Boundary Liquation Research Articles

Electron beam welding of P91 steel and incoloy 800HT and their microstructural studies for advanced ultra super critical (AUSC) power plants

This study characterizes an electron beam welded dissimilar joint between P91 ferritic/martensitic steel and Incoloy 800HT Nickel-based alloy. Such dissimilar metal welds (DMW) have applications in nuclear and thermal powerplants where steam-carrying pipes lead to header sections operating at around 600–650 °C. The study employed an advanced welding process to obtain a clean, high-quality dissimilar metal. The microstructure characterization of the DMW involving Nickel-based alloys is prone to solidification cracking in the weld fusion zone (WFZ), which affects the overall weld properties and leads to failure. In the present study, the WFZ microstructure revealed a mixed solidification mode with a homogenous presence of equiaxed dendritic and cellular-type structures. Advanced analyses of WFZ microstructure were done to account for microsegregation. Nb, Ti, Cr, and Mo microsegregation was found in the WFZ. The Ti-rich grain boundaries in the WFZ provided the pinning action during plastic deformation. The evolution of Cr, Mo-based carbide phases affected the microhardness distribution. Microsegregation under rapid solidification of the WFZ resulted in the formation of secondary phases in the interdendritic region. The effect of the evolution of precipitates deteriorated the mechanical strength. High and uneven dislocation distribution was observed in the WFZ near the IN 800HT weld interface. Under tensile loading, the DMW failed from the IN 800HT weld interface due to coarsening of precipitated in the IN 800HT HAZ region near the weld interface and grain boundary thickening due to the deposition of Ti over them. The tensile strength of WFZ was 592 MPa. The tensile strength of WFZ was 592 MPa. The post-weld heat treatment (PWHT) homogenized the microhardness across the weld. Impact toughness after PWHT slightly increased from 63 J to 69 J. The dissimilar joint obtained using the EBW process had sufficient mechanical strength to be considered for Generation IV power plants under the American Society for Testing and Materials (ASTM) standards. The study concludes that the dissimilar metal welded joint was highly prone to failure despite sufficient strength, as IN 800HT weld interface was susceptible to grain boundary liquation.

Effects of welding layer arrangement on microstructure and mechanical properties of gas metal arc welded 5083/6005A aluminium alloy butt joints

The effects of welding layer arrangement on the microstructure and mechanical properties of 5083/6005A aluminium alloy gas metal arc welded (GMAW) joints were studied. High-quality butt joints were obtained under different welding layer arrangements, including two layers and two passes, two layers and three passes, and three layers and three passes. The results indicated that the softening zone located in the heat-affected zone (HAZ) of the 6005A aluminium alloy side was the weakest part of the 5083/6005A alloy joints. The highest level of recrystallization and precipitate dissolution was observed in the HAZ of the two-layer and two-pass joints; however, the lowest level was verified in the three-layer and three-pass joints. With the increase in the number of welding layers and passes, grain boundary liquation in the partially melted zone (PMZ) and strength loss of the softened zone were improved. The tensile strength of the two-layer and two-pass joints was 161 MPa, while that of the three-layer and three-pass joints increased to 217 MPa.

Microstructural modification and enhanced mechanical properties of wire-arc additive manufactured 6061 aluminum alloy via interlayer friction stir processing

In this study, an innovative hybrid additive manufacturing technology that combines wire arc additive manufacturing (WAAM) and interlayer friction stir processing (FSP) is designed to overcome the hot cracking and other solidification issues observed in WAAM of 6061 aluminum alloy. The grain boundary liquation and porosity defects are effectively eliminated by the interlayer FSP technique, and the coarse Si-rich phases are broken up and redistributed, and the coarse columnar grains are transformed into fine equiaxed grains, resulting in superior mechanical properties to the initial WAAM alloy. Furthermore, the anisotropy in strength of the WAAM sample can be eliminated by the WAAM + interlayer FSP technique. It demonstrates that the WAAM + interlayer FSP technique has great potential for use in the fabrication of aluminum alloys susceptible to hot cracking with high performance.

Microstructural and Mechanical Characterization of Ledeburitic AISI D2 Cold-Work Tool Steel in Semisolid Zones via Direct Partial Remelting Process

The success of the thixoforming process largely depends on the created microstructure, which must be globular in the liquid phase. The solid–liquid structural changes that occur on as-annealed D2 tool steel when it is subjected to the so-called DPRM are described in this work (direct partial remelting method). The paper discusses phase changes and how carbide dissolution affects grain boundary liquation and grain spheroidization. Equiaxed grains with undissolved carbide particles have been found in the microstructural analysis at 1250 °C; however, the carbides gradually disappear as the temperature rises. Additionally, the equiaxed grains were transformed to a globular structure, which improves the shape factor and grain size for the thixoforming process. For AISI D2 thixoforming, which produced grains with a diameter of 50 μm and a shape factor of 1.18, temperatures of 1300 °C and a holding period of 5 min were the optimum parameters. The outcomes also showed that the mechanical properties of the AISI D2 were greatly enhanced after using direct partial remelting, where hardness was increased from 220 Hv to 350 Hv and tensile strength from 791 MPa to 961 MPa.

The Microstructure and Mechanical Properties of 5083, 6005A and 7N01 Aluminum Alloy Gas Metal Arc-Welded Joints for High-Speed Train: A Comparative Study

This study aimed to conduct a comparative study on the microstructure and mechanical performance of 5083, 6005A and 7N01 Al joints used in China Railway High-speed (CRH) trains. We connected 10 mm-thick plates by three-layer and three-pass gas metal arc welding (GMAW). The results indicated that 6005A and 7N01 Al joints were more sensitive to grain boundary liquation in the partially melted zone (PMZ) than 5083 Al joins. Besides, recrystallization was obtained in heat-affected zones (HAZ). The 5083 Al joints experienced the most severe recrystallization and the grain size changed from 6.32 (BM) to 32.44 (HAZ) μm duo to intracrystalline strain induced by cold-rolled processes. The 7N01 Al alloys experienced the lowest extent of recrystallization and the grain size increased from 5.32 (BM) to 22.31 (HAZ) μm. Furthermore, significant precipitate evolution in the HAZ was observed. Original thin β” precipitates dissolved in HAZ of 6005A Al joints and transformed to the softer β phase. However, less precipitation transition was examined in 5083 and 7N01 Al joints. The precipitates’ evolution produced a softening region in HAZ of 6005A joints where the hardness was only 55 HV. The microhardness profile of the other two Al joints was less affected. The tensile strength of 5083, 6005A, and 7N01 Al alloy joints reached 323, 206 and 361 MPa, respectively. The 5083 Al and 6005A Al joints failed at HAZ near the fusion line while 7N01 Al joints failed at the fusion zone owing to the high strength of the base metal. The liquation, coarse grains by recrystallization, and precipitate evolution all decreased local strength, resulting in the fracture at HAZ.

Processing window for laser metal deposition of Al 7075 powder with minimized defects

There is a relative lack of data on the processing parameters suitable for laser metal deposition (LMD) of Al alloys, particularly high strength alloys. The objective of this study is to outline a systematic investigation to define the processing window for LMD of Al alloy 7075. Single linear tracks were fabricated by varying the powder mass flow rate, laser power, and scan speed according to a 3 3 factorial experimental design, in order to determine conditions that produce deposits with suitable cross section geometries. Single layer multi-track deposits were fabricated using parameters selected from the linear track experiments, and with varying track overlap values. Low energy input and powder deposition conditions were found to produce single track and multi-track deposits with porosity less than 0.3 and 1 %, respectively. Cuboid samples were produced using parameters selected from the single layer multi-track processing window. However, porosity of at least 4 % was observed in the cross section for all cuboid samples. Liquation cracking was not observed in the samples, despite clear signs of grain boundary liquation. Microcracks with a width of ∼1 μm were observed in the deposited material, and the results indicate that these are solidification cracks due to insufficient eutectic liquid to backfill and ‘heal’ incipient cracks. It is concluded that low energy density and powder mass deposition conditions can produce crack-free single layer deposits of 7075 with low porosity, but the elimination of interlayer porosity and microcracks in multi-layer deposits requires further study.

Investigation of broom-straw fracture behavior of aluminum alloy 2024 debris recovered from Space Shuttle Columbia

Materials investigations of Space Shuttle Columbia debris hold great relevance for understanding materials response to atmospheric re-entry. Sections of the Xo 582 ring-frame bulkhead, recovered from the Columbia accident, exhibited highly localized thermal degradation and material loss. Unique delamination features were observed on fracture surfaces of the bulkhead, as well as on other aluminum debris. The unique nature of the fracture surfaces, termed broom-straw, is not a well described phenomenon, and takes central focus of this research. This paper presents a forensic materials characterization of the bulkhead that substantiates and expands upon the previous notions that broom-straw fracture is the result of grain boundary liquation. Observed features were consistent with material exposure to elevated temperatures and subsequent material property degradation, followed by mechanical overload. This research attempts to advance the understanding of broom-straw fracture and determine the influence of microstructure and thermo-mechanical processing on the broom-straw fracture mechanism.

Electron microscopy study of laser welded GH909 superalloy joint

The formation process of gamma/Laves eutectic constituents were systematically investigated in both heat-affected zone (HAZ) and fusion zone (FZ) of laser-welded GH909 superalloy. Solidification sequences and eutectic reactions were proposed by morphologically and chemically analyzing phase and chemical composition by utilizing various characterization methods. In FZ, the ultimate eutectic constituents experienced three stages: 1) liquid (L)→Primary gamma (γ) (1360 °C), forming a γ dendritic morphology; 2) following L→γ+(Nb, Ti)C precipitation at mushy zone; 3) terminal L→γ+Laves (1184 °C) at γ interdendritic regions. Enrichment of Nb in Laves and surrounding γ produced a high dislocation density in eutectic constituents. Continuous distribution of such γ/Laves structures promoted the formation of micro interdendritic solidification cracks under accumulated tensile stress. In HAZ, however, the fraction of γ/Laves constituents was heat input depended and increased as the reduction of distance from fusion line. An interactionally and simultaneously integrated process NbC+γ→Laves (~1323 °C) caused the formation of eutectic constituents with NbC decomposition, Laves formation, grain boundary liquation and subsequently eutectic reactions.

Complete dissolution of gamma prime via severe plastic deformation in a precipitation-hardened nickel-base superalloy

The effect of multiple friction stir processing on suppressing liquation cracking of a cast precipitation-hardened nickel-base superalloy was investigated. The severe plastic deformation and simultaneous high temperature with rapid cooling resulted in dramatic changes of the liquation-cracking-related microstructures: high volume fraction gamma prime precipitations were completely dissolved into the matrix, carbide dendrites were fully disintegrated and matrix grain size was remarkably refined to submicron scale. These modifications significantly improved ductility of heat-affected zone and effectively decreased grain boundary liquation simultaneously, thus liquation cracking was completely suppressed in laser surface remelting.

Influence of homogenisation treatments on the hot ductility of cast ATI® 718Plus®: Effect of niobium and minor elements on liquation characteristics

The hot ductility of cast ATI® 718Plus® was assessed using the Gleeble thermo-mechanical simulator after being subjected to different homogenisation heat treatments. The hot ductility deteriorated significantly after long-dwell homogenisation heat treatments for 24 h at temperatures of 1120 and 1190 °C as compared with those treated at a short dwell time of 4 h at the same temperatures. The observed ductility deterioration was related to more extensive liquation along the grain boundaries caused by different mechanisms, e.g., liquation by solute segregation mechanism, Laves melting, constitutional liquation of MC carbides and supersolidus grain boundary melting, with the effect and extent depending on the solute changes after the homogenisation heat treatments. Furthermore, the role of Nb as the solute element and as the precipitate former, as well as the effect of minor alloying elements segregating along the grain boundaries, is discussed in connection to grain boundary liquation, which contributes to a better understanding of heat-affected zone liquation cracking susceptibility of cast ATI® 718Plus®.

Combining alloy and process modification for micro-crack mitigation in an additively manufactured Ni-base superalloy

The additive manufacturing (AM) of the γ` precipitation strengthened Ni-base superalloys still remains a challenge due to their susceptibility to micro-cracking. Post-processing, such as HIPing, has been shown to heal the micro-cracks but it remains desirable to prevent the micro-cracking from even occurring. Numerous studies highlighting potential mechanisms for micro-cracking exist but few solutions have been demonstrated. The intent of this study was to identify the micro-crack mechanisms and demonstrate how process and alloy modifications can reduce the micro-cracking. The micro-crack surfaces exhibit a dendritic appearance that is indicative of solidification cracking. Additionally, Gleeble experiments, simulating the L-PBF induced Heat Affected Zone (HAZ), were conducted below the γ` solvus temperature and reveal the existence of grain boundary liquation, indicative of liquation cracking. Two cracking mechanisms are thus coexisting during Laser Powder Bed Fusion (L-PBF) of CM247LC. Based on experimental evidence, reduction in the solidification interval of CM247LC was investigated as a candidate for micro-crack mitigation and a new alloy was developed. As Hf is found to have a significant influence on the freezing range of the alloy, a new CM247LC without Hf was produced and tested. The study also involved two separate and distinct processing conditions to highlight the importance of melt pool geometry on micro-crack density. Samples fabricated with the Hf-free CM247LC, CM247LC NHf, in combination with optimized processing conditions exhibit a reduction in crack density of 98 %. This study demonstrates the importance of both processing conditions and alloy chemistry on micro-cracking in L-PBF fabricated γ` hardening Ni-base superalloys.

Microstructure and Performance of Laser-Welded GH3128/Mo Dissimilar Joints

Laser beam offset welding of GH3128 and molybdenum (Mo) dissimilar joints was studied with an emphasis on the role of laser beam offset (LBO). It was found that the change of LBO significantly affected the properties of GH3128/Mo dissimilar joints. When the LBO increased from − 0.1 mm (i.e., laser spot was centered on Mo surface) to + 0.3 mm (i.e., laser spot was centered on GH3128 surface) at the step size of 0.1 mm, the amount of melted Mo gradually declined; the welding mode gradually changed from fusion welding to welding–brazing; macro-segregation of Mo-rich phases in fusion zone (FZ) gradually disappeared; the average microhardness of FZ significantly decreased, and therefore, microhardness distribution in FZ became more uniformly; the quantity of grain boundary liquation observed in heat-affected zone (HAZ) of GH3128 side gradually decreased; and recrystallization of grains in the HAZ of Mo was gradually suppressed. Joints with the optimal LBO of + 0.2 mm reached the maximum tensile strength of about 560 MPa. When using a small LBO, i.e., LBOs were + 0.1, 0, − 0.1 mm, the HAZ of Mo was greatly softened and embrittled. When adopting a large LBO, i.e., LBO was + 0.3 mm, the melt near Mo-FZ interface might fail to moisten Mo surface and spread on Mo surface, thus resulting in the reduction in tensile strength.

Metallurgical modeling of intergranular HAZ liquation cracking during laser welding newly developed crystallography-dependent aerospace materials. Part II: Grain boundary alloying segregation of microstructure modification

Metallurgical modeling of the nonequilibrium grain boundary segregation was coupled with a nonequilibrium weld pool solidification to numerically analyze the contributing factors to liquation cracking resistance. The effect of the welding conditions, material composition, and average grain size on the kinetics of the grain boundary segregation in the Inconel 718-type nickel-based superalloy during laser welding were optimized. The grain boundary segregation of boron near the fusion boundary is of particular interest, and is closely correlated to the intergranular heat-affected zone (HAZ) liquation cracking. It is indicated that the low heat input, fine-grained material and full penetration contribute to the reversible desegregation of boron and suppress the grain boundary segregation of boron to significantly minimize the segregation-induced grain boundary liquation and improve the weldablity. The subsolidus temperature due to nonequilibrium weld pool solidification is capable of ameliorating the segregation kinetics to reduce the HAZ grain boundary segregation. Intergranular HAZ liquation cracking is evitable with a crack-free weld without using filler wire. The grain boundary segregation decreases and the threshold tensile stress of solid-liquid interfacial decohesion across the intergranular liquid film simultaneously increases in the recrystallized HAZ during the weld shape change from partial penetration to full keyhole penetration. Furthermore, the numerical analysis results are verified indirectly by the experiment results of the total crack length. In addition, this metallurgical modeling is also applicable for the alloying segregation behavior prediction of other nickel-based superalloys with similar metallurgical properties.

Porosity and liquation cracking of dissimilar Nd:YAG laser welding of SUS304 stainless steel to T2 copper

In this study, we present experimental laser welding SUS304 stainless steel to T2 copper. The forming mechanism of fusion zone polygonal porosity and heat affected zone (HAZ) liquation cracking were systemically investigated and possible solutions to the welding process were also proposed. We deemed that the generation of HAZ liquation cracking mainly underwent three stages: crack incubation, crack initiation and crack growth. Formation of HAZ liquation cracking was closely related to precipitation of Cu-Fe compounds at grain boundaries and grain boundary liquation. The occurrence of porosity was determined by the keyhole instability correlated with fluid flow, keyhole free surface evolutions and composition segregation in a welding process. The susceptibility to HAZ liquation cracking can be effectively lowered by controlling the heat input during laser welding. In addition, the porosity in the fusion zone can be eliminated by reasonably adjusting laser deflection angle in the experiments, which greatly improved the microstructure and mechanical properties of welded joints.

Hot ductility behavior of AD730™ nickel-base superalloy

Hot ductility of the newly developed AD730™ nickel-base superalloy was investigated in the temperature interval 1050–1240 °C. The nil strength and nil ductility temperatures were determined by hot tensile testing using the Gleeble™ 3800 weld thermal simulation method. The influence of heating rate, representing the weld thermal cycle, on hot ductility behavior of the alloy was also investigated. The microstructure and the fracture mode of samples were examined by optical and scanning electron microscopy. The influence of heating rate on the extent of grain boundary liquation and void formation was determined and it is shown that the significant ductility loss near the NDT point could be related to the reduction of surface tension at the grain boundary-matrix interface. In addition, the contribution of hard precipitates, such as grain boundary MC carbides, voids, and cavities as other damage mechanisms responsible for ductility loss at high temperature, are discussed.

Pulse MIG Welding of 6061-T6/A356-T6 Aluminum Alloy Dissimilar T-joint

Dissimilar T-joints between 4-mm-thick wrought 6061-T6 and cast A356-T6 aluminum alloy plates are made with pulse metal inert gas welding (P-MIG) process, and the weld geometry, microstructure and mechanical properties of the joints are investigated. The average tensile load of the joints is 9488 N at travel speed 12 mm/s and welding current 165 A with 6061 aluminum alloy as web plate and A356 aluminum alloy as flange plate. The weld zone (WZ) is composed of dendrite zone and columnar zone, which is wide at bottom of weld making it become a weak zone. Besides, grain boundary (GB) liquation, consisting of GB eutectic and solute-depleted α-Al, occurs in the partially melted zone on 6061 aluminum alloy side. The T-joint with highest tensile load fails at the root of WZ and the flange plate near the weld with different fracture modes.

The effect of pre-cold treatment on microstructure, weldability and mechanical properties in laser welding of superalloys

The purpose of this study is to evaluate the effects of pre-cold treatment on weldability of superalloys. The results of numerical calculations showed that the length of different welding regions including mushy zone (MZ), partially melted zone (PMZ) and heat affected zone (HAZ) decreased by 46, 46 and 56 percent, respectively. The experimental calculations indicated that the length of PMZ and HAZ, as well as the HAZ area, decreased by 2.1, 2.5 and 2.5 times, respectively. These results showed that significant improvement in weldability can be obtained by performing pre-cold treatment. Using pulsed Nd:YAG laser, grains boundary liquation was observed, followed by HAZ liquation cracking. The mechanism of HAZ liquation cracking is constitutional liquation of Nb-rich carbides and delta precipitates that encourage the formation of liquid films in the grain boundaries. With the accumulation of micro-pores in liquated grain boundaries, the cracks can be formed. With reducing the temperature of specimen from +200 to −30 °C, the penetration depth of laser welds decreased from 1624 to 1450 μm and the WTOP/WNECK ratio reduced from 1.45 to 1.24. The hardness profile at temperatures of -30 and + 200 °C indicates that the hardness of the weld metal increased using pre-cold conditions.

Resistance spot welding of dissimilar austenitic/duplex stainless steels: Microstructural evolution and failure mode analysis

The paper addresses the microstructural evolution and failure mode analysis for dissimilar resistance spot welding of austenitic and duplex stainless steels. Microstructural examination by optical and scanning electron microscopes revealed that pre-existing TiC particles in the base metal microstructure of the duplex stainless steel were responsible for changing the nature of solid state phase transformations in heat affected zone and columnar to equiaxed transition grain structure in fusion zone due to heterogeneous nucleation. Moreover, it was found that simultaneous occurrence of constitutional liquation of these TiC particles as well as grain boundary sweeping mechanisms led to grain boundary liquation in the heat affected zone of the investigated duplex stainless steel. Failure analysis after the tensile-shear testing revealed that in the partial thickness-partial pullout failure mode fracture initiates in a similar manner as pullout failure mode; and it was found that the work hardening ability of the parent metals is the key controlling factor for crack initiation site.

Microstructure and Mechanical Properties of Pulse MIG Welded 6061/A356 Aluminum Alloy Dissimilar Butt Joints

The microstructure and mechanical properties of pulse metal inert-gas (MIG) welded dissimilar joints between 4 mm thick wrought 6061-T6 and cast A356-T6 aluminum alloy plates were investigated. The tensile strength of the joints reached 235 MPa, which is 83% of that of 6061 aluminum alloy, and then decreased with the increase of travel speed while keeping other welding parameters constant. The microstructure, composition and fractography of joints were examined by the optical microscopy (OM), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Grain boundary liquation and segregation occurred in the partially melted zone (PMZ) on 6061 aluminum alloy side, and brittle Fe-rich phases were observed in partially melted zone on A356 aluminum alloy side. The minimum microhardness appeared in heat-affected zone (HAZ) near A356 aluminum alloy substrate. The samples during tensile test failed mainly in PMZ and HAZ on A356 aluminum alloy side through mixed fracture mode with quasi cleavage and dimples on fracture surface.

Nano-volcanic Eruption of Silver.

Silver (Ag) is one of the seven metals of antiquity and an important engineering material in the electronic, medical, and chemical industries because of its unique noble and catalytic properties. Ag thin films are extensively used in modern electronics primarily because of their oxidation-resistance. Here we report a novel phenomenon of Ag nano-volcanic eruption that is caused by interactions between Ag and oxygen (O). It involves grain boundary liquation, the ejection of transient Ag-O fluids through grain boundaries, and the decomposition of Ag-O fluids into O2 gas and suspended Ag and Ag2O clusters. Subsequent coating with re-deposited Ag-O and the de-alloying of O yield a conformal amorphous Ag coating. Patterned Ag hillock arrays and direct Ag-to-Ag bonding can be formed by the homogenous crystallization of amorphous coatings. The Ag “nano-volcanic eruption” mechanism is elaborated, shedding light on a new mechanism of hillock formation and new applications of amorphous Ag coatings.