The effects of process variables on pulsed Nd:YAG laser spot welds: Part II. AA 1100 aluminum and comparison to AISI 409 stainless steel

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.     

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.     

Some aspects of thermomechanical fatigue of AISI 304L stainless steel: Part I. creep- fatigue damage

Thermomechanical fatigue (TMF) tests on the austenitic stainless steel AISI 304L have been conducted under “true≓ plastic-strain control in vacuum. This report considers the damage oc-curring during TMF loading. It is shown how the temperature interval and the phasing (in-phase, out-of-phase) determine the mechanical response and the lifetime of the specimens. If creep-fatigue interaction takes place during in-phase cycling, the damage occurs inside the ma-terial, leading to intergranular cracks which reduce the lifetime considerably. Out-of-phase cy-cling inhibits creep-induced damage, and no lifetime reduction occurs, even if the material is exposed periodically to temperatures in the creep regime. A formula is proposed which allows prediction of the failure mode, depending on whether creep-fatigue damage occurs or not. At a given strain rate, the formula is able to estimate the temperature of transition between pure fatigue and creep-fatigue damage. H.J.Christ formerly with the University of Erlangen.     

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 I. Mathematical model of the process

Some available mathematical models for the argon-oxygen decarburization (AOD) stainless steelmaking process have been reviewed. The actual situations of the AOD process, including the competitive oxidation of the elements dissolved in the molten steel and the changes in the bath composition, as well as the nonisothermal nature of the process, have been analyzed. A new mathematical model for the AOD refining process of stainless steel has been proposed and developed. The model is based on the assumption that the blown oxygen oxidizes C, Cr, Si, and Mn in the steel and Fe as a matrix, but the FeO formed is also an oxidant of C, Cr, Si, and Mn in the steel. All the possible oxidation-reduction reactions take place simultaneously and reach a combined equilibrium in competition at the liquid/bubble interfaces. It is also assumed that at high carbon levels, the oxidation rates of elements are primarily related to the supplied oxygen rate, and at low carbon levels, the rate of decarburization is mainly determined by the mass transfer of carbon from the molten steel bulk to the reaction interfaces. It is further assumed that the nonreacting oxygen blown into the bath does not accumulate in the liquid steel and will escape from the bath into the exhaust gas. The model performs the rate calculations of the refining process and the mass and heat balances of the system. Also, the effects of the operating factors, including adding the slag materials, crop ends, and scrap, and alloy agents; the nonisothermal conditions; the changes in the amounts of metal and slag during the refining; and other factors have all been taken into account. []—metal phase; ()—slag phase; {}—gaseous phase; and 〈〉—solid phase     

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.     

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.     

Grain-size effects on tensile behavior of nickel and AISI 316L stainless steel

The aim of this work is to provide experimental results to understand the grain-size effects on tensile hardening of fcc polycrystalline materials. The contribution of grain size on hardening rate is discussed in terms of backstress (X) and effective-stress (Σ _ef) evolutions in the different hardening stages. Based on this stress partition, the origin of the classical Hall-Petch relationship is clarified at the different levels of microstructural heterogeneities. If the backstresses and effective stresses verified the Hall-Petch formulation, however, the effective stress is less dependent on grain size than the backstress. The grain-size effect on short-range internal stresses (effective stress) is well explained in terms of a mean path length using classical dislocation modeling. The backstress dependence on grain size seems to be mainly the result of intergranular plastic-strain incompatibilities in relation with the formation of a grain-boundary layer in stage I. In others stages (higher plastic strain), the interactions between intergranular and intragranular long-range internal stresses have been illustrated. The degree of these interactions remains unclear.     

The constitutive behavior of laser welds in 304L stainless steel determined by digital image correlation

A digital image correlation (DIC) method has been used to characterize the constitutive tensile stress-strain response in 304L austenitic stainless steel weldments produced by both continuous-wave (CW) and pulsed-wave (PW) laser welding. The method provides quantitative two-dimensional (2-D) strain maps of the deformation field across the transverse weld samples throughout the tensile test. Local stress-strain response was extracted from regions within the fusion zone and compared to base metal response. The weldments were found to have a higher yield strength than the base metal. The metallurgical origin for the fusion zone strengthening was largely attributed to Hall-Petch and ferrite content effects. While failures localized in the fusion zone with little appreciable necking, the material within the fusion zone retained considerable local ductility: more than 45 pct strain at failure. Significant weld root porosity found in the PW condition and absent in the CW condition appeared to have no deleterious effect on the mechanical performance under the present test conditions in this very ductile, flaw-tolerant alloy.     

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.     

Physical-mechanical effects of Nd:YAG laser on the surface of sound dental enamel

We present our experience with 55 children in which we performed flexible fiberoptic bronchoscopy (FFB) using an Olympus BF3C20 instrument and by using sedation and local anaesthesia or laryngeal mask airway. Indications for performing this procedure were stridor, opportunist or recurrent pneumonia, persistent atelectasis, a suspected foreign body, confirmation of endobronchial tuberculosis and evaluation of tracheostomy. In 70% of the cases, the diagnosis was made by the FFB and 14 cases were normal. One child with severe hypoxia presented respiratory arrest and need intubation. Our results suggest that FFB is safe, has advantages over rigid bronchoscopy, avoids general anaesthetic and with laryngeal mask airway is possible to perform in patients of every age.     

Effect of welding processes on tensile, impact, hardness and microstructure of AISI 409M ferritic stainless joints fabricated using duplex stainless steel filler metal

 The present investigation is aimed at to study the effect of welding processes such as shielded metal arc welding, gas metal arc welding and gas tungsten arc welding on tensile and impact properties of the ferritic stainless steel conforming to AISI 409M grade. Rolled plates of 4 mm thickness were used as the base material for preparing single pass butt welded joints. Tensile and impact properties, micro hardness, microstructure and fracture surface morphology of the welded joints have been evaluated and the results are compared. From this investigation, it is found that gas tungsten arc welded joints of ferritic stainless steel showed superior tensile and impact properties compared with shielded metal arc and gas metal arc welded joints and this is mainly due to the presence of finer grains in fusion zone and heat affected zone.     

A process model for the heat-affected zone microstructure evolution in duplex stainless steel weldments: Part I. the model

The present investigation is concerned with modeling of the microstructure evolution in duplex stainless steels under thermal conditions applicable to welding. The important reactions that have been modeled are the dissolution of austenite during heating, subsequent grain growth in the delta ferrite regime, and finally, the decomposition of the delta ferrite to austenite during cooling. As a starting point, a differential formulation of the underlying diffusion problem is presented, based on the internal-state variable approach. These solutions are later manipulated and expressed in terms of the Scheil integral in the cases where the evolution equation is separable or can be made separable by a simple change of variables. The models have then been applied to describe the heat-affected zone microstructure evolution during both thick-plate and thin-plate welding of three commercial duplex stainless steel grades: 2205, 2304, and 2507. The results may conveniently be presented in the form of novel process diagrams, which display contours of constant delta ferrite grain size along with information about dissolution and reprecipitation of austenite for different combinations of weld input energy and peak temperature. These diagrams are well suited for quantitative readings and illustrate, in a condensed manner, the competition between the different variables that lead to structural changes during welding of duplex stainless steels.     

Decarburization of stainless steel: Part I. A mathematical model for laboratory scale results

A mathematical model is presented for describing the reaction of iron-chromium-carbon melts with pure oxygen, air and oxygen-argon mixtures. The model is based on the generalization of the Asai-Muchi model for oxygen steelmaking to systems containing chromium and where the oxygen blown is diluted by an inert gas. The predictions based on the model were compared to the experimental measurements of Barnhardt obtained with heats of about 1.2 to 1.5 kg, having carbon contents ranging from 0.3 to 0.6 wt pct and chromium contents of 0.0 to 21.0 wt pct. The agreement between measurements and predictions was quite good for a variety of blowing arrangements which included top blowing with pure oxygen or air and bottom blowing with air. The fact that this good agreement was obtained by using a single value of the adjustable parameter in the model for all runs, renders very promising the extension of the model to more complicated systems. On leave from Department of Iron and Steel Engineering, Nagoya University, Nagoya, Japan.     

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.     

Effects of pulsed Nd:YAG laser radiation on action potential conduction in nerve fibres inside teeth in vitro

OBJECTIVE: This study aimed to simulate the effects of lasing dentine on pulpal nerve function. METHODS: Rat spinal nerve roots were threaded through the prepared pulp canal of a 10 mm long tooth root segment which was mounted in a perspex bath. The protruding ends of the nerve were placed on platinum wire electrodes used to elicit and to record compound nerve action potentials (CAPs). Laser energy (average power = 0.3-3.0 W) was applied to the surface of the root segment using a pulsed Nd:YAG dental laser (dLase 300). RESULTS: With the laser probe tip placed in static contact with the tooth surface, the nerve CAP was irreversibly abolished within 60 s of lasing at 1.0-3.0 W power. When the laser tip was moved to and fro over the root surface in a scanning mode, similar levels of radiation produced less marked effects. In the latter mode, CAP attenuation increased with increasing power and duration of lasing. After 60 s lasing at 0.3 W, the CAP size was 95% (+/- 5, S.D.) of the prelasing controls value; with 2.0 W the CAP was reduced to 54% (+/- 33). The CAP recovered to 90% of control levels after lasing at powers up to 1.5 W, but reached only 72% of control values after lasing at 2.0 W power. CONCLUSIONS: Laser radiation applied to dentine caused a dose-dependent block of action potential conduction in nerve fibres in the underlying pulp chamber.     

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.