Numerical simulation of the liquating behavior of niobium carbide in heat-affected-zone during welding of a superalloy

In order to predict the propensity of a superalloy to heat-affected-zone (HAZ) liquation cracking, Visual FORTRAN procedures were developed based on a heat transfer and mass diffusion model and the constitutional liquation of precipitate (NbC) at grain boundaries was simulated numerically. The results show that with the increase in the rate of welding thermal cycle, the solid dissolution of precipitate prior to liquation decreases and the thickness of liquid film produced by constitutional liquation increases. Higher heating rate inhibits the further melting of adjacent matrix and the solidification of liquid by liquid-to-γ mode in the subsequent thermal cycle. As a result, the residual liquid film still maintains a great thickness at the moment when temperature is down to the eutectic point, which will promote HAZ micro-fissuring. Finally, hot ductility tests on a low-expansion superalloy were performed to verify indirectly the conclusions drawn from the numerical simulation.     

Microstructural analysis of fusion and heat affected zones in electron beam welded ALLVAC 718PLUS™ superalloy

The fusion zone and heat affected zone (HAZ) microstructures of electron beam welded superalloy 718PLUS™ (718 Plus) that has been newly developed by ATI ALLVAC were examined. The microsegregation pattern during solidification of the fusion zone indicated that while Fe, Co, W, and Cr segregated to the core of the gamma dendrites, Nb, Ti, and Al were extensively rejected into the interdendritic liquid. Electron diffraction and X-ray microanalysis using transmission electron microscopy (TEM) of the fusion zone showed that the major secondary phases that formed from the interdendritic liquid were gamma/MC type carbide eutectic and gamma/Laves eutectic constituents. HAZ microstructure showed partially melted zone immediately adjacent to the fusion zone and intergranular microfissuring associated with resolidified products which suggested that HAZ cracking in this alloy occurred by liquation cracking. Microstructural examination of the HAZ using analytical scanning electron microscope showed resolidified gamma/Laves eutectic on the cracked and backfilled grain boundaries. Fine resolidified MC type carbide particles were also observed in the HAZ. Causes of grain boundary liquation were identified and the solidification of intergranular liquid in the HAZ was discussed.     

热处理工艺对GH909板材TIG焊缝析出相的影响

为改善GH909板材的焊接工艺及使用性能,采用扫描电镜和EDX分析研究了GH909板材在TIG焊后不同热处理工艺焊缝析出相的变化规律,对提高GH909板材TIG焊接接头质量具有重要的应用价值.研究结果表明：焊缝形成的相有针片状ε/ε″相、γ/Laves低熔点共晶相,且都在γ晶间分布;焊缝中心低熔点共晶相所占比例均比离焊...     

On preventing HAZ cracking in laser welded DS Rene 80 superalloy

Abstract

The heat affected zone (HAZ) cracking behaviour in a laser beam directionally solidified (DS) Rene 80 nickel based superalloy subjected to preweld heat treatments was studied. The HAZ cracks in the alloy are grain boundary liquation cracks caused by liquation reaction of both non-equilibrium secondary solidification product, MC carbides and equilibrium solid state reaction product, γ′ precipitates. In contrast to theoretical prediction based a preweld heat treatment that reduced grain boundary liquid film thickness did not result in a lower HAZ cracking, which can be related to concomitant reduction in the ability of the base alloy to relax welding stress. In addition, formation of intergranular M₅B₃ boride particles in preweld alloy appeared to have aided cracking susceptibility by lowering grain boundary liquation temperature and widening the brittle temperature range in the HAZ during cooling. Based on the analysis of the results, application of a new preweld heat treatment that prevents the formation of the intergranular borides and induces moderate base alloy hardness resulted in a nearly crack free HAZ in laser welded DS Rene 80 superalloy.     

Effect of rhenium on solidification of Inconel 718 alloy

Abstract

The effect of rhenium (Re) on the solidification of standard Inconel 718 (St-In718) has been investigated by using experimental alloys containing 2·4, 3·5, and 6·0%Re. Rhenium is one of the most powerful refractory elements that improve the high temperature mechanical properties of Ni base superalloys. Results indicate that solidification starts with precipitation of primary γ phase, which is followed by (γ+ NbC) eutectic and (γ+ Ni₂Nb)eutectic. The solidification temperature of the St-In718 is increased by 30 K with the addition of 6%Re. Moreover, the volume fraction of the primary γ is increased, while the volume fraction of eutectic (γ+ NbC) and (γ+ Ni₂Nb) are decreased by the addition of Re. Also, increasing Re content enlarges the secondary dendrite arm spacing (SDAS). Finally, the effects of Re on the partition coefficients k of alloying elements to primary γ and to eutectic γ+ NbC were evaluated and Re was found to segregate preferentially to the primary γ.     

高钒铁碳合金凝固过程及元素的分布

通过定向凝固 液淬试验、金相分析、电子探针面扫描、微区成分能谱分析和差热分析等手段,研究了高钒铁碳耐磨合金的凝固过程。结果表明:该合金在凝固时首先从液相中析出初生VC相;随着VC的析出,液相中V含量降低,达到共晶成分时,合金将发生共晶反应L→γ+MC;之后将按照L→γ+MC+MC2的反应进行三元共晶凝固。V主要分布于MC型的碳化物,Mo和Cr主要分布于MC2型的碳化物中。     

Study of grain refinement and eutectic transformation of undercooled IN718 superalloy

Abstract

A substantial undercooling up to 250 K was produced in the IN718 superalloy melt by employing the method of molten salt denucleating, and the microstructure evolution with undercooling was investigated. Within the achieved undercooling, 0–250 K, the solidification microstructure of IN718 undergoes two grain refinements: the first grain refinement occurs in a lower range of undercooling, which results from the ripening and remelting of the primary dendrite, and at a larger range of undercooling, grain refinement attributes to solidification shrinkage stress and lattice distortion energy originating from the rapid solidification process. A ‘lamellar eutectic anomalous eutectic’ transition was observed when undercooling exceeds a critical value of ～250 K. When undercooling is small, owing to niobium enrichment in interdendrite, the remaining liquid solidifies as eutectic (γ+Laves phase); whereas, if the undercooling achieves 250 K, the interdendrite transforms from eutectic (γ+Laves phase) to Laves phase, which results from the formation of divorced eutectic arising from the huge variance of the growth velocities of γ and Laves phases.     

Microstructural evolution and defect formation in a powder metallurgy nickel-based superalloy processed by selective laser melting

In this study,the selective laser melting(SLM)technology has been employed to manufacture a nickelbased superalloy which was conventionally prepared through powder metallurgy(PM)route.The microstructural features and defects were systematically investigated both prior to and after heat treatment and compared with the PM counterpart.Both solidification cracking and liquation cracking were observed in the SLM specimen in which the grain misorientation and low melting point(γ+γ')eutectic played a vital role in their formation mechanism.Columnar grains oriented along building direction were ubiquitous,corresponding to strong<001>fiber texture.Solidification cell structures and melt pools are pervasive and noγ'precipitates were detected at about 10 nm scale before heat treatment.After supersolvus solution and two-step aging treatments,high volume fractionγ'precipitates emerged and their sizes and morphologies were comparable to those in PM alloy.<001>texture is relieved and columnar grains tend to become more equiaxed due to static recrystallization process and grain boundary migration events.Significant annealing twins formed in SLM alloy and are clarified as a consequence of recrystallization.Our results provide fundamental understandings for the SLM PM nickel-based superalloy both before and after heat treatment and demonstrate the potential to fabricate this group of alloys using SLM technology.     

A study on the formation of liquation cracks in the weld heat-affected zone of HY-80 quenched and tempered steel

Liquation cracking of heat-affected zones (HAZs) is often encountered during the welding of HY-80 steels. To reduce the sensitivity of this defect, the content of impurities, such as P and S, must be kept as low as possible. However, in the development of HY-80 steel, HAZ grain boundary liquation is still found even at very low impurity contents. In order to clarify the cause of this defect, the Gleeble hot ductility test and electron probe X-ray microanalysis (EPMA) were carried out. From the results of EPMA, it was evident that the grain boundary liquation in the heat-affected zone of HY-80 steel was due to the low-melting-point eutectic reaction between Cr, Ni and Mn, which had been swept up by the migrating grain boundaries in the welding-heating thermal cycle, and hence enriched at the grain boundary. In addition, the Gleeble hot ductility test results revealed that the HAZ liquation cracking sensitivity of HY-80 steel could be decreased by reducing the C, Ni and Cr contents of base metal, and by decreasing the dwell time at high temperatures during the welding thermal cycle.     

Relationship between η phase formation and solidification rate in directionally solidified IN792 + Hf alloy

Abstract

The effect of solidification rate on the formation of η phase in a directionally solidified IN792 + Hf superalloy was studied. A Cr rich ‘precipitate free zone’ adjacent to the eutectic (γ + γ′) was noticed in the quenched directionally solidified microstructure of the alloy. The formation of the precipitate free zone was due to the solidification of eutectic (γ + γ′) which consumed a large amount of the γ′ forming elements and increased the ratio of (Ti + Hf + Ta + W)/Al in the residual liquid. The high ratio of (Ti + Hf + Ta + W)/Al in the final residual liquid induced the formation of η phase and hence η phase always appeared at the boundary between the eutectic regions and the dendrite core regions. The ratio of (Ti + Hf + Ta + W)/Al was decreased by the diffusion of elements between the residual liquid and the surrounding area. A lower solidification rate favoured the back diffusion and decreased the ratio of (Ti + Hf + Ta + W)/Al in the residual liquid, and hence reduced the formation of η phase.     

Analysis of laser beam weldability of Inconel 738 superalloy

The susceptibility of pre-weld heat treated laser beam welded IN 738 superalloy to heat affected zone (HAZ) cracking was studied. A pre-weld heat treatment that produced the minimal grain boundary liquation resulted in a higher level of cracking compared to those with more intergranular liquation. This deviation from the general expectation of influence of intergranular liquation extent on HAZ microfissuring is attributable to the reduction in the ability of the base alloy to accommodate welding tensile stress that accompanied a pre-weld heat treatment condition designed to minimize intergranular liquation. Furthermore, in contrast to what has been generally reported in other nickel-based superalloys, a decrease in laser welding speed resulted in increased HAZ cracking in the IN 738, which can be attributed to exacerbated process instability at lower welding speeds.     

Laser arc hybrid weld microstructure in nickel based IN738 superalloy

Abstract

A study of the microstructural characteristics of laser arc hybrid welded nickel based IN738 superalloy was performed. Laser arc hybrid welding produced a desirable weld profile in the alloy, similar to what is usually obtained during laser beam welding, and no cracking occurred exclusively in the fusion zone. Elemental partitioning pattern in the fusion zone was studied by electron probe microanalysis and calculating the volume fraction of the weld metal that resulted from the consumption of the filler wire. The result showed that Ti, Ta, Nb, Mo, Al and Zr partitioned into the interdendritic regions of the fusion zone. SEM and TEM examination of the fusion zone showed the presence of secondary solidification reaction constituents, which consists of MC type carbides. The study further revealed that non-equilibrium liquation of various second phases that were present in the alloy prior to welding contributed to intergranular liquation in the heat affected zone (HAZ), which consequently resulted in extensive HAZ intergranular cracking during welding. Although laser arc hybrid welding appears promising for improving the weldability of nickel based IN738 superalloy, a suitable weldability improvement procedure is required in order to minimise HAZ intergranular cracking and thereby enhance the applicability of this technology to the joining of the superalloy.     

原位观察不同冷却速率下GH3625合金的凝固过程

利用带有红外加热炉的共聚焦激光扫描显微镜(CLSM)对GH3625高温合金在不同冷却速率(30℃/min、100℃/min和200℃/min)下的凝固过程进行了动态原位观察,通过差示扫描量热分析仪(DSC)、扫描电镜(SEM)和能谱仪(EDS)研究了凝固后的组织形态及相的析出规律。结果表明,GH3625合金的液相线温度在1 356.5℃,在凝固过程中自由表面液相分数随温度和时间的变化关系满足Avrami方程;凝固过程中主要相的析出顺序依次为γ基体相、碳化物和Laves相;在GH3625合金凝固过程中,随着冷却速率的增大,枝晶细化,枝晶间距减小,成分偏析减轻,Laves相分布更加弥散,且以析出共晶Laves相为主;凝固末期大量的Nb元素富集在枝晶间和晶界,这是形成Laves相的主要原因。     

Preferred growth orientation and microsegregation behaviors of eutectic in a nickel-based single-crystal superalloy

Abstract

A nickel-based single-crystal superalloy was employed to investigate the preferred growth orientation behavior of the (γ + γ′) eutectic and the effect of these orientations on the segregation behavior. A novel solidification model for the eutectic island was proposed. At the beginning of the eutectic island’s crystallization, the core directly formed from the liquid by the eutectic reaction, and then preferably grew along [100] direction. The crystallization of the eutectic along [110] always lagged behind that in [100] direction. The eutectic growth in [100] direction terminated on impinging the edge of the dendrites or another eutectic island. The end of the eutectic island’s solidification terminates due to the encroachment of the eutectic liquid/solid interface at the dendrites or another eutectic island in [110] direction. The distribution of the alloying elements depended on the crystalline axis. The degree of the alloying elements’ segregation was lower along [100] than [110] direction with increasing distance from the eutectic island’s center.     

Microstructural response to heat affected zone cracking of prewelding heat-treated Inconel 939 superalloy

The microstructural response to cracking in the heat-affected zone (HAZ) of a nickel-based IN 939 superalloy after prewelding heat treatments (PWHT) was investigated. The PWHT specimens showed two different microstructures: 1) spherical ordered γ′ precipitates (357–442 nm), with blocky MC and discreet M₂₃C₆ carbides dispersed within the coarse dendrites and in the interdendritic regions; and 2) ordered γ′ precipitates in “ogdoadically” diced cube shapes and coarse MC carbides within the dendrites and in the interdendritic regions. After being tungsten inert gas welded (TIG) applying low heat input, welding speed and using a more ductile filler alloy, specimens with microstructures consisting of spherical γ′ precipitate particles and dispersed discreet MC carbides along the grain boundaries, displayed a considerably improved weldability due to a strong reduction of the intergranular HAZ cracking associated with the liquation microfissuring phenomena.     

镍基高温合金电子束焊接热影响区微裂纹特征分析

利用金相分析和扫描电镜对镍基高温合金电子束焊接热影响区微裂纹行为进行了分析.研究发现,熔合线附近的热影响区产生大量液化裂纹和沿晶扩展的固相裂纹.液化裂纹起源于MC碳化物的组份液化而形成的晶界连续或半连续的低熔点共晶液化膜,固相裂纹形成的则是高能电子束流的快速瞬态热冲击效应的直接结果.通过改善焊缝成形和提高焊接线能量有助于减小两类热影响区微裂纹倾向.     

Thermodynamics and structure of solidification in the fusion zone of CO2 laser welds of 15-5 PH stainless steel

The microstructure in the fusion zone of 15-5 PH autogeneous laser beam welds revealed that the solidification mode is primary ferrite. Both primary ferrite and eutectic ferrite existed in the fusion zone. Primary ferrite was found to be in the vermicular morphology and located at the axes of the cells. Eutectic ferrite was roundish in nature and was located at the cell boundaries. Eutectic ferrite was identified with its location in the structure and it is associated with NbC and a niobium-containing Z-phase precipitate.

The formation of a eutectic and the thermal phase stability of 15-5 PH was predicted from the Fe-Ni-Cr phase diagram for the nominal composition of the alloy from equal chemical potentials and differential mass balance equations. These equations were used to predict the solidification path and microsegregation patterns for iron, nickel and chromium for the case of complete mixing in the liquid. It was shown that the solidification mode is ferrite and that the liquid reaches the eutectic valley when the fraction of solid is 0.7.

The precipitation hardening element and the phase were pure copper, and were coherent with the matrix both in the fusion zone and in the base metal. Generally, the fusion zone was found to be harder than the base metal.     

Solutal configuration during coupled two-phase [α (Al) + θ-Al2Cu] planar univariant eutetic growth in Al-Cu-(Ag, Si) ternary eutetic alloys

The solutal configuration, both in the solid as in the liquid, during coupled two-phase planar growth as it can be obtained during unidirectional solidification of ternary Al-Cu-Ag and Al-Cu-Si alloys with a composition close to the univariant eutectic reaction L → α (Al) + θ?Al₂Cu has been investigated during preparatory ground experiments. During solidification, both Al and Cu can be redistributed in the liquid by cross-diffusion parallel to the planar solid/liquid interface. However the third element, i.e. Ag and Si, must be segregated over a much longer distance. In the case of Ag, a diffusive stable solute boundary layer has been observed. Si however is found to be solutal unstable and gravity driven solutal convection breaks up the boundary layer. In both cases, however, the interface is prone to a Mullins-Sekerka instability giving rise to two-phase cellular growth at a critical value of the growth velocity. In the solid, the α(Al) composition, determined by the solidus line, changes with changing growth velocity.     

Effect of silver addition on formation of secondary phases in squeeze cast Al–Cu–Mg alloys

Abstract

The effect of silver addition on the formation of secondary phases in squeeze cast Al–4.0Cu–1.5Mg and Al–4.0Cu–1.5Mg–0.7Ag (all wt-%) alloys has been investigated using optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffractometry, and transmission electron microscopy. The as cast microstructure of both alloys consists of primary dendritic α-Al and various types of secondary solidification phase, e.g. Al₂Cu, Al₂CuMg, Al(Cu,Ag)Mg, and icosahedral (I) and decagonal (D) quasicrystalline phases. However, the solidification path in the interdendritic region during squeeze casting is different for each alloy, i.e. L→ternary α-Al–Al₂Cu–Al₂CuMg eutectic in Al–4.0Cu–1.5Mg and L→L′+Al₂Cu→α-Al–Al₂Cu–Al(Cu_0.75Ag_0.25)Mg eutectic in Al–4.0Cu–1.5Mg–0.7Ag. This indicates that silver acts as an alloying element stabilising the formation of Al(Cu,Ag)Mg Laves phase. The remaining copper and iron rich liquid in the interdendritic region at the final stage of solidification solidifies into a mixed structure of α-Al, Al₂Cu, and AlCuFe I (or D) phases. The composition of the I and D phases, measured by energy dispersive X-ray spectroscopy, is in the range Al–(27～28)Cu–(9～10)Fe and Al–(26～27)Cu–(7～9)Fe (all at.-%) respectively.     

Microstructure and Mechanical Properties of Fiber Laser Welded GH3535 Superalloy

As a primary material of the thorium molten salt reactor(TMSR) that is a suitable candidate reactor of the Generation IV nuclear reactors, GH3535 superalloy was successfully welded. The effect of laser beam welding(LBW) on microstructure evolution of fusion zone(FZ) and heat affected zone(HAZ), such as element segregation, precipitate behavior and grain evolution, was investigated. The microhardness and tensile properties were tested and discussed. The results of microstructure evolution showed that a number of fine M_6C-y eutectic phases precipitated at solidification grain boundaries and interdendritic region in FZ. Compared to base metal zone(BMZ), the grain size of HAZ has no obvious change. While a few of M_6C-y eutectic phases were observed in partially melted zone(PMZ) of HAZ. The results of microhardness indicated that the hardness of FZ was higher than that of HAZ and BMZ. The results of tensile test showed that the ultimate tensile strength of joints at room temperature, 650 and 700?C were98%, 97% and 99% of that of BM, respectively. All the tensile specimens of joints failed in BMZ rather than in PMZ where M6 C carbides had been transformed into M_6C-y eutectic phases.