共查询到20条相似文献,搜索用时 15 毫秒
1.
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
2.
In this two-part article, the weldabilities of AA 1100 aluminum and AISI 409 stainless steel by the pulsed Nd:YAG laser welding
process have been examined experimentally and compared. The effects of laser pulse time and power density on laser spot weld
characteristics, such as weld diameter, penetration, melt area, melting ratio, porosity, and surface cratering, have been
studied and explained qualitatively in relation to material-dependent variables such as absorptivity and thermophysical properties.
The weldability of AISI 409 stainless steel was reported in Part I of this article. In the present article, the weldability
of AA 1100 aluminum is reported and compared to that of AISI 409 stainless steel. Weld pool shapes in aluminum were found
to be influenced by the mean power density of the laser beam and the laser pulse time. Both conduction-mode and keyhole-mode
welding were observed in aluminum. Unlike stainless steel, however, drilling was not observed. Conduction-mode welds were
produced in aluminum at power densities ranging from 3.2 to 10 GW/m2. The power density required for melting aluminum was approximately 4.5 times greater than stainless steel. The initial transient
in weld pool development in aluminum occurred within 2 ms, and the aspect ratios (depth/width) of the steady-state conduction-mode
weld pools were approximately 0.2. These values are about half those observed in stainless steel. The transition from conduction-
to keyhole-mode welding occurred in aluminum at a power density of about 10 GW/m2, compared to about 4 GW/m2 for stainless steel. Weld defects such as porosity and cratering were observed in both aluminum and stainless steel spot
welds. In both materials, there was an increased propensity for large occluded vapor pores near the root of keyhole-mode welds
with increasing power density. In aluminum, pores were observed close to the fusion boundary. These could be eliminated by
surface milling and vacuum annealing the specimens, suggesting that such pores were due to hydrogen. Finally, excellent agreement
was obtained between experimental data from both alloys and an existing analytical model for conduction-mode laser spot welding.
Two nondimensional parameters, the Fourier number and a nondimensional incident heat flux parameter, were derived and shown
to completely characterize weld pool development in conduction-mode welds made in both materials. 相似文献
3.
Mathematical models capable of accurate prediction of the weld bead and weld pool geometry in gas metal arc (GMA) welding
processes would be valuable for rapid development of welding procedures and empirical equations for control algorithms in
automated welding applications. This article introduces a three-dimensional (3-D) model for heat and fluid flow in a moving
GMA weld pool. The model takes the mass, momentum, and heat transfer of filler metal droplets into consideration and quantitatively
analyzes their effects on the weld bead shape and weld pool geometry. The algorithm for calculating the weld reinforcement
and weld pool surface deformation has been proved to be effective. Difficulties associated with the irregular shape of the
weld bead and weld pool surface have been successfully overcome by adopting a boundary-fitted nonorthogonal coordinate system.
It is found that the size and profile of the weld pool are strongly influenced by the volume of molten wire, impact of droplets,
and heat content of droplets. Good agreement is demonstrated between predicted weld dimensions and experimently measured ones
for bead-on-plate GMA welds on mild steel plate. 相似文献
4.
5.
The effects of process variables on pulsed Nd:YAG laser spot welds: Part I. AISI 409 stainless steel
The weldabilities of AA 1100 aluminum and AISI 409 stainless steel by the pulsed Nd:YAG laser welding process have been examined
experimentally and compared. The effects of Nd:YAG laser welding parameters, including laser pulse time and power intensity,
and material-dependent variables, such as absorptivity and thermophysical properties, on laser spot-weld characteristics,
such as weld diameter, penetration, melt area, melting ratio, porosity, and sur-face cratering, have been studied experimentally.
The results of this work are reported in two parts. In Part I, the weldability of AISI 409 stainless steel by the pulse laser
welding process is reported. In Part II, the weldability of A A 1100 aluminum under the same operating con-ditions is reported
and compared to those of the stainless steel. When welding AISI 409 stainless steel, weld pool shapes were found to be influenced
most by the power intensity of the laser beam and to a lesser extent by the pulse duration. Conduction mode welding, keyhole
mode welding, and drilling were observed. Conduction mode welds were produced when power in-tensities between 0.7 and 4 GW/m2 were used. The initial transient in weld pool development occurred in the first 4 ms of the laser pulse. Following this,
steady-state conditions existed and conduction mode welds with aspect ratios (depth/width) of about 0.4 were produced. Keyhole
mode welds were observed at power intensities greater than 4 GW/m2. Penetration of these keyhole mode welds increased with increases in both power intensity and pulse time. The major weld
defects observed in the stainless steel spot welds were cratering and large-occluded gas pores. Significant metal loss due
to spatter was measured during the initial 2 ms of keyhole mode welds. With increasing power intensity, there was an increased
propensity for occluded gas pores near the bottom of the keyhole mode welds.
Formerly Graduate Student. 相似文献
6.
Sindo Kou 《Metallurgical and Materials Transactions A》1981,12(12):2025-2030
Based on the finite difference method and the enthalpy model of Shamsundar, a computer model was developed to describe the
steady state, two-dimensional heat flow during the welding of thin plates. In order to allow accurate computations of the
weld pool configuration, the size of the mushy zone and the temperature distribution near the heat source, a grid mesh of
variable spacings was used. The heat of fusion, the size and distribution of the heat source, the temperature dependence of
thermal properties, the heat conduction in the welding direction and the surface heat loss during welding were considered.
The model was first checked with Rosenthal’s analytical solution of welding heat flow using pure aluminum for examples. Experimental
results of 6061 aluminum, including the width of the fusion zone and the thermal cycles at positions in both the fusion and
the heat affected zones, were then compared with the calculated results of the heat flow model. The agreement was very good.
Finally, in order to demonstrate systematically the quantitative effect of welding parameters such as the heat input, the
welding speed and the preheating of the workpiece, a series of computations were made based upon 6061 aluminum. 相似文献
7.
Heat transfer during Nd: Yag pulsed laser welding and its effect on solidification structure of austenitic stainless steels 总被引:2,自引:0,他引:2
T. Zacharia S. A. David J. M. Vitek T. Debroy 《Metallurgical and Materials Transactions A》1989,20(5):957-967
Theoretical and experimental investigations were carried out to determine the effect of process parameters on weld metal microstructures
of austenitic stainless steels during pulsed laser welding. Laser welds made on four austenitic stainless steels at different
power levels and scanning speeds were considered. A transient heat transfer model that takes into account fluid flow in the
weld pool was employed to simulate thermal cycles and cooling rates experienced by the material under various welding conditions.
The weld metal thermal cycles and cooling rates are related to features of the solidification structure. For the conditions
investigated, the observed fusion zone structure ranged from duplex austenite (γ)+ferrite (δ) to fully austenitic or fully
ferritic. Unlike welding with a continuous wave laser, pulsed laser welding results in thermal cycling from multiple melting
and solidification cycles in the fusion zone, causing significant post-solidification solid-state transformation to occur.
There was microstructural evidence of significant recrystallization in the fusion zone structure that can be explained on
the basis of the thermal cycles. The present investigation clearly demonstrated the potential of the computational model to
provide detailed information regarding the heat transfer conditions experienced during welding. 相似文献
8.
The grain structure of the weld metal can significantly affect its resistance to solidification cracking during welding and
its mechanical properties after welding. An experimental study was conducted to investigate the effect of two basic welding
parameters,i.e., the heat input and the welding speed, on the grain structure of aluminum-alloy welds. Gas-tungsten arc welding was performed
under various heat inputs and welding speeds, with thermal measurements in the weld pool being carried out during welding
and the amounts and nuclei of equiaxed grains in the resultant welds being examined using optical and electron microscopy.
The experimentally measuredG/R ratios and the clearly revealed heterogeneous nuclei together demonstrated the thermodynamic effect of the heat input and
welding speed on the weld metal grain structure. 相似文献
9.
《Canadian Metallurgical Quarterly》1998,37(3-4):293-303
A comprehensive thermofluids numerical model of stationary gas tungsten arc (GTA) welding has been developed and used to examine the effects of thermofluids phenomena on the temperatures and flow velocities in the weld pool and their impact on the resultant weld dimensions for welds produced in high vs low thermal conductivity metals. A dynamic grid re-mapping technique was used to map a block of finite elements into the liquid weld pool to permit use of a k–ε turbulence model in the liquid. Simulations for low conductivity AISI 304 stainless steel show that good correlation with experimental data was only possible if the effects of fluid flow and turbulent mixing in the weld pool were modeled accurately. Conversely, simulations in higher conductivity metals, 6061 and 1100 aluminum, showed that while the average flow velocities and level of turbulence were higher, their effect on the final temperatures and weld pool dimensions were less significant. 相似文献
10.
J. C. Villafuerte E. Pardo H. W. Kerr 《Metallurgical and Materials Transactions A》1990,21(7):2009-2019
The columnar-equiaxed transition (CET) was investigated in full penetration gas-tungsten arc (GTA) welds on ferritic stainless
steel plates containing different amounts of minor elements, such as titanium and aluminum, for a range of welding conditions.
In general, the fraction of equiaxed grains increased, and the size of the equiaxed grains decreased with increasing titanium
contents above 0.18 wt pct. At a given level of titanium, the equiaxed fraction increased, and the size of the equiaxed grains
decreased with increased aluminum content. The CET was ascribed to heterogeneous nucleation of ferrite on Ti-rich cuboidal
inclusions, since these inclusions were observed at the origin of equiaxed dendrites in the grain refined welds. Titanium-rich
cuboidal inclusions, in turn, were found to contain Al-Ca-Mg-rich inclusions at their centers, consistent with observations
by previous investigators for other processes. The welding conditions, in particular, the welding speed, were observed to
affect the occurrence of the CET. Increasing the welding speed from 3 to 8 mm/s increased the equiaxed fraction noticeably,
but a further increase in speed to 14 mm/s had a smaller additional effect. A finite element model (FEM) of heat transfer
was used to examine the role of the welding conditions on the local solidification conditions along the weld pool edge. The
results are compared with existing models for the CET.
Formerly Postdoctoral Fellow, Department of Mechanical Engineering, University of Waterloo 相似文献
11.
CO2 laser beam welding of 6061-T6 aluminum alloy thin plate 总被引:1,自引:0,他引:1
Akio Hirose Kojiro F. Kobayashi Hirotaka Todaka 《Metallurgical and Materials Transactions A》1997,28(12):2657-2662
Laser beam welding is an attractive welding process for age-hardened aluminum alloys, because its low heat input minimizes
the width of weld fusion and heat-affected zones (HAZs). In the present work, 1-mm-thick age-hardened Al-Mg-Si alloy, 6061-T6,
plates were welded with full penetration using a 2.5-kW CO2 laser. Fractions of porosity in the fusion zones were less than 0.05 pct in bead-on-plate welding and less than 0.2 pct in
butt welding with polishing the groove surface before welding. The width of a softened region in the-laser beam welds was
less than 1/4 times that of a tungsten inert gas (TIG) weld. The softened region is caused by reversion of strengthening β″
(Mg2Si) precipitates due to weld heat input. The hardness values of the softened region in the laser beam welds were almost fully
recovered to that of the base metal after an artificial aging treatment at 448 K for 28.8 ks without solution annealing, whereas
those in the TIG weld were not recovered in a partly reverted region. Both the bead-on-plate weld and the butt weld after
the postweld artificial aging treatment had almost equivalent tensile strengths to that of the base plate. 相似文献
12.
Weld metal composition change during conduction mode laser welding of aluminum alloy 5182 总被引:6,自引:0,他引:6
Selective vaporization of volatile elements during laser welding of automotive aluminum alloys affects weld metal composition
and properties. An experimental and theoretical study was carried out to seek a quantitative understanding of the influences
of various welding variables on vaporization and composition change during conduction mode laser welding of aluminum alloy
5182. A comprehensive model for the calculation of vaporization rate and weld metal composition change was developed based
on the principles of transport phenomena, kinetics, and thermodynamics. The calculations showed that the vaporization was
concentrated in a small high-temperature region under the laser beam where the local vapor pressure exceeded the ambient pressure.
The convective vapor flux driven by the pressure gradient was much higher than the diffusive vapor flux driven by the concentration
gradient. The computed weld pool geometry, vaporization rates, and composition changes for different welding conditions agreed
well with the corresponding experimental data. The good agreement demonstrates that the comprehensive model can serve as a
basis for the quantitative understanding of the influences of various welding variables on the heat transfer, fluid flow,
and vaporization occurring during conduction mode laser welding of automotive aluminum alloys. 相似文献
13.
A theoretical and experimental study of heat flow and solidification during the autogenous GTA welding of aluminum plates
was carried out. The theoretical part of the study involves the development of a computer model which describes three-dimensional
heat flow during welding. The model, though valid for any plate thickness, is particularly useful for moderately thick plates
since both full- and partial-penetration welds can be considered. The experimental part of the study, on the other hand, involves
the measurement of the thermal response of the workpiece during welding, and the examination of the configuration, grain structure,
and subgrain structure of the fusion zone. The experimental results were compared with the calculated ones and the agreement
was very good. With the help of the computer model, the effects of welding parameters on weld penetration in moderately thick
plates were discussed. These parameters are the heat input per unit length of the weld, the thickness of the workpiece, the
preheating of the workpiece, and the power-density distribution of the heat source.
Formerly with the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 相似文献
14.
15.
16.
Prediction of weld pool and reinforcement dimensions of GMA welds using a finite-element model 总被引:5,自引:0,他引:5
Mathematical models of the gas metal arc (GMA) welding process may be used to study the influence of various welding parameters
on weld dimensions, to assist in the development of welding procedures, and to aid in the generation of process control algorithms
for automated applications. In this work, a three-dimensional (3-D), steady-state thermal model of the GMA welding process
has been formulated for a moving coordinate framework and solved using the finite-element method. The model includes temperature-dependent
material properties, a new finite-element formulation for the inclusion of latent heat of fusion, a Gaussian distribution
of heat flux from the arc, plus the effects of mass convection into the weld pool from the melted filler wire. The influence
of weld pool convection on the pool shape was approximated using anisotropically enhanced thermal conductivity for the liquid
phase. Weld bead width and reinforcement height were predicted using a unique iterative technique developed for this purpose.
In this paper, the numerical model is shown to be capable of predicting GMA weld dimensions for individual welds, including
those with finger penetration. Also, good agreement is demonstrated between predicted weld dimensions and experimentally derived
relations that describe the effects of process variables and their influence on average weld dimensions for bead-onplate GMA
welds on steel plate.
E. PARDO, formerly Postdoctoral Fellow, Department of Mechanical Engineering, University of Waterloo, Waterloo, ON, Canada
N2L 3G1, 相似文献
17.
S. A. David J. M. Vitek M. Rappaz L. A. Boatner 《Metallurgical and Materials Transactions A》1990,21(6):1753-1766
Previously developed techniques by the authors for the microstructural analysis of welds, that included the effects of both
the growth crystallography and the weld pool shape, are applied to several cases involving the single-crystal electron beam
welding of an Fe-15Ni-15Cr alloy. This evaluation of weld microstructures and associated dendritic growth patterns is based
on a three-dimensional (3-D) geometrical analysis. The present study includes examination of the effects observed in overlapping,
multipass autogenous welds and butt welds of two single crystals with different orientations, as well as effects due to variations
in the welding speed. Weld pool shapes were found to change significantly with increasing welding speed—becoming narrower
in cross section but more elongated in the welding direction. Additionally, all electron beam welds showed evidence of a plateau
region in the center of the weld pool. The pool shapes, however, were found to be independent of the crystallographic orientation.
Therefore, it is possible to extend the pool shape results to crystals welded in any orientation and even to polycrystals.
The over-lapping multipass welds showed remarkable reproducibility from pass to pass and duplicated the structural patterns
found in single-pass welds. The similarity in dendritic patterns within each pass indicated that the weld pool shapes were
identical in all of the passes. The micro-structure of butt welds of two single crystals with different relative orientations
showed a remarkable relationship to that associated with each individual crystallographic orientation, and the micro structure
was, in effect, simply a composite of two single-pass microstructures. Additional microstructural details were also examined.
The tendency toward branching of dendrites was associated with the transition from one dendrite growth orientation to another.
It was also found that the nonpenetrating welds exhibited a small protrusion at the bottom of the weld. It is suggested that
the modeling of weld pool shapes can be directly evaluated by comparing the predicted dendritic growth patterns based on the
modeled shapes with the actual experimentally observed dendritic growth patterns.
Formerly Visiting Scientist, Solid State Division, Oak Ridge National Laboratory. 相似文献
18.
The effect of Nd:YAG laser welding aluminum alloys 6061, 5456, and 5086 was studied from a perspective of alloying element vaporization, hot cracking susceptibility, and resultant mechanical properties. Both continuous wave and pulsed Nd.YAG laser welds were investigated. It was found that Mg was vaporized during welding, the extent of which was a function of the weld travel speed. Calculations based upon evaporation theory, and assuming a regular solution model, resulted in an estimation of weld pool surface temperatures from 1080 to 1970 K for the continuous wave welds. Pulsed Nd:YAG laser welds were observed to be extremely susceptible to weld metal hot cracking whereas continuous wave Nd:YAG laser welds were crack-free. The hardness of 6061 welds was affected by the Mg vaporization such that base metal strengths could not be achieved by subsequent re-heat treatment to the T6 condition. This loss in hardness was attributed to a reduced ability of the alloy to precipitation harden due to a lower Mg concentration. In the cases of 5456 and 5086, when samples containing welds were processed to the O condition, the weld metal had reduced hardness relative to the base metal. This loss of hardness was also attributed to the loss of Mg in these welds, resulting in reduced solid solution strengthening. 相似文献
19.
The effect of Nd:YAG laser welding aluminum alloys 6061, 5456, and 5086 was studied from a perspective of alloying element
vaporization, hot cracking susceptibility, and resultant mechanical properties. Both continuous wave and pulsed Nd.YAG laser
welds were investigated. It was found that Mg was vaporized during welding, the extent of which was a function of the weld
travel speed. Calculations based upon evaporation theory, and assuming a regular solution model, resulted in an estimation
of weld pool surface temperatures from 1080 to 1970 K for the continuous wave welds. Pulsed Nd:YAG laser welds were observed
to be extremely susceptible to weld metal hot cracking whereas continuous wave Nd:YAG laser welds were crack-free. The hardness
of 6061 welds was affected by the Mg vaporization such that base metal strengths could not be achieved by subsequent re-heat
treatment to the T6 condition. This loss in hardness was attributed to a reduced ability of the alloy to precipitation harden
due to a lower Mg concentration. In the cases of 5456 and 5086, when samples containing welds were processed to the O condition,
the weld metal had reduced hardness relative to the base metal. This loss of hardness was also attributed to the loss of Mg
in these welds, resulting in reduced solid solution strengthening. 相似文献
20.
Grain structures in aluminum alloy GTA welds 总被引:2,自引:0,他引:2
The grain structures in two dimensional GTA welds of a number of commercial aluminum alloys have been studied in order to
clarify the mechanisms of grain refinement in welds. At low welding speeds and heat inputs the structures were either axial
(continuous grains along the weld centerline) or stray (intermittent new grains). At higher speeds and heat inputs the structures
were generally either columnar to the weld centerline, or contained some equiaxed grains at the center. Regression analyses
indicated that both stray grains and equiaxed grains were favored by increased titanium content. In several alloys, titanium-rich
compounds, and in one alloy, zirconium-rich compounds were found at the centers of dendrites. It is concluded that both stray
and equiaxed grains originate by heterogeneous nucleation, with possible secondary effects due to constitutional undercooling.
Formerly Graduate Student, University of Waterloo 相似文献