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1.
This study examines procedures for consistently producing sound (crack and void free) welds using the autogenous (without
filler metal) gas tungsten arc (GTA) welding process. Cast alloy Ti−48Al−2Cr−2Nb (at. pct) and extruded alloy Ti−46Al−2Cr−2Nb−0.9Mo
(at. pct) have been examined to determine if sound welds can be produced using autogenous GTA welding without any preheat.
Experimentation consisted of GTA spot welding samples of gamma titanium aluminide at weld current levels of 45, 55, 65, and
75 A for a duration of 3 seconds. For the cast alloy, current levels of 45, 55, and 65 A for 3 seconds produced similar fusion
zone microstructures, which consisted of a dendritic solidification structure. The fusion zone microstructure of the 75A for
3 seconds current level differed significantly from the lower current levels. It also consisted of a dendritic solidification
structure; however, the morphology was quite different. For the extruded alloy, current levels of 45 and 55 A for 3 seconds
produced fusion zone microstructures similar to the lower current level samples of the cast γ-TiAl, which consisted of a dendritic
solidification structure. The fusion zone microstructures of the 65 and 75 A samples were similar to each other, but they
had a dendritic solidification structure of a different morphology than that of the 45 and 55 A samples. For both alloys at
all current levels, microhardness profiles showed an increase in hardness from the base metal to the fusion zone. There were
no significant differences in the average fusion zone hardness as a function of increasing current level. However, nanoindentation
testing did show that certain phases and microconstituents in the fusion zone did have significant variations in hardness
in relation to the enrichment and depletion of chromium.
This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the
TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations
Committees. 相似文献
2.
T. Zacharia S. A. David J. M. Vitek H. G. Kraus 《Metallurgical and Materials Transactions B》1991,22(2):243-257
A systematic study was carried out to verify the predictions of a transient multidimensional computational model by comparing
the numerical results with the results of an experimental study. The welding parameters were chosen such that the predictions
of the model could be correlated with the results of an earlier experimental investigation of the weld pool surface temperatures
during spot gas-tungsten-arc (GTA) welding of Type 304 stainless steel (SS). This study represents the first time that such
a comprehensive attempt has been made to experimentally verify the predictions of a numerical study of weld pool fluid flow
and heat flow. The computational model considers buoyancy and electromagnetic and surface tension forces in the solution of
convective heat transfer in the weld pool. In addition, the model treats the weld pool surface as a truly deformable surface.
Theoretical predictions of the weld pool surface temperature distributions, the cross-sectional weld pool size and shape,
and the weld pool surface topology were compared with corresponding experimental measurements. Comparison of the theoretically
predicted and the experimentally obtained surface temperature profiles indicated agreement within ±8 pct for the best theoretical
models. The predicted surface profiles were found to agree within ±20 pct on dome height and ±8 pct on weld pool diameter
for the best theoretical models. The predicted weld cross-sectional profiles were overlaid on macrographs of the actual weld
cross sections, and they were found to agree very well for the best theoretical models. 相似文献
3.
This study examines procedures for consistently producing sound (crack and void free) welds using the autogenous (without
filler metal) gas tungsten arc (GTA) welding process. Cast alloy Ti-48Al-2Cr-2Nb (at. pct) and extruded alloy Ti-46Al-2Cr-2Nb-0.9Mo
(at. pct) have been examined to determine if sound welds can be produced using autogenous GTA welding without any preheat.
Experimentation consisted of GTA spot welding samples of gamma titanium aluminide at weld current levels of 45, 55, 65, and
75 A for a duration of 3 seconds. For the cast alloy, current levels of 45, 55, and 65 A for 3 seconds produced similar fusion
zone microstructures, which consisted of a dendritic solidification structure. The fusion zone microstructure of the 75 A
for 3 seconds current level differed significantly from the lower current levels. It also consisted of a dendritic solidification
structure; however, the morphology was quite different. For the extruded alloy, current levels of 45 and 55 A for 3 seconds
produced fusion zone microstructures similar to the lower current level samples of the cast γ-TiAl, which consisted of a dendritic solidification structure. The fusion zone microstructures of the 65 and 75 A samples
were similar to each other, but they had a dendritic solidification structure of a different morphology than that of the 45
and 55 A samples. For both alloys at all current levels, microhardness profiles showed an increase in hardness from the base
metal to the fusion zone. There were no significant differences in the average fusion zone hardness as a function of increasing
current level. However, nanoindentation testing did show that certain phases and microconstituents in the fusion zone did
have significant variations in hardness in relation to the enrichment and depletion of chromium.
This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the
TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations
Committees. 相似文献
4.
E. Cerreta S. R. Chen G. T. GrayIII T. M. Pollock 《Metallurgical and Materials Transactions A》2004,35(9):2557-2566
The dynamic deformation, damage evolution, and cracking in four cast gamma titanium aluminide alloys have been investigated
experimentally and theoretically. In this study, the dynamic compressive constitutive properties of several cast TiAl alloys
were measured at quasi-static and dynamic rates. Using the measured high-strain-rate constitutive properties, the deformation,
damage evolution, and postmortem specimen geometries observed in Taylor cylinder experiments were predicted using the mechanical
threshold stress (MTS) constitutive model. The utility of the Taylor impact test for validation of high-rate constitutive
models for intermetallics is discussed.
This article is based on a presentation given in the symposium “Dynamic Deformation: Constitutive Modeling, Grain Size, and
Other Effects: In Honor of Prof. Ronald W. Armstrong,” March 2–6, 2003, at the 2003 TMS/ASM Annual Meeting, San Diego, California,
under the auspices of the TMS/ASM Joint Mechanical Behavior of Materials Committe. 相似文献
5.
The use of high-power density laser beam for welding of many important alloys often leads to appreciable changes in the composition
and properties of the weld metal. The main difficulties in the estimation of laser-induced vaporization rates and the resulting
composition changes are the determination of the vapor condensation rates and the incorporation of the effect of the welding
plasma in suppressing vaporization rates. In this article, a model is presented to predict the weld metal composition change
during laser welding. The velocity and temperature fields in the weld pool are simulated through numerical solution of the
Navier-Stokes equation and the equation of conservation of energy. The computed temperature fields are coupled with ve-locity
distribution functions of the vapor molecules and the equations of conservation of mass, momentum, and the translational kinetic
energy in the gas phase for the calculation of the evap-oration and the condensation rates. Results of carefully controlled
physical modeling experi-ments are utilized to include the effect of plasma on the metal vaporization rate. The predicted
area of cross section and the rates of vaporization are then used to compute the resulting com-position change. The calculated
vaporization rates and the weld metal composition change for the welding of high-manganese 201 stainless steels are found
to be in fair agreement with the corresponding experimental results. 相似文献
6.
7.
High-temperature deformation behavior of a gamma TiAl alloy—Microstructural evolution and mechanisms
Jeoung Han Kim Graduate Ph.D. Candidate Student Young Won Chang Chong Soo Lee Tae Kwon Ha 《Metallurgical and Materials Transactions A》2003,34(10):2165-2176
The present investigation was carried out in the context of the internal-variable theory of inelastic deformation and the
dynamic-materials model (DMM), to shed light on the high-temperature deformation mechanisms in TiAl. A series of load-relaxation
tests and tensile tests were conducted on a fine-grained duplex gamma TiAl alloy at temperatures ranging from 800 °C to 1050
°C. Results of the load-relaxation tests, in which the deformation took place at an infinitesimal level (ε ≅ 0.05), showed that the deformation behavior of the alloy was well described by the sum of dislocation-glide and dislocation-climb
processes. To investigate the deformation behavior of the fine-grained duplex gamma TiAl alloy at a finite strain level, processing
maps were constructed on the basis of a DMM. For this purpose, compression tests were carried out at temperatures ranging
from 800 °C to 1250 °C using strain rates ranging from 10 to 10−4/s. Two domains were identified and characterized in the processing maps obtained at finite strain levels (0.2 and 0.6). One
domain was found in the region of 980 °C and 10−3/s with a peak efficiency (maximum efficiency of power dissipation) of 48 pct and was identified as a domain of dynamic recrystallization
(DRx) from microstructural observations. Another domain with a peak efficiency of 64 pct was located in the region of 1250
°C and 10−4/s and was considered to be a domain of superplasticity.
This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented
at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint
Committee on Mechanical Behavior of Materials. 相似文献
8.
Temperature and velocity fields, and weld pool geometry during gas metal arc welding (GMAW) of commercially pure aluminum were predicted by solving equations of conservation of mass, energy and momentum in a three-dimensional transient model. Influence of welding speed was studied. In order to validate the model, welding experiments were conducted under the similar conditions. The calculated geometry of the weld pool were in good agreement with the corresponding experimental results. It was found that an increase in the welding speed results in a decrease peak temperature and maximum velocity in the weld pool, weld pool dimensions and width of the heat-affected zone (HAZ). Dimensionless analyses were employed to understand the importance of heat transfer by convection and the roles of various driving forces in the weld pool. According to dimensionless analyses droplet driving force strongly affected fluid flow in the weld pool. 相似文献
9.
The current study has investigated the influence of alternating current pulsing on the structure and mechanical properties of AZ31 magnesium alloy gas tungsten arc (GTA) weldments. Autogenous full penetration bead-on-plate GTA welds were made under a variety of conditions including variable polarity (VP), variable polarity mixed (VPM), alternating current (AC), and alternating current pulsing (ACPC). AC pulsing resulted in significant refinement of weld metal when compared with the unpulsed conditions. AC pulsing leads to relatively finer and more equiaxed grain structure in GTA welds. In contrast, VP, VPM, and AC welding resulted in predominantly columnar grain structures. The reason for this grain refinement may be attributed to the periodic variations in temperature gradient and solidification rate associated with pulsing as well as weld pool oscillation observed in the ACPC welds. The observed grain refinement was shown to result in an appreciable increase in fusion zone hardness, tensile strength, and ductility. 相似文献
10.
C. T. Fujii 《Metallurgical and Materials Transactions A》1981,12(6):1099-1105
The stress-corrosion cracking (SCC) behavior of high-strength steel weld metals, as indexed by KIscc, was examined with emphasis on the relative influences of yield strength, electrochemical potential, welding process, and
weld metal composition. The weld metals were from weldments fabricated by the gas metal arc (GMA) or gas tungsten arc (GTA)
process. Filler metals with four different compositions—designated 120S, 140S, AX140 and HY-130—were used. The multi-pass
welding procedures and their associated thermal cycles produced very complex martensitic-bainitic type microstructures. The
GTA weld metals were considerably more fine-grained and more highly tempered than the GMA weld metals. This enhanced the fracture
toughness of all four of the higher strength GTA weld metals but improved the SCC properties of only two GTA weld metals—HY-130
and 140S. The effectiveness of microstructural influences on SCC behavior is correlated with the sulfur content of the weld
metals assuming that hydrogen is the cause of SCC in these materials. The role of sulfur is presumed to be that of catalytic
poison for the hydrogen recombination reaction which increases opportunities for nascent hydrogen absorption. The results
show that the weld metals with improved SCC properties contain the lower concentrations of sulfur. 相似文献
11.
Effect of evaporation and temperature-dependent material properties on weld pool development 总被引:2,自引:0,他引:2
This paper evaluates the effect of weld pool evaporation and thermophysical properties on the development of the weld pool.
An existing computational model was modified to include vaporization and temperature-dependent thermophysical properties.
Transient, convective heat transfer during gas tungsten arc (GTA) welding with and without vaporization effects and variable
properties was studied. The present analysis differs from earlier studies that assumed no vaporization and constant values
for all of the physical properties throughout the range of temperature of interest. The results indicate that consideration
of weld pool vaporization effects and variable physical properties produce significantly different weld model predictions.
The calculated results are consistent with previously published experimental findings. 相似文献
12.
By combining a mathematical model of the welding arc and of the weld pool, calculations are presented to describe the free
surface temperature of weld pools for spot welding operations. The novel aspects of the treatment include the calculation
of the heat and current fluxes falling on the free weld pool surface from first principles, a realistic allowance for heat
losses due to vaporization, and a realistic allowance for the temperature dependence of the surface tension. The most important
finding reported in this article is that the free surface temperature of weld pools appears to be limited by Marangoni convection,
rather than heat losses due to vaporiza-tion. Furthermore, it was found that once thermocapillary flow can produce high enough
surface velocities (>25 cm/s), the precise nature of the relationship between temperature and surface tension will become
less important. 相似文献
13.
This article presents a mathematical model simulating the effects of surface tension (Maragoni effect) on weld pool fluid
flow and weld penetration in spot gas metal arc welding (GMAW). Filler droplets driven by gravity, electromagnetic force,
and plasma arc drag force, carrying mass, thermal energy, and momentum, periodically impinge onto the weld pool. Complicated
fluid flow in the weld pool is influenced by the droplet impinging momentum, electromagnetic force, and natural convection
due to temperature and concentration gradients, and by surface tension, which is a function of both temperature and concentration
of a surface active element (sulfur in the present study). Although the droplet impinging momentum creates a complex fluid
flow near the weld pool surface, the momentum is damped out by an “up-and-down” fluid motion. A numerical study has shown
that, depending upon the droplet’s sulfur content, which is different from that in the base metal, an inward or outward surface
flow of the weld pool may be created, leading to deep or shallow weld penetration. In other words, it is primarily the Marangoni
effect that contributes to weld penetration in spot GMAW. 相似文献
14.
A mathematical model of gas tungsten arc welding considering the cathode and the free surface of the weld pool 总被引:4,自引:0,他引:4
A two-dimensional axisymmetric numerical model, including the influence of the cathode and the free surface of the weld pool,
is developed to describe the heat transfer and fluid flow in gas tungsten arc (GTA) welding. In the model, a boundary-fitted
coordinate system is adopted to precisely describe the cathode shape and deformed weld-pool surface. The current continuity
equation has been solved with the combined arc plasma-cathode system, independent of the assumption of current density distribution
on the cathode surface, which was essential in the previous studies of arc plasma. It has been shown that the temperature
profile, the current, and the heat flux to the anode show good agreement with the experimental data. Moreover, the current
and the heat-flux distributions may be affected by the shape of the cathode and the free surface of the weld pool. 相似文献
15.
Manabu Tanaka Hidenori Terasaki Ph.D. Masao Ushio John J. Lowke 《Metallurgical and Materials Transactions A》2002,33(7):2043-2052
In order to clarify the formative mechanism of weld penetration in an arc welding process, the development of a numerical
model of the process is quite useful for understanding quantitative values of the balances of mass, energy, and force in the
welding phenomena because there is still lack of experimentally understanding of the quantitative values of them because of
the existence of complicated interactive phenomena between the arc plasma and the weld pool. The present article is focused
on a stationary tungsten-inert-gas (TIG) welding process for simplification, but the whole region of TIG arc welding, namely,
tungsten cathode, arc plasma, workpiece, and weld pool is treated in a unified numerical model, taking into account the close
interaction between the arc plasma and the weld pool. Calculations in a steady state are made for stationary TIG welding in
an argon atmosphere at a current of 150 A. The anode is assumed to be a stainless steel, SUS304, with its negative temperature
coefficient of surface tension. The two-dimensional distributions of temperature and velocity in the whole region of TIG welding
process are predicted. The weld-penetration geometry is also predicted. Furthermore, quantitative values of the energy balance
for the various plasma and electrode regions are given. The predicted temperatures of the arc plasma and the tungsten-cathode
surface are in good agreement with the experiments. There is also approximate agreement of the weld shape with experiment,
although there is a difference between the calculated and experimental volumes of the weld. The calculated convective flow
in the weld pool is mainly dominated by the drag force of the cathode jet and the Marangoni force as compared with the other
two driving forces, namely, the buoyancy force and the electromagnetic force. 相似文献
16.
17.
T. Hong T. Debroy S. S. Babu S. A. David 《Metallurgical and Materials Transactions B》2000,31(1):161-169
The composition, size distribution, and number density of oxide inclusions in weld metal are critical factors in determining
weldment properties. A computational model has been developed to understand these factors, considering fluid flow and the
temperature field in the weld pool during submerged are (SA) welding of low-alloy steels. The equations of conservation of
mass, momentum, and energy are solved in three dimensions to calculate the velocity and temperature fields in the weld pool.
The loci and corresponding thermal cycles of thousands of oxide inclusions are numerically calculated in the weld pool. The
inclusions undergo considerable recirculatory motion and experience strong temperature gyrations. The temperature-time history
and the computed time-temperature-transformation (TTT) behavior of inclusions were then used to understand the growth and
dissolution of oxide inclusions in the weld pool. The statistically meaningful characteristics of inclusion behavior in the
weld pool, such as the residence time, number of temperature peaks, etc., were calculated for several thousand inclusions. The calculated trends agree with experimental observations and indicate
that the inclusion formation can be described by combining thermodynamics and kinetics with the fundamentals of transport
phenomena. 相似文献
18.
R. T. C. Choo J. Szekely R. C. Westhoff 《Metallurgical and Materials Transactions B》1992,23(3):357-369
A mathematical formulation has been developed and computed results are presented describing the temperature profiles in gas
tungsten arc welding (GTAW) arcs and, hence, the net heat flux from the welding arc to the weld pool. The formulation consists
of the statement of Maxwell's equations, coupled to the Navier-Stokes equations and the differential thermal energy balance
equation. The theoretical predictions for the heat flux to the workpiece are in good agreement with experimental measurements
— for long arcs. The results of this work provide a fundamental basis for predicting the behavior of arc welding systems from
first principles. 相似文献
19.
Shanping Lu Hidetoshi Fujii Kiyoshi Nogi 《Metallurgical and Materials Transactions A》2004,35(9):2861-2867
Small CO2 additions of 0.092 to 10 vol pct to the Ar shielding gas dramatically change the weld shape and penetration from a shallow
flat-bottomed shape, to a deep cylindrical shape, to a shallow concave-bottomed shape, and back to the shallow flat-bottomed
shape again with increasing CO2 additions in gas thermal arc (GTA) welding of a SUS304 plate. Oxygen from the decomposition of CO2 transfers and becomes an active solute element in the weld pool and reverses the Marangoni convection mode. An inward Marangoni
convection in the weld pool occurs when the oxygen content in the weld pool is over 80 ppm. Lower than 80 ppm, flow will change
to the outward direction. An oxide layer forms on the weld pool in the welding process. The heavy oxide layer on the liquid-pool
surface will inhibit the inward fluid flow under it and also affects the oxygen transfer to the liquid pool. A model is proposed
to illustrate the interaction between the CO2 gas and the molten pool in the welding process. 相似文献
20.
Type 316 LN stainless steel is the major structural material used in the construction of nuclear reactors. Activated flux
tungsten inert gas (A-TIG) welding has been developed to increase the depth of penetration because the depth of penetration
achievable in single-pass TIG welding is limited. Real-time monitoring and control of weld processes is gaining importance
because of the requirement of remoter welding process technologies. Hence, it is essential to develop computational methodologies
based on an adaptive neuro fuzzy inference system (ANFIS) or artificial neural network (ANN) for predicting and controlling
the depth of penetration and weld bead width during A-TIG welding of type 316 LN stainless steel. In the current work, A-TIG
welding experiments have been carried out on 6-mm-thick plates of 316 LN stainless steel by varying the welding current. During
welding, infrared (IR) thermal images of the weld pool have been acquired in real time, and the features have been extracted
from the IR thermal images of the weld pool. The welding current values, along with the extracted features such as length,
width of the hot spot, thermal area determined from the Gaussian fit, and thermal bead width computed from the first derivative
curve were used as inputs, whereas the measured depth of penetration and weld bead width were used as output of the respective
models. Accurate ANFIS models have been developed for predicting the depth of penetration and the weld bead width during TIG
welding of 6-mm-thick 316 LN stainless steel plates. A good correlation between the measured and predicted values of weld
bead width and depth of penetration were observed in the developed models. The performance of the ANFIS models are compared
with that of the ANN models. 相似文献