High-Efficiency Dynamic Scanning Strategy for Powder Bed Fusion by Controlling Temperature Field of the Heat-Affected Zone

Improvement of fabrication efficiency and part performance was the main challenge for the large-scale powder bed fusion(PBF)process.In this study,a dynamic monitoring and feedback system of powder bed temperature field using an infrared thermal imager has been established and integrated into a four-laser PBF equipment with a working area of 2000 mm×2000 mm.The heat-affected zone(HAZ)temperature field has been controlled by adjusting the scanning speed dynamically.Simultaneously,the relationship among spot size,HAZ temperature,and part performance has been established.The fluctuation of the HAZ temperature in four-laser scanning areas was decreased from 30.85℃to 17.41℃.Thus,the consistency of the sintering performance of the produced large component has been improved.Based on the controllable temperature field,a dynamically adjusting strategy for laser spot size was proposed,by which the fabrication efficiency was improved up to 65.38%.The current research results were of great significance to the further industrial applications of large-scale PBF equipment.

Effect of Dynamic Strain Aging on Hardness in the Heat-Affected Zone of SUS316 Steel Welds

DSA(dynamic strain aging)phenomenon in SUS316 steel was investigated using isothermal and non-isothermal tensile tests of simulated HAZ(heat-affected zone)thermal cycles.Isothermal tensile tests were performed on SUS316 in the peak temperature range of 20-700°C,with strain rates varying from 4.2×10^(-3)to 4.2×10^(-5)s^(-1).Based on the appearance of discontinuous plastic flows,expressed as serrations,and the hardening phenomenon of the tensile samples,the conditions for the occurrence of DSA in the SUS316 steel were investigated.Furthermore,the extent of hardening due to DSA was evaluated by comparing the hardness values of the SUS316 and SUS316EHP steels after the tensile tests.To confirm the effect of DSA on hardness in the HAZ of the welded SUS316 steel,non-isothermal tensile tests of the simulated HAZ thermal cycles were performed using a Thermec Master.The relationship between the increase in Vickers hardness due to DSA and the strain in the HAZ was determined;the effect of DSA on hardness in the HAZ could be predicted.The DSA in SUS316 steel was found to be mainly attributed to the dynamic interaction of dislocations with C and N interstitial atoms during high-temperature deformation.

Influence of different microstructural features on impact toughness and crack initiation behavior of coarse grain heat-affected zone in X80 pipeline steel

Coarse grain heat-affected zone samples of X80 pipeline steel under different heat inputs were obtained through thermal welding simulation experiments with Gleeble 3500.Charpy impact tests and a combination of multiscale characterizations were conducted to investigate the influence of various microstructural features on impact toughness and crack initiation behavior.The results prove that, as the heat input increases, the number of M/A components increases, thereby degrading toughness and increasing hardness.Meanwhile, more M/A constituents tend to aggregate on prior austenite grain boundaries(PAGBs),and the overall dimensions of M/A and the width and volume fraction of the lath martensite substructure inside M/A islands would increase as well.These changes make intersections between boundary M/As and PAGBs become one of the preferred sites for crack initiation.In addition, only large-sized grotesque inclusions can act as a direct inducement of crack initiation.

Liquation cracking in the heat-affected zone of IN939 superalloy tungsten inert gas weldments

The main aim of this study was to investigate liquation cracking in the heat-affected zone(HAZ)of the IN939 superalloy upon tungsten inert gas welding.A solid solution and age-hardenable filler metals were further studied.On the pre-weld heat-treated samples,upon solving the secondaryγ′particles in the matrix,primaryγ′particles in the base metal grew to"ogdoadically diced cubes"of about 2μm in side lengths.The pre-weld heat treatment reduced the hardness of the base metal to about HV 310.Microstructural studies using optical and fieldemission scanning electron microscopy revealed that the IN939 alloy was susceptible to liquation cracking in the HAZ.The constitutional melting of the secondary,eutectic,and Zr-rich phases promoted the liquation cracking in the HAZ.The microstructure of the weld fusion zones showed the presence of fine spheroidalγ′particles with sizes of about 0.2μm after the post-weld heat treatment,which increased the hardness of the weld pools to about HV 350 and 380 for the Hastelloy X and IN718 filler metals,respectively.Application of a suitable solid solution filler metal could partially reduce the liquation cracking in the HAZ of IN939 alloy.

Heat-affected zone in pulsed laser cutting of titanium alloy

For the machining of bladed ring parts of an aeroengine, experiments on Nd∶YAG pulsed laser cutting of TC1 titanium alloy sheet were carried out to investigate the influences of laser cutting parameters on heat-affected zone (HAZ). The selected laser cut samples were photographed by scanning electron microscopy. The results show that the microstructure in HAZ layers is characterized by metastable acicular martensite grains with a slight increase of hardness by 10%. The thickness of HAZ layers was studied as a function of laser cutting parameters. Medium pulse energy, higher pulse rate, higher cutting speed and argon at higher pressures help to reduce the thickness of HAZ layers.

High-temperature creep properties of fine grained heat-affected zone in P92 weldment

The simulated fine grained heat-affected zone (FGHAZ) specimens for P92 welded joints were prepared by heat treatment, then the creep tests were carried out at 650 ℃ under the applied stress of 90-120 MPa to investigate high-temperature creep behavior of FGHAZ. The results show that the creep property of FGHAZ is much inferior to that of the base metal, which exhibits the much higher steady creep rate and shorter time to creep fracture. The power law equation can describe the steady creep rate dependence on applied stress, indicating that the stress exponent n of FGHAZ is distinguished between two regions with n=15.1 at high stresses (more than 100 MPa) and n=8.64 at lower stresses. Based on Monkman-Grant equation, the relationship between the secondary creep rate and time to rupture is obtained to evaluate the creep life of FGHAZ with the applied stress above 100 MPa.

Effects of Electrode Types on the Microstructure, Tensile and Hardness Properties of 304 L Austenitic Stainless Steel Heat-Affected Zone (HAZ)

Submerged arc welding (SAW) has been well utilised for the production of weld joints in 304 L ASS for various industrial application. However, effective performance of the material in service has been hampered by improper choice of electrode. Therefore, in this study, effects of different types of electrode on the microstructure and tensile property of type 304 L austenitic stainless steel heat-affected zone (HAZ) were studied. Chemical composition of the as-received sample was determined. A number of samples were cut from the as-received sample. Afterwards, two half were joined together with 308 L, 312 L and 316 electrodes at a controlled welding speed, current and voltage of 4.6 mm/s, 160 A and 30 V to produce a constant heat input of 626.09 J/mm. An automatic SAW machine with Model Type: DX3-301, and Frequency: 50 Hz was used. And based on ASTM standard, tensile and hardness samples were prepared from the as-received and HAZs. Tensile and hardness measurements were made. Also, specimens for microscopy studies were prepared from the HAZ and as-received samples. From the results, microstructures of the HAZs revealed varied volume fraction of austenite and ferrite phases and grain sizes, and at austenite and ferrite grain boundaries, chromium carbide formation and precipitation were observed. The weld joint produced with 308 L electrode revealed optimum UTS value and YS value of 475 and 325 respectively. While weld joint produced with 316 L electrode has superior ductility of value 41%. Irrespective of the types of electrode used, the as-received sample revealed superior tensile properties over the weld joints. Also, optimum hardness value of 45.7 HRA was obtained with 308 L. Hardness value of the as-received sample was higher than that of HAZ samples.

HEAT-AFFECTED ZONE FRACTURE TOUGHNESS OF HIGH STRENGTH STEELS EVALUATED USING THE WEAKEST LINK MODEL

A probabilistic model is derived based on the weakest link theory to evaluate the specimen thickness effect and the size effect of coarse-grained heat-affected zone (CGHAZ) on CTOD toughness values in heat-affected zones (HAZ) of high strength steels. The scatter of CTOD fracture toughness values in heterogeneous weldments is predicted using this model. The specimen thickness has influence on the CTOD toughness values of HAZ. With increase in the specimen thickness the estimated and experimental mean CTOD values decrease while the lower boundary CTOD values remain almost unchanged. The size of CGHAZ has strong negative effect on the mean CTOD values of HAZ. The estimated mean CTOD values and lower boundary CTOD values agree with experimental CTOD testing results of HAZ notched specimens. The facture toughness scatter is dependent on the Weibull shape parameter a and specimen thickness.

In situ observation of austenite grain growth behavior in the simulated coarse-grained heat-affected zone of Ti-microalloyed steels

The austenite grain growth behavior in a simulated coarse-grained heat-affected zone during thermal cycling was investigated via in situ observation. Austenite grains nucleated at ferrite grain boundaries and then grew in different directions through movement of grain boundaries into the ferrite phase. Subsequently, the adjacent austenite grains impinged against each other during the α→γ transformation. After the α→γ transformation, austenite grains coarsened via the coalescence of small grains and via boundary migration between grains. The growth process of austenite grains was a continuous process during heating, isothermal holding, and cooling in simulated thermal cycling. Abundant finely dispersed nanoscale TiN particles in a steel specimen containing 0.012wt% Ti effectively retarded the grain boundary migration, which resulted in refined austenite grains. When the Ti concentration in the steel was increased, the number of TiN particles decreased and their size coarsened. The big particles were not effective in pinning the austenite grain boundary movement and resulted in coarse austenite grains.

Study on the microstructure of 12%chromium low carbon stainless steel in a high temperature heat-affected zone

Coarsening,embrittlement and corrosion sensitization in a high temperature heat-affected zone(HTHAZ) are the major problems when 12%chromium low carbon stainless steel is being welded,which induce the deterioration of the impact toughness at a low temperature and intergranular corrosion resistance property.This study investigates the corresponding microstructures in HTHAZ with different chemical compositions and heat inputs through thermal simulation tests.The results show that the martensite content increases with the descending of ferrite factor(FF) when FF is below 9.0 and heat input influences the microstructure of high FF steel in HTHAZ.Martensite of 12%Cr stainless steel in HTHAZ with only Nb stabilization reticularly distributes at ferrite grain boundaries.

Microstructure and embrittlement of the fine-grained heat-affected zone of ASTM4130 steel

The mechanical properties and microstructure features of the fine-grained heat-affected zone(FGHAZ) of ASTM4130 steel was investigated by optical microscope(OM),scanning electron microscope(SEM),transmission electron microscope(TEM),and welding thermal simulation test.It is found that serious embrittlement occurs in the FGHAZ with an 81.37% decrease of toughness,compared with that of the base metal.Microstructure analysis reveals that the FGHAZ is mainly composed of acicular,equiaxed ferrite,granular ferrite,martensite,and martensite-austenite(M-A) constituent.The FGHAZ embrittlement is mainly induced by granular ferrite because of carbides located at its boundaries and sub-boundaries.Meanwhile,the existence of martensite and M-A constituent,which distribute in a discontinuous network,is also detrimental to the mechanical properties.

Effect of welding parameters on the heat-affected zone of AISI409 ferritic stainless steel

One of the main problems during the welding of ferritic stainless steels is severe grain growth within the heat-affected zone(HAZ).In the present study,the microstructural characteristics of tungsten inert gas(TIG) welded AISI409 ferritic stainless steel were investigated by electron backscattered diffraction(EBSD),and the effects of welding parameters on the grain size,local misorientation,and low-angle grain boundaries were studied.A 3-D finite element model(FEM) was developed to predict the effects of welding parameters on the holding time of the HAZ above the critical temperature of grain growth.It is found that the base metal is not fully recrystallized.During the welding,complete recrystallization is followed by severe grain growth.A decrease in the number of low-angle grain boundaries is observed within the HAZ.FEM results show that the final state of residual strains is caused by competition between welding plastic strains and their release by recrystallization.Still,the decisive factor for grain growth is heat input.

Study on laser welded heat-affected zone in new ultralow carbon bainitic steel

800 MPa grade ultralow carbon bainitic (NULCB) steel is the recently developed new generation steel, which was produced by thermo mechanical controlled processing & relaxation-precipitation controlling transformation (TMCP&RPC) technique. The microstructure and the mechanical properties of the heat-affected zone (HAZ) in NULCB steel under laser welding condi- tions were investigated by using a Gleeble-1500 thermal simulator. The experimental results indicate that the simplex microstructure in the HAZ is granular bainite that consists of bainite-ferrite (BF) lath and M-A constituent when the cooling time from 800 to 500°C (t_(8/5)) is 0.3-30 s, and the M-A constituent consists of twinned martensite and residual austenite. As t_(8/5) increases, the hardness and tensile strength of HAZ decreases, but they are higher than that of the base metal, indicating the absence of softened zone after laser welding. The impact toughness of HAZ increases at first and then decreases when t_(8/5) increases. The impact energy of HAZ is much higher than that of the base metal when t_(8/5) is between 3 and 15 s. It indicates that excellent low temperature toughness can be obtained under ap- propriate laser welding conditions.

Improving creep strength of the fine-grained heat-affected zone of novel 9Cr martensitic heat-resistant steel via modified thermo-mechanical treatment

The infamous type Ⅳ failure within the fine-grained heat-affected zone (FGHAZ) in G115 steel weldments seriously threatens the safe operation of ultra-supercritical (USC) power plants.In this work,the traditional thermo-mechanical treatment was modified via the replacement of hot-rolling with cold rolling,i.e.,normalizing,cold rolling,and tempering (NCT),which was developed to improve the creep strength of the FGHAZ in G115 steel weldments.The NCT treatment effectively promoted the dissolution of preformed M_(23)C_(6)particles and relieved the boundary segregation of C and Cr during welding thermal cycling,which accelerated the dispersed reprecipitation of M_(23)C_(6) particles within the fresh reaustenitized grains during post-weld heat treatment.In addition,the precipitation of Cu-rich phases and MX particles was promoted evidently due to the deformation-induced dislocations.As a result,the interacting actions between precipitates,dislocations,and boundaries during creep were reinforced considerably.Following this strategy,the creep rupture life of the FGHAZ in G115 steel weldments can be prolonged by 18.6%,which can further push the application of G115 steel in USC power plants.

Effect of cathodic potential on stress corrosion cracking behavior of different heat-affected zone microstructures of E690 steel in artificial seawater

In this work,the stress corrosion cracking(SCC)behavior of E690 steel base metal(BM)and different heat-affected zone(HAZ)microstructures,i.e.,coarse grain HAZ(CGHAZ),fine grain HAZ(FGHAZ),and intercritical HAZ(ICHAZ),was investigated at different cathodic potentials in artificial seawater by slow strain rate tensile tests,scanning electron microscopy and electron back-scattered diffraction measurements.The results show that the HAZ microstructures and BM exhibit different SCC susceptibilities:FGHAZ<ICHAZ<BM<CGHAZ,which are controlled by anodic dissolution(AD)at the open circuit potential.With the cathodic potential equaling to-750 mV,the SCC susceptibility of the four microstructures increases because of the synergistic effect of AD and weak hydrogen embrittlement(HE).At-850 mV,AD is inhibited,and the SCC susceptibility of BM decreases,while the SCC susceptibility of the HAZ microstructures increases.At a potential below-850 mV,the SCC susceptibility of the four microstructures gradually increases because of the augment of HE,and the SCC susceptibility of the HAZ microstructures is higher than that of BM.The distinction reveals that the HAZ microstructures have the greater HE susceptibility than BM.

Impact Toughness of Heat-Affected Zones of 11Cr Heat-Resistant Steels

Aiming at the requirements of structural steel in Gen-IV nuclear reactor, the high-chromium martensitic heat-resistant steels containing 10–12% chromium were developed. The toughness of heat-affected zones(HAZs) is one of the important factors for evaluating the weldability of steels. In this paper, the simulated HAZs were fabricated using tempered SIMP steels. The effects of microstructures on the impact toughness of materials were analyzed using Vickers hardness tester, optical microscope, transmission electron microscope. Experimental results demonstrated that the HAZs of weldment were poor in toughness, much lower than that of the base metal. However, after experiencing post-weld heat treatment, the toughness of the HAZs increased greatly. The toughness became better in terms of CG-HAZ, FG-HAZ and IC-HAZ for the two steels, regardless of as-welded or after PWHT. Compared with SIMP7 steel, chemical compositions, such as C, Si, Mn and Cr, were adjusted to a lower content;the toughness of base metal and simulated HAZs was better in the case of SIMP11. The conjunct roles of dislocation density and carbon contents retained in the martensite led to poor impact toughness of the aswelded HAZs, because dislocations and carbon atoms affected the inner stresses within lattices.