive manufacturers to improve the fuel efficiency of cars and light trucks by using lightweight materials such as aluminium. The focus of the present work is to develop welding procedures for autogenous CO₂ (continuous mode) and Nd–YAG (continuous mode) laser beam welding of 5754–O aluminium alloy. The mechanical and microstructural characteristics of the welded joints were evaluated using tensile tests, microhardness tests, optical microscopy, and chemical analysis. Results indicate that this alloy can be autogenously laser welded with full penetration, minimum surface discontinuities, and little if any loss of magnesium through vaporisation from the fusion zone. The total elongation (all weld metal) in the longitudinal direction for 5754–O laser welds produced using 5 kW CO₂ and 3 kW continuous wave (CW) Nd–YAG shows a slight decrease with increasing travel speed. Studies indicate that the decreasing tendency is probably due to the orientation of the grains with respect to the loading direction. The welds produced using the 5 kW CO₂ laser at travel speeds between 127 and 212 mm s^-1 displayed a total longitudinal elongation of 19.13–15.12% and those produced using the 3 kW CW Nd–YAG laser at travel speeds between 85 and 148 mm s^-1 displayed a total longitudinal elongation of 22.6–18.15%, compared with the base metal value of 28.1%. An observation of great interest was that the weld surface condition did not have any effect on the ductility of the 5754–O aluminium alloy studied in the present investigation.     

Plasma Electrolytic Synthesis and Characteristics of WO3–FeO–Fe2O3 and WO3–FeO–Fe2(WO4)3 Heterostructures

Protection of Metals and Physical Chemistry of Surfaces - Fe- and W-containing oxide heterostructures were formed on a titanium surface by plasma electrolytic oxidation in an alkaline...     

Microstructures and mechanical properties of Ti–24Al–17Nb (at.%) laser beam welding joints

Laser beam welding of Ti–24Al–17Nb (at.%) alloy was conducted to investigate the microstructures and the mechanical properties of its joints. The results indicated that the weld microstructure consisted primarily of retained ordered β phase (namely B2 phase) and was independent of the laser welding parameters, while the size and the orientation of the weld solidification structures and then the bend ductility of the joints were related to the welding conditions. The microstructures became coarser and the strains of inducing crack and fracturing decreased as the heat input increased. The fracture occurred in the base metal when the transverse tensile test of the joints was conducted. The tensile strength of the joints was equal to that of the base material and the tensile ductility could reach 12∼17%, which was near to that of the base material.     

Processing and microstructure control of (α2+B2+O) alloy sheet in Ti–Al–Nb system

Rolling processing and microstructure evolution during rolling and heat treatment for two typical α₂+B2+O alloys, Ti–24Al–14Nb–3V and Ti–23Al–17Nb (at%), were investigated. The experimental results showed that the alloys have good workability for rolling at temperatures both in the α₂+B2 and in α₂+B2+O fields. The thickness reduction up to 99% was obtained for the sheets rolled above 900°C in a quasi-isothermal condition without cracking. A typical duplex microstructure of the α₂+B2+O phases formed when the sheets were rolled and then solution treated at temperatures in the α₂+B2 phase field plus aging in the O+B2 field. Such duplex microstructure was proved to have good mechanical properties both in room and elevated temperatures. A microstructure of fine equiaxed α₂ and O phases distributed in B2 matrix was obtained for the sheets rolled and then solution treated at the temperatures in the α₂+B2+O phase field, which possess excellent room temperature ductility and superplasticity in temperatures of 900∼1000°C. An advanced cold rolling processing plus a proper vacuum heat-treatment used for the production of high-quality foils of Ti–24Al–14Nb–3V (at%) alloy are also reported.     

Preparation and Characterization of K_n|Ga(H_2O)MW_(11)O_(39)|·xH_2O

In this paper the heteropoly complexes K_n[Ga(H_20)MW_(11)O_39)]·xH_20 [M=Cu(Ⅱ), Co(Ⅱ), Cr(Ⅲ),Fe(Ⅲ), Mn(Ⅳ)] have been prepared in this laboratory firstly. The products were characterized by chemicalanalysis, ionexchange, IR, UV and X-ray powder diffraction.     

Some Thermodynamic and Kinetic Properties of the HCl–H3PO4–H2O–Fe(III) System

Protection of Metals and Physical Chemistry of Surfaces - To characterize the oxidizing ability of the HCl—H3PO4–H2O–Fe(III) system, the electrode potentials of the Fe(III)/Fe(II)...     

Relaxor ferroelectric and dielectric properties of (1–x)Ba(Zr0.1Ti0.9)O3–xBa(Mg1/3Ta2/3)O3 ceramics

The environmentally-friendly (1–x)Ba(Zr_0.1Ti_0.9)O₃–xBa(Mg_1/3Ta_2/3)O₃ (x=0, 0.02, 0.04, 0.06, 0.08) relaxor ferroelectric ceramics were prepared by the conventional solid-state method and sintered in air at 1400 °C for 2 h. SEM and XRD analyses were utilized to study the surface morphologies and the crystalline structures, respectively. The effects of Ba(Mg_1/3Ta_2/3)O₃ on the phase transformation, dielectric and ferroelectric properties of Ba(Zr_0.1Ti_0.9)O₃ ceramics were also investigated. It is found that the average grain size of (1–x)Ba(Zr_0.1Ti_0.9)O₃–xBa(Mg_1/3Ta_2/3)O₃ (BZT–BMT) perovskite single-phase ceramics decreases as the content of Ba(Mg_1/3Ta_2/3)O₃ (BMT) increases. The relaxor ferroelectric behavior with diffuse phase transition and well-defined frequency dispersion of dielectric maximum temperature is found for the ceramic with increasing x values. 0.98BZT–0.02BMT ceramic shows very good dielectric properties with the relative permittivity and the dielectric loss, measured at 100 kHz as 6034 and 0.01399 respectively at room temperature. Both remnant polarization and coercive field decreased with increasing BMT content, indicating a transition from the ferroelectric phase to the paraelectric phase at room temperature.     

Dielectric properties and relaxor ferroelectric behavior of (1–y)Ba(Zr0.1Ti0.9)O3–yBa(Zn1/3Nb2/3)O3 ceramics

The microstructure, dielectric and ferroelectric properties of (1–y)Ba(Zr_0.1Ti_0.9)O₃–yBa(Zn_1/3Nb_2/3)O₃ (y=0–0.05) ceramics prepared by traditional solid state method were investigated by X-ray diffractometer, scanning electron microscope, electric parameter testing system and ferroelectric tester. It is found that the barium zirconate titanate based ceramics are single-phase perovskites as y increases up to 0.05 and their average grain size decreases with the increase of y. The permittivity maximum ε_r,max is suppressed from 8948 to 1611 at 1 kHz with increasing y, and the ferroelectric–paraelectric phase transition temperature T_m decreases from 93 to –89 °C at 1 kHz as y increases. The composition-induced diffuse phase transition is enhanced with increasing y. The relaxor-like ferroelectric behavior with a strong frequency dispersion of T_m and permittivity at T<T_m accompanied by a strong diffuse phase transition is found for the system with high y value. The remnant polarization decreases with increasing y, while the coercive field decreases remarkably and then increases with the increase of y.     

Alumina Scale Formation on a Powder Metallurgical FeCrAl Alloy (Kanthal APMT) at 900–1,100 °C in Dry O2 and in O2 + H2O

A Rapidly Solidified Powder (RSP) metallurgical FeCrAl alloy, Kanthal APMT, was exposed in dry and humid O₂ for 72 h at 900–1,100 °C. The formed oxide scales were characterized using gravimetry in combination with advanced analysis techniques (SEM, EDX, TEM, XRD, AES and SIMS). The oxide scales were at all exposures composed of two-layered α-Al₂O₃ scales exhibiting a top layer of equiaxed grains and a bottom layer containing elongated grains. A Cr-rich zone, originating in the native oxide present before exposure, separated these two layers. The top α-Al₂O₃ layer is suggested to have formed by transformation of outwardly grown metastable alumina, while the inward-grown bottom α-Al₂O₃ layer had incorporated small Zr-, Hf- and Ti-rich oxide particles present in the alloy matrix. The scale also contained larger Y-rich oxide particles. Furthermore, in the temperature range studied, the presence of water vapour accelerated alloy oxidation somewhat and affected scale morphology.     

Electrical conductivity and thermopower studies on Fex[Pt(C2O2)2]·6H2O (where x ⋍ 0.8)

The electrical conductivity (d.c. and 35 GHz) and thermopower of Fe_x[Pt(C₂O₄)₂]·6H₂O, FeOP, are presented and compared with those of other partially oxidized bis(oxalato)platinate salts of divalent cations. At room temperature FeOP is metallic with a conuctivity of 6 S cm⁻¹ and a thermopower of 15 μV/K. Below room temperature there is a structural transition, which has little influence on transport properties. Below 160 K FeOP is a semiconductor with an activation energy of 55 meV.     

Ceramic and single-crystal (1 – x)PMN–xPT constitutive behavior under combined stress and electric field loading

Pb(Mg_1/3Nb_2/3)O₃–0.32(PbTiO₃), PMN–0.32PT, single crystals have been characterized under combined stress and electric field loading [McLaughlin EA, Liu T, Lynch CS. Relaxor ferroelectric PMN–32%PT crystals under stress and electric field loading: I-32 mode measurements. Acta Mater 2004;52:3849, McLaughlin EA, Liu T, Lynch CS. Relaxor ferroelectric PMN–32%PT crystals under stress, electric field and temperature loading: II-33-mode measurements. Acta Mater 2005;53:4001] [1], [2], [3]. This approach is extended to PMN–0.26PT single crystals to explore the effect of composition on field driven phase transformations and to PMN–0.32PT ceramic specimens to compare with polycrystalline behavior. Electric displacement and strain were measured as a function of combinations of stress and both unipolar and bipolar electric fields. The single-crystal results indicate that compositions further from the morphotropic phase boundary require higher driving forces for field induced phase transformations. Evidence of these transformations is not apparent in the results from the ceramic specimens.     

Study on syntheses and structures of nine-coordinated mononuclear K[TbⅢ(edta)(H2O)3]·5H2O and binuclear K4[TbⅢ2(Httha)2]·14H2O complexes

The crystal and molecular structures of the K[TbⅢ(edta)(H2O)3]-5H2O (edta = ethylenediaminetetraacetic acid) and K4[TbⅢ2(Httha)2]+14H2O (ttha = triethylenetetraminehexaacetic acid) complexes have been determined by sin-gle-crystal X-ray diffraction analyses. The crystal of the K[TbⅢ(edta)(H2O)3]·5H2O complex belongs to orthorhombic crys-tal system and Fdd2 space group. The crystal data are as follows: a = 1.9373(5) nm, b = 3.5429(10) nm, c = 1.2114(3) nm, V = 8.315(4) nm3, Z= 16, M = 630.35, Dc = 2.014 g cn-3, m = 3.683 mm-1 and F(000) = 5024. The final R and wR values are 0.0224 and 0.0557 for 3189 [I> 2.0o(I)] unique reflections, and 0.0245 and 0.0567 for all 8206 reflections, respectively. The [TbⅢ(edta)(H2O)3]- complex anion has a nine-coordinated pseudo-monocapped square antiprismatic structure in which the nine coordinate atoms, two N and seven O come from one edta ligand and three water molecules. The crystal of the K4[TbⅢ2(Httha)2]·14H2O complex belongs to monoclinic system and P2(1)/n sp     

Microstructure,mechanical properties and creep resistance of Mg–(8%–12%)Zn–(2%–6%)Al alloys

The microstructural characteristics, mechanical properties and creep resistance of Mg–(8%–12%)Zn–(2%–6%)Al alloys were investigated to get a better overall understanding of these series alloys. The results indicate that the microstructure of the alloys ZA82, ZA102 and ZA122 with the mass ratio of Zn to Al of 4–6 is mainly composed of α-Mg matrix and two different morphologies of precipitates (block τ-Mg₃₂(Al, Zn)₄₉ and dense lamellar ε-Mg₅₁Zn₂₀), the alloys ZA84, ZA104 and ZA124 with the mass ratio of 2–3 contain α-Mg matrix and only block τ phases, and the alloys ZA86, ZA106 and ZA126 with the mass ratio of 1–2 consist of α-Mg matrix, block τ precipitates, lamellar ?-Al₂Mg₅Zn₂ eutectics and flocculent β-Mg₁₇Al₁₂ compounds. The alloys studied with the mass ratio of Zn to Al of 2–3 exhibit high creep resistance, and the alloy ZA124 with the continuous network of τ precipitating along grain boundaries shows the highest creep resistance.     

Investigation of the effect of Al2O3–Y2O3–CaO (AYC) additives on sinterability,microstructure and mechanical properties of SiC matrix composites: A review

Appropriate properties of SiC ceramic such as high hardness, low density, high melting point and high elastic modulus make this material as a favorite candidate for different industrial applications. Although some disadvantages including high sintering temperature, low sinterability, and low fracture toughness have restricted the use of this material, previous studies showed that using Al₂O₃-Y₂O₃ additives plays an effective role in the improvement of sinterability as well as the enhancement of the properties of these composites. Moreover, the addition of CaO results in the acceleration of the formation of molten phase and the improvement of sinterability. In addition, the use of these additives cause the formation of the intermetallic phases of Al₅Y₃O₁₂ (YAG) and CaY₂O₄ and by activating the mechanisms of crack deflection, crack bridging, phase transformation, strengthening the grain boundary and changing the fracture mode from intergranular to transgranular results in improved mechanical properties. This paper attempts to investigate the effect of using Al₂O₃–Y₂O₃–CaO (AYC) additives on sinterability, microstructure, and mechanical properties of SiC matrix composites including the composites reinforced with SiC fibers and SiC matrix nano-composites. Finally, the effect of the post-sintering annealing process under two conditions i.e., with and without applying pressure (pressureless sintering) on microstructure and mechanical properties has been studied.