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.     

Texture characterization of autogenous Nd:YAG laser welds in AA5182-O and AA6111-T4 aluminum alloys

This article reports a study of texture characterization in Nd:YAG laser welds of AA5182-O and AA6111-T4 alloys. Electron backscattering diffraction (EBSD) in the scanning electron microscope was used to determine the texture. The determination was made as a function of thickness through the sample. The results show that the welds can develop significant texture. In particular, the columnar grains that grow from the base metal into the weld have a strong 001 texture along the direction of growth. The work at Brown University was supported by GM through the GM Collaborative Research Lab at Brown University.     

Investigation of striation formation in thin stainless steel tube during pulsed Nd:YAG laser cutting process by numerical simulation

The formation of a striation pattern in a thin stainless steel tube was investigated by numerical simulation during a pulsed Nd:YAG laser cutting process. The simulated results were compared with the experimental results, which were performed under the same conditions for simulation. The simulated results showed good agreement with the experimental results. Although the formation of the striation pattern was influenced by various laser parameters, the laser power density had become the most important factor in the formation of striation patterns, since the laser power density is the most influential in the heating of metal, and the striation formation is caused by the ejection of molten metal and evaporation during laser cutting process. Although a high power density resulted in clear regular striation patterns, relatively low power density caused the formation of a hot spot, which hindered the formation of regular striation patterns and caused less striation. The numerical simulation calculations can be used to predict the shape of striation patterns and to offer a way to provide a smooth cut wall.     

On the weldability,composition, and hardness of pulsed and continuous Nd:YAG laser welds in aluminum alloys 6061,5456, and 5086

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.     

On the weldability, composition, and hardness of pulsed and continuous Nd:YAG laser welds in aluminum alloys 6061,5456, and 5086

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.     

Dissimilar friction stir welds in AA5083-AA6082. Part II: Process parameter effects on microstructure

The aim of this study is to explore the bounds of the so-called processing window, within which good-quality welds can be produced, for the friction stir welding of AA5083 to AA6082 using a systematic set of rotation and traverse speeds. The first paper in this series examined the thermal and macroscopic aspects. In this paper, several microstructurally related characteristics, including hardness, grain size, and precipitate distribution, have been examined in greater detail. The observed variations are correlated and contrasted with the observed and predicted thermal distributions. In addition, the thermal model developed in part I has been coupled to hardness models based on classical isothermal aging studies for each alloy to predict the hardness variations across the welds.     

X-ray-based measurement of composition during electron beam melting of AISI 316 stainless steel: Part II. Evaporative processes and simulation

An energy dispersive X-ray (EDX) detector mounted on a laboratory scale electron beam furnace (30 kW) was employed to assess the potential use of X-rays as a means of on-line composition monitoring during electron beam (E B) melting of alloys. The design and construction of the collimation and protection systems used for the EDX are described in Part I. In Part II, a mathematical simulation of the heat, mass, and momentum transfer was performed for comparison to the EDX and vapor deposition results. The predicted flow patterns and evaporation rates are used to explain the differences between the two experimental methods. For the EDX spectra measured, the X-rays generated were from the center of the hearth where fluid flow rising from the bulk of the pool is sufficient to maintain the bulk composition despite the high evaporative flux from the surface. The flow moves radially outward from the center of the pool, with the volatile species being depleted. The vapor deposition technique measures the entire region, giving an average surface composition, and it therefore differs from the EDX results, which gave a near bulk composition. This combined study using in-situ EDX measurements and numerical simulations both provided an insight into the phenomena controlling the evaporation in an EB-heated system and demonstrated the viability of using EDX to measure the bulk composition during EB melting processes.     

X-ray-based measurement of composition during electron beam melting of AISI 316 stainless steel: Part II. Evaporative processes and simulation

An energy dispersive X-ray (EDX) detector mounted on a laboratory scale electron beam furnace (30 kW) was employed to assess the potential use of X-rays as a means of on-line composition monitoring during electron beam (E B) melting of alloys. The design and construction of the collimation and protection systems used for the EDX are described in Part I. In Part II, a mathematical simulation of the heat, mass, and momentum transfer was performed for comparison to the EDX and vapor deposition results. The predicted flow patterns and evaporation rates are used to explain the differences between the two experimental methods. For the EDX spectra measured, the X-rays generated were from the center of the hearth where fluid flow rising from the bulk of the pool is sufficient to maintain the bulk composition despite the high evaporative flux from the surface. The flow moves radially outward from the center of the pool, with the volatile species being depleted. The vapor deposition technique measures the entire region, giving an average surface composition, and it therefore differs from the EDX results, which gave a near bulk composition. This combined study using in-situ EDX measurements and numerical simulations both provided an insight into the phenomena controlling the evaporation in an EB-heated system and demonstrated the viability of using EDX to measure the bulk composition during EB melting processes.     

Decarburization of stainless steel: Part II. A mathematical model and a process optimization for industrial scale systems

A mathematical model is presented for describing the bath temperature and composition trajectories for the decarburization of stainless steel for processes operating on an indus-trial scale. The model is based on a set of component balances, written with the aid of driving force expressions, the appropriate equilibrium relationships and the heat balance. The resultant set of ordinary differential equations were solved numerically. The predic-tions based on the model were compared with experimentally measured bath composition and temperature paths obtained for the operation of a 40 ton electric furnace. The predic-tions and measurements showed very good agreement. The mathematical model was then combined with a trajectory optimization technique to compute the optimal blowing programi .e., oxygen-argon content of the gas supplied) such that the total cost of the operation is minimized. This calculation was repeated for a series of cost factors. On leave from Department of Iron and Steel Engineering, Nagoya University, Nagoya, Japan     

The effects of cold working on sensitization and intergranular corrosion behavior of AISI 304 stainless steel

The effects of prior cold rolling of up to an 80 pct reduction in thickness on the sensitization-desensitization behavior of Type AISI 304 stainless steel and its susceptibility to intergranular corrosion have been studied by electrochemical potentiokinetic reactivation (EPR) and Strauss-test methods. The results indicate that the prior deformation accelerated the sensitization as compared to the undeformed stainless steel. The deformed Type 304 stainless steel experienced desensitization at higher temperatures and times, and it was found to be enhanced by increased cold deformation. This could be attributed to the increased long-range chromium diffusion, possibly brought on by increasing pipe diffusion and vacancies. The role of the deformation-induced martensite (DIM) and texture, introduced by uniaxial cold rolling, on the sensitization-desensitization kinetics has also been discussed. This study could not reveal any systematic relationship between texture and the degree of sensitization (DOS) obtained. The effect of DIM on DOS seems to be pronounced at 500 °C when the steel retained significant amounts of DIM; however, the retained DIM is insignificant at higher sensitization times and temperatures.     

Resistance spot welding of galvanized steel: Part I. Material variations and process modifications

Material variations and process modifications have been studied to determine their effects on the acceptable range of resistance spot welding conditions for galvanized steel sheet. The material variations studied include zinc coating integrity, structure, composition, thickness, roughness, oil, and the amount and type of Fe-Zn intermetallics. Process modifications studied include upsloping and downsloping of the weld current, preheat current, postheat current, electrode tip geometry, and applied force. It was found that hot-dipped galvanized materials with coatings which have a very thin Fe-Zn alloy layer have a wider range of acceptable welding conditions than the commercial galvannealed products, which have a fully alloyed Fe-Zn coating. The decreased lobe width of the galvannealed material is due to the discontinuous Fe-Zn coating structure and morphology. Small variations in the thickness of the coatings studied have no significant effect on the welding current range. Surface roughness of the coating has no effect on lobe width. Upsloping and downsloping of the weld current increase the welding range of hot-dipped products when using truncated cone electrodes, whereas sloped current has no advantage for galvannealed or uncoated materials. Radiused electrodes can increase the lobe width of hot-dipped products but are not beneficial when using sloped current or when welding galvannealed or uncoated materials.     

Mathematical modeling of the argon-oxygen decarburization refining process of stainless steel: Part II. Application of the model to industrial practice

The mathematical model proposed and presented in Part I of the present work has been used to deal with and analyze the austenitic stainless steel making (including ultralow-carbon steel) and has been tested on data of 32 heats obtained in producing 18Cr9Ni-grade steel in an 18-t argon-oxygen decarburization (AOD) vessel. The results indicated that the carbon concentrations and bath temperatures at the endpoints of blowing periods, calculated by the model, are in excellent agreement with the determined data, and the Cr content after the predeoxidization, obtained from the model predictions, also agrees very well with the observed value. The Gibbs free energies of the oxidation reactions of elements can be used to characterize fully the competitive oxidation among the elements during the refining process and to determine reasonably the corresponding distribution ratios of oxygen. The critical carbon concentration of decarburization (after which the decarburization changes to become controlled by the mass transfer of carbon in molten steel) for the AOD refining process of austenitic stainless steel in an 18-t AOD vessel is in the range of 0.25 to 0.40 mass pct. The model can provide some very useful information and a reliable basis for optimization of the technology of the AOD refining process of stainless steel and control of the process in real time and online.     

Dissimilar friction stir welds in AA5083-AA6082. Part I: Process parameter effects on thermal history and weld properties

The aim of this study was to explore the so-called processing window, within which good-quality welds can be produced, for the friction stir welding of AA5083 to AA6082. To that end a systematic set of nine instrumented welds were made using rotation speeds of 280, 560, and 840 rpm and traverse speeds of 100, 200, and 300 mm/min with AA5083 on the advancing side and another nine with the materials reversed. For comparison a smaller series of AA5083-AA5083 and AA6082-AA6082 welds were also made. Thermocouple measurements, tool torque, extent of material mixing, and macrostructural observations all indicate that the temperature under the tool is more strongly dependent on the rotation than the traverse speed. It was found that in the current case, the power (energy/s) and heat input (energy/mm) do not correlate simply with the weld temperature. As a result, such metrics may not be suitable for characterizing the conditions under which welds are produced.     

A process model for the heat-affected zone microstructure evolution in duplex stainless steel weldments: Part II. Application to electron beam welding

In the present investigation, a process model for electron beam (EB) welding of different grades of duplex stainless steels (i.e. SAF 2205 and 2507) has been developed. A number of attractive features are built into the original finite element code, including (1) a separate module for prediction of the penetration depth and distribution of the heat source into the plate, (2) adaptive refinement of the three-dimensional (3-D) element mesh for quick and reliable solution of the differential heat flow equation, and (3) special subroutines for calculation of the heat-affected zone (HAZ) microstructure evolution. The process model has been validated by comparison with experimental data obtained from in situ thermocouple measurements and optical microscope examinations. Subsequently, its aptness to alloy design and optimization of welding conditions for duplex stainless steels is illustrated in different numerical examples and case studies pertaining to EB welding of tubular joints.     

A process model for the heat-affected zone microstructure evolution in duplex stainless steel weldments: Part II. Application to electron beam welding

In the present investigation, a process model for electron beam (EB) welding of different grades of duplex stainless steels (i.e. SAF 2205 and 2507) has been developed. A number of attractive features are built into the original finite element code, including (1) a separate module for prediction of the penetration depth and distribution of the heat source into the plate, (2) adaptive refinement of the three-dimensional (3-D) element mesh for quick and reliable solution of the differential heat flow equation, and (3) special subroutines for calculation of the heat-affected zone (HAZ) microstructure evolution. The process model has been validated by comparison with experimental data obtained from in situ thermocouple measurements and optical microscope examinations. Subsequently, its aptness to alloy design and optimization of welding conditions for duplex stainless steels is illustrated in different numerical examples and case studies pertaining to EB welding of tubular joints.     

Heat transfer during Nd: Yag pulsed laser welding and its effect on solidification structure of austenitic stainless steels

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.     

Precipitate effects on the mechanical behavior of aluminum copper alloys: Part II. Modeling

This work focuses on a new hardening formulation accounting for precipitate-induced anisotropy in a binary aluminum-copper precipitation-hardened alloy. Different precipitates were developed upon aging at 190°C and 260°C, and corresponding work hardening characteristics were predicted for single and polycrystals. The use of single crystals facilitated the demonstration of the effect of precipitates on the flow anisotropy behavior. Pure aluminum was also studied to highlight the change in deformation mechanisms due to the introduction of precipitates in the matrix. The influence of precipitate-induced anisotropy on single-crystal flow behavior was clearly established, again relating to the precipitate character. Simulations are presented for several single-crystal orientations and polycrystals, and they display good agreement with experiments. The work demonstrates that precipitate-induced anisotropy can dominate over the crystal anisotropy effects in some cases. T. FOGLESONG formerly with the Department of Mechanical and Industrial Engineering, University of Illinois, Urbana, IL 61801     

Precipitate effects on the mechanical behavior of aluminum copper alloys: Part II. Modeling

This work focuses on a new hardening formulation accounting for precipitate-induced anisotropy in a binary aluminum-copper precipitation-hardened alloy. Different precipitates were developed upon aging at 190 °C and 260 °C, and corresponding work hardening characteristics were predicted for single and polycrystals. The use of single crystals facilitated the demonstration of the effect of precipitates on the flow anisotropy behavior. Pure aluminum was also studied to highlight the change in deformation mechanisms due to the introduction of precipitates in the matrix. The influence of precipitate-induced anisotropy on single-crystal flow behavior was clearly established, again relating to the precipitate character. Simulations are presented for several single-crystal orientations and polycrystals, and they display good agreement with experiments. The work demonstrates that precipitate-induced anisotropy can dominate over the crystal anisotropy effects in some cases.     

X-ray-based measurement of composition during electron beam melting of AISI 316 stainless steel: Part I. Experimental setup and processing of spectra

An energy dispersive X-ray (EDX) detector mounted on a laboratory scale electron beam furnace (30 kW) was employed to assess the potential use of X-rays as a means of on-line liquid alloy composition monitoring during electron beam (EB) melting of alloys. The design and construction of the collimation and protection systems used for the EDX are described in Part I. X-ray spectra are obtained from a sample of AISI 316 stainless steel at both beam idle (in the absence of liquid metal) and high power (in the presence of liquid metal). Two different types of molds are employed: (1) a water-cooled copper mold and (2) a ceramic lined water-cooled copper mold. Various strategies for signal processing and filtration are presented and compared. Correction factors for beam voltage were developed and applied in order to develop correlations between the mole fraction and normalized X-ray intensity for Ni−K _α, Cr−K _α, and Fe−K _α based on an analysis of the vapor condensate. Correlations were also developed relating the change in the X-ray intensities to time for (a) Mo−L, (b) Cr−K _α, (c) Fe−K _α, and (d) Ni−K _α. The stability of the electron beam was found to be the principal source of error, and suggestions for further improvements are also discussed. The study confirms the feasibility of the method and is the first reported study of on-line analysis of a high-temperature liquid alloy. In Part II, the technique is applied to the study of the complex evaporation processes occurring during EB melting.     

X-ray-based measurement of composition during electron beam melting of AISI 316 stainless steel: Part I. Experimental setup and processing of spectra

An energy dispersive X-ray (EDX) detector mounted on a laboratory scale electron beam furnace (30 kW) was employed to assess the potential use of X-rays as a means of on-line liquid alloy composition monitoring during electron beam (EB) melting of alloys. The design and construction of the collimation and protection systems used for the EDX are described in Part I. X-ray spectra are obtained from a sample of AISI 316 stainless steel at both beam idle (in the absence of liquid metal) and high power (in the presence of liquid metal). Two different types of molds are employed: (1) a water-cooled copper mold and (2) a ceramic lined water-cooled copper mold. Various strategies for signal processing and filtration are presented and compared. Correction factors for beam voltage were developed and applied in order to develop correlations between the mole fraction and normalized X-ray intensity for Cr-K _α, and Fe-K _α based on an analysis of the vapor condensate. Correlations were also developed relating the change in the X-ray intensities to time for (a) Mo-L, (b) Cr-K _α, (c) Fe-K _α, and (d) Ni-K _α. The stability of the electron beam was found to be the principal source of error, and suggestions for further improvements are also discussed. The study confirms the feasibility of the method and is the first reported study of on-line analysis of a high-temperature liquid alloy. In Part II, the technique is applied to the study of the complex evaporation processes occurring during EB melting.