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The hot cracking behaviour in the diode laser braze joint of 13Cr–4Ni stainless steel using Au–18Ni, Ag–10Pd and Ag–21Cu–25Pd filler metals has been investigated. The types of joint investigated were the T fillet joint and the L fillet joint which simulated the second braze bead in the T fillet joint. Tandem beam brazing was also carried out in order to prevent the hot cracking by post-heating treatment with a trailing beam. A centreline crack, characterised as a ductility dip crack, occurred in the second braze bead of the T fillet joint using Au–Ni filler metal. On the other hand, no cracks occurred in either the first or the second braze bead in T fillet braze joints using Ag–Pd and Ag–Cu–Pd filler metals. The hot cracking susceptibility of the Au–Ni braze metal was evaluated by the spot Varestraint test. Most of the cracks observed in the spot Varestraint test specimen were also characterised as ductility dip cracks, and the susceptibility to such cracking increased with increasing the augment...

The effect of Ce addition on microcracking susceptibility in the multipass weld metal of alloy 690 was investigated in order to improve the microcracking susceptibility in it. The ductility-dip cracking susceptibility in the reheated weld metal could be greatly improved by adding 0.015-0.025mass%Ce to the weld metal. Conversely, the excessive Ce addition to the weld metal led to liquation and solidification cracking in the weld metal. Hot ductility of the weld metal at the cracking temperature was greatly improved by adding 0.01-0.03mass%Ce, implying that the ductility-dip cracking susceptibility was decreased as a result of the desegregation of impurity elements of P and S to grain boundaries due to the scavenging effect of Ce. The excessive Ce addition to the weld metal resulted in the liquation and solidification cracking attributed to the formation of liquefiable Ni-Ce intermetallic compound. The multipass welding test confirmed that microcracks in the multipass welds were completely prevented by using the filler metal added 0.032mass%Ce.

The wetting and flowing behaviors of the filler metal during laser brazing process were analyzed by the computer simulation. Two situations of the wetting and flowing during laser brazing were modelled, i.e., the metled Au-18%Ni and Ag-10%Pd filler metals on the butt joint of Inconel 600, and the melted Cu-8%Sn filler metal on the dissimilar butt joint of type 304 stainless steel to Cu. The filler metal droplet wetted and spread on the base metals and simultaneously infiltrated into the joint gap with the lapse of time. The Au-Ni and Ag-Pd filler metal infiltrated into the 0.3mm wide joint gap at the completion of brazing even in the single beam brazing. The Au-Ni filler metal did not infiltrate into the joint gap completely at the brazing clearances of 0.1-0.2mm in the single beam brazing, however, it could be filled up in the joint gap in the tandem beam brazing. The Cu-Sn filler metal wetted on the both base metals of stainless steel and Cu and filled up the 0.3mm wide joint gap when the location of preheating beam deviated in 0.5mm to Cu substrate, however, it did not infiltrate into the joint gap completely at the deviation distance of preheating beam to Cu substrate being 1.0mm. It followed that the wetting and flowing behaviors of the filler metal during laser brazing process could be predicted by the computer simulation.

The occurrence of microcracks, especially ductility-dip crack in multipass weld metal during GTAW and laser overlay welding processes of Ni-base alloy 690 was predicted by the mechanical approach. The stress/strain analysis in multipass welds was conducted using the thermo elasto-plastic finite element method. The brittle temperature range for ductility-dip cracking (DTR) of the reheated weld metal was determined by the Varestraint test. Plastic strain in the weld metal accumulated with applying the weld thermal cycle in multipass welding. The plastic strain-temperature curve in the La free weld metal did not cross the DTR in the cooling stage of GTAW process, however, it crossed the DTR in the cooling stage of reheating process by subsequent welding. On the other hand, the plastic strain-temperature curves of any weld passes in the La added weld metal did not cross the DTR. Ductility-dip cracks occurred in the La free weld metal except for the final layer, however, any ductility-dip cracks did not occur in the La added weld metal during multipass welding. It could be understood that ductility-dip crack would occur during not only single-pass welding but also multipass welding when plastic strain intersected the DTR.

Crystal growth behaviours in the laser surface-melted region and overlay weld metal of/on Ni-base single crystal superalloy CMSX-4 has been investigated. The Ni-base superalloy Rene 142 was used as the filler metal for overlay welding. Surfaces of the specimens were prepared as to be coincident with the (001) plane. The surface melting and overlay welding were conducted using a 2 kW diode laser along the [100] direction. Electron back scattering analysis for the crystal orientation revealed that, under low heat input conditions, the surface-melted region solidified as a single crystal with unidirectional dendrites which grew from the base metal epitaxially along the [001] direction. For intermediate heat input conditions, the surface-melted region was also found to be single-crystalline; however, it consisted of orthogonal dendrites along the [001], [010] and [100] directions. In contrast, under high heat input conditions, stray crystals were observed in some parts of the surface-melted region resulting in the melted region becoming polycrystalline. A numerical analysis of the dendritic growth direction and the constitutional supercooling at solidification front indicated that the overlay weld metal grew epitaxially on the substrate and stray crystals tended to form under conditions of higher laser power and (V f / V w) ratio (V f: wire feeding speed, V w: laser scanning speed), which agreed with the metallographic observation. A numerical calculation combining CET (columnar to equiaxed transition) theory with an inverse heat conduction analysis allowed us prediction of the conditions needed to suppress the formation of stray crystals and to promote single-crystal production in the overlay welded region with epitaxial dendritic growth of [001] orientation and/or dendritic growth of [100] and [010] orientations. Using these predictions, a single crystalline layer of 3 mm thickness on a single crystal substrate could be achieved by laser overlay welding with ten passes. Furthermore, creep rupture properties of single-crystalline surface-melted region were comparable or slightly better than that of the CMSX-4 base metal.

Recrystallization behavior in postheat treatment of the surface-melted regions of Ni-base single crystal superalloy CMSX-4 have been investigated. In, addition, mechanical properties of surface-melted regions after postheat treatment were evaluated. The surface melting was conducted using 2kW diode laser. The microstructure and crystal orientation in the melted regions were analyzed by using optical microscopy, scanning electron microscopy (SEM) and electron backscattered pattern analysis (EBSP). The effect of postheat treatment condition on microstructure was investigated using the specimens whose surface-melted regions were single-crystallized. The surface-melted regions were recrystallized when the temperature of postheat treatment was higher than 1513K. On the basis of this result, the partial solution treatment condition was proposed as 1493K×3.6ks to maintain single crystal structure without recrystallization in the surface-melted regions. Furthermore, the mechanical properties of surface-melted regions after partial solution treatment and subsequent aging were investigated by the creep rupture test. The creep rupture strength of surface-melted specimens was comparable to those of the base metal. It could be concluded that the crystalline controlled repairing technique would be applicable to the repair of single crystal superalloy turbine blades.

Crystal growth behavior in the laser surface-melted region of Ni-base single crystal superalloy CMSX-4 have been investigated to clarify the effect of heat input on the conditions for achieving single-crystallization and/or preventing the formation of stray crystals. The crystal growth in the surface-melted region was numerically analyzed based on the geometric model and CET (columnar to equiaxed transition) theory in combination with an inverse heat conduction analysis. A theoretical analysis of the dendritic growth direction suggested that the surface-melted region consisted of [001], [100] and [010] dendrites, when the laser beam was scanned along the [100] or [110] direction on the (001) surface. Furthermore, it was predicted that the occurrence of the [001] dendrite could be observed at the bottom of the melted region and the [100] and [010] dendrites existed only at the upper part of the melted region. On the other hand, theoretical analysis of the stray crystal formation indicated that stray crystals tended to form under conditions of higher laser power and lower scanning speed, i.e. higher heat input conditions. These calculation results agreed with the metallographic observation. As the results, it can be said that the conditions needed to suppress the formation of stray crystals and to promote single-crystallization in the surface-melted region with epitaxial dendritic growth of [001] orientation and/or the dendritic growth of [100] and [010] orientations is predictable by calculation.

Ni-base single crystal superalloy CMSX-4 has been widely used for turbine blades and vanes. However, since they are used under severe environments, the blades sometime suffer from erosion. Therfore, it is of great interest to develop repairing techniques that allow the life of single crystal turbine blades and vanes to be extended. So, in this study, the applicability of repairing techniques of the eroded blades by directional solidification treatment was investigated. Ni-Cr-B and Ni-Cr-Si filler metals were used as built-up repairing for the eroded region of the blade. The built-up repairing treatment was developed by utilizing unidirectionally solidification of a repaierd region under temperature gradient using a vertical type furnace with a movable heater. Microstructure and crystal orientaion in the repaired regions under various heat treatment conditions were examined by SEM and EBSP. The microstructure of the repaired region could be classified into several zones; eutectic zone, normally solidified zone, isothermally solidified zone and substrate. The solidification morphology of the normally solidified zone changes from dendoritic to planar with decreasing the moving speed of the heater. The single-crystallization could be achieved in the isothermally solidified zone and the normally solidified zone when its solidification morphology was planar. The thickness of the built-up region of about 2590μm can be obtained when the moving speed of heater is 0.28μm/s using Ni-Cr-Si filler metal.

The microcracking susceptibility in dissimilar multipass welds of alloy 690 to type 316L stainless steel was investigated by the Varestraint test and the multipass welding test using the four different stainless steels varying the amounts of impurity elements such as P and S, and using the nine different filler metals of alloy 690 varying the La content. In order to simulate the dissimilar weld metals, stainless steels were gas tungsten arc (GTA) welded using alloy 690 filler metals with varying the dilution ratio. Microcracks occurring in the reheated weld metals were classified into ductility dip, liquation and solidification cracks. The ductility dip cracking susceptibility decreased with increasing the La content in the weld metal, while the liquation and solidification cracking susceptibilities increased contrarily when the La content in the weld metal exceeded ∼0·03 mass-%. The relation among the microcracking susceptibility, the (P + S) and La contents in every weld pass of the multipass we...

The theoretical approaches to the erosion and wetting/flowing phenomena during the laser brazing process were made by the computer simulation. The erosion kinetics of the Inconel 600 base metal was investigated using Au-18%Ni filler metal. The erosion profile during the laser brazing was estimated by applying the additivity rule to the Nernst-Brunner theory based on the measured thermal cycle of the filler metal droplet. The computed erosion profiles in the laser brazed joints were fairly consistent with the experimental ones. The wetting and flowing of the melted Au-18%Ni filler metal on the Inconel 600 butt joint during the laser brazing were analyzed by the metal flow model. The filler metal droplet wetted and spread on the base metal and simultaneously infiltrated into the joint gap with the lapse of time. In the single beam brazing, the filler metal did not infiltrate into the 0.1mm wide joint gap at the completion of brazing, however, the filler metal could be filled up in it in the tandem beam brazing.

The diode laser brazing of heat-resistant alloys with precious brazing filler metals has been conducted using the tandem beam consisted of preheating beam and main brazing beam. The 1mm thick plates of Inconel 600 and A286 alloys were butt-brazed with the 0.5mm diameter Au-18%Ni, Ag-10%Pd and Ag-21%Cu-25%Pd filler metals using a brazing flux. The sound butt joints which were free from brazing defects such as porosity and lack of penetration could be obtained at the brazing clearances of 0.1-1.5mm. Fracture strength of braze joints using Au-Ni and Ag-Cu-Pd filler metals was comparable to the base metal strength at any brazing clearance between 0.1-1.5mm, whereas that using Ag-Pd filler metal increased with decreasing the brazing clearance and attained about 70% of the base metal strength at brazing clearance of 0.1mm. The computer simulations of the braze metal flowing and the base metal erosion suggested that the preheating effect during the tandem beam brazing resulted in the superior brazability at the narrow-gap as well as wide-gap brazing attributed to the improvement in the wetting, spreading, infiltrativity and the erosion resistance of melted filler metal.

Ni-base single crystal superalloy CMSX-4 has been widely used for turbine blades and vanes because of its superior heat-resistance. However, since they are usually used under severe environments, cracks sometime occur. Since this material is so expensive, there exists strong demand for repairing technique which enables to form singlecrystal in the repaired region to prevent deterioration of fatigue resistance. Therefore, in this study, the applicability of repairing technique utilizing iso-thermal solidification treatment with using Ni-Cr-B and Ni-Cr-Si alloys as filler metals to cracks occurred in single crystal superalloy CMSX-4 was investigated. The microstructure and crystal orientaion in the repaired regions under various heat treatment conditions were examined by SEM and EBSP. SEM and EBSP analyses have revealed that isothermal solidification and single-crystallization in the repaired region was achieved by repairing with using Ni-4Cr-2.5B filler metal under the condition of 1448K×176.4ks. The same result could be obtained also by using Ni-4Cr-10Si filler metal under the condition of 1473K×90ks. Creep rupture test for the repaired regions with Ni-4Cr-2.5B and Ni-4Cr-10Si filler metals has shown that the creep rupture strentgh of repaired region with Ni-4Cr-10Si filler metal reached about 2/3 of that of the base metal.

Hot cracking behavior in laser braze joint of 13Cr-4Ni stainless steel has been investigated using BAu-4 (Au-18%Ni) filler metal. The diode laser brazing was carried out for the T-fillet joint as well as the L-fillet joint which simulates the second bead in the T-fillet joint. The tandem beam brazing with a post-heating beam was also carried out in order to prevent the hot cracking. The laser powers of main and trailing beams were varied. The center line crack occurred in the second braze bead of the T-fillet joint, and was characterized as ductility-dip crack. Hot cracking susceptibility in Au-Ni braze metal was evaluated by spot-Varestraint test. The ductility-dip cracking temperature range (DTR) was about 1000-1250K. The FEM analysis of thermal stress/strain revealed that the rise curve of plastic strain intersected the DTR in the single beam brazing, however, it would not intersect the DTR by post-heating during laser brazing. The tandem beam brazing confirmed that no cracks occurred in the L-fillet braze bead made with trailing beam at the laser power of 200-300W.

Microcracking behaviour in the gas tungsten arc multipass weld metal of alloy 690 was investigated. The majority of microcracks occurred within about 300 μm from the fusion line of the subsequent weld bead and propagated along the solidification boundaries in the multipass weld metal. The morphology of the crack surface indicated the characteristic texture of ductility dip cracking. The microcracking susceptibility of the reheated weld metal was evaluated via the spot Varestraint test using three different filler metals having varying contents of impurity elements such as P and S. Microcracking occurring in the spot Varestraint tests consisted predominantly of ductility dip cracking, with a small amount of liquation cracking. The ductility dip cracking temperature range was about 1350–1600 K in the weld metal FF1, and narrowed in the order of weld metals FF1>FF3>FF5. The ductility dip cracking susceptibility was reduced with decreasing contents of impurity elements in the filler metal. It was conc...

The effect of addition of La to a filler metal on microcracking (ductility dip cracking) in the multipass weld metal of alloy 690 was investigated with the aim of improving its microcracking susceptibility. The susceptibility to ductility dip cracking in the reheated weld metal could be greatly improved by adding 0·01–0·02 wt-%La to the weld metal. Conversely, excessive La addition to the weld metal led to liquation and solidification cracking in the weld metal. Hot ductility of the weld metal at the cracking temperature was greatly improved by adding 0·01–0·02 wt-%La to the weld metal, implying that the ductility dip cracking susceptibility was decreased as a result of the desegregation of impurity elements of P and S to grain boundaries due to the scavenging effect of La. The liquation and solidification cracking resulting from excessive addition of La to the weld metal is attributed to the formation of liquefiable Ni–La intermetallic compound. A multipass welding test confirmed that microcracks...

To elucidate the microcracking (ductility dip cracking) mechanism in the multipass weld metal of alloy 690, the hot ductility of the reheated weld metal was evaluated using three different filler metals with varying contents of impurity elements such as P and S. Hot ductility of the weld metal decreased at temperatures over 1400 K, and the weld metal containing a low quantity of impurity elements showed much higher ductility than that containing a high quantity of impurity elements. Local deformability at high temperature of the alloy 690 reheated weld metal was compared with that of Invar alloy. Grain boundary sliding in alloy 690 occurred not in the intermediate temperature range (800–1000 K), where grain boundary sliding was activated in Invar alloy, but at high temperatures just below the melting temperature of alloy 690. The computer simulation of microsegregation suggested that the deterioration of hot ductility is caused by the grain boundary segregation of impurity elements during the mult...

This work investigated microstructure in the surface-melted region of Ni-base single crystal superalloy CMSX-4 by using a diode laser beam as a heating source. Such processing parameters as laser power and scanning speed in laser surface melting were varied while defocusing distance and shielding gas (Ar) flow rates were fixed. Specimen surfaces were arranged parallel to the (001) of base alloy. The microstructure in the melted region was analyzed by optical microscopy and SEM. Crystal orientation of the melted region was analyzed using electron backscattered pattern analysis. The microstructure was remarkably changed when the heat input of surface melting was varied. The surface-melted region was found to solidify into a single crystal with directional dendrites that grew along the [001] directions under low heat input conditions. The surface-melted region was also a single crystal with disoriented dendrites that grew along the [100] or [010] directions under medium heat input conditions. In contrast, the melted region consisted of poly crystals with stray crystals under high heat input conditions. Such tendencies were also observed in the melted region with gas tungsten arc. These results demonstrate that the surface-melted region can solidify into a single crystal under conditions in which larger temperature gradient and higher solidification rates can be achieved.

In order to predict the sigma (σ) phase embrittlement in multipass weldment of duplex stainless steels, the precipitation behaviours of the σ phase in the base metal and the weld metal were investigated using super duplex stainless steels (SAF2507 and DP3W) and a conventional duplex stainless steel (NAS64). In all the steels, the amount of the σ phase precipitated during an isothermal ageing process sigmoidally increased with the length of the ageing time. The kinetics of σ phase precipitation in the base metal and the weld metal could be expressed approximately by the Johnson-Mehl type equation, however, the initiating time of σ phase precipitation in the weld metal was longer than that in the base metal. The precipitation of the σ phase in the DP3W base metal was much delayed and depressed compared with the SAF2507 and NAS64 base metals. Increasing the amount of the σ phase drastically decreased the absorbed impact energies of aged steels. They fell to as low as 1/4 of the unaged values when the amount of the σ phase exceeded 5%. The precipitation of the σ phase in duplex stainless steels during multiple thermal cycle process were predicted using the additivity rule. We predicted that the largest amount of the σ phase and the most significant degradation of impact toughness would occur in the base metal region (HAZ) for peak temperatures of 1 100–1 300 K in multiple thermal cycle.

The optimisation of TLP-bonding process for Ni-base superalloy, IN738LC has been carried out using the Bayesian Expert System for TLP-bonding. The evaluation factors introduced as an index of bonding performance of an insert metal involved the melting point of insert metal, hardness and formability of brittle phases in the bond layer. The chemical composition of Ni-3.0Cr-8.1Si-1.0B (mass%) was determined as the optimal composition of an insert metal. The processing parameters have been optimised with two evaluation factors; the joint strength and toughness. The optimal bonding condition was determined as 1433K×5.6ks. Brittle phases were not formed in the bond layer using a newly developed insert metal, and the joint strength and toughness was superior to the average of the base metal properties.

The development of TLP-bonding process for Ni-base superalloy, Inconel 718 has been carried out using the Bayesian Expert System for TLP-bonding. In order to skip the homogenising treatment, alloy system for insert metal was selected as Ni-Mo-Cr-Nb-Si-B system. The evaluation factors introduced as an index of bonding performance of an insert metal involved the melting point of insert metal, hardness and formability of brittle phases in the bond layer. The chemical composition of Ni-3.0Mo-6.4Cr-4.7Nb-2.0Si-1.0B (mass%) was determined as the optimal composition of an insert metal. The processing parameters have been optimised with three evaluation factors; the joint strength, reduction of area and toughness. The optimal bonding condition was determined as 1459K×3.3ks. Brittle phases were not formed in the bond layer by using a newly developed insert metal, and the joint strength and toughness was over 80% of the base metal properties.

The applicability of built-up repairing by isothermal solidification to Ni-base single crystal superalloy CMSX-4 was investigated using Ni-Cr-B filler metals. Base metal dissolution and isothermal solidification behaviors were examined under various heat treatment conditions. Microstructure of the repaired region could be classified into eutectic zone, normally solidified zone, isothermally solidified zone, precipitated zone in the base metal and substrate. Isothermally solidified zone and precipitated zone in the base metal were found to possess the identical crystal orientation with the base metal one. The dissolution width of base metal and the isothermal solidification width increased with repairing temperature and holding time. The single-crystallization could be achieved in the isothermally solidified zone of the repaired region. The built-up repairing up to 38μm in width could be made at the repairing condition of 1448K×176.4ks by using a Ni-Cr-B filler metal.

A new conceptual bonding technique, " hyper-interfacial bonding" has been proposed as the most effective bonding technique for ultra-fine grained steels. The hyper-interfacial bonding process was characterized by the instantaneously surface-melted pressure-bonding which involved a series of steps, namely, surface heating by high frequency induction, rapid removing of a heating coil and simultaneously pressing of specimens together by an oil pressure system. All equences were typically completed within a second under vacuum/gas atmosphere. An ultra-fine grained high strength steels with the average grain size of 1.0-1.5µm were used for bonding. A bainitic structure and MAconstituents were confirmed in the HAZ, while the base metal indicated the fine ferrite-cementite/ pearlite texture. The maximum hardness in HAZ was limited at HV320-400 and the softening in the HAZ did not occur consequently. Prior austenitic grains were coarsened at the vicinity of the bond interface, however, the grain growth in the bond layer could be depressed below about 11-16µm attributed to the dynamic recrystallization during pressure-bonding process. The tensile strength of joints attained to 83% of the base metal strength. The upper-shelf absorbed energy and DBTT were improved to approx. 60J/cm2 and 223K respectively in the case of Ni-plated UFG steel joints. It could be resulted that the hyper-interfacial bonding technique was a feasible joining method for ultra finegrained steels.

The diode laser brazing of heat-resistant alloys with precious filler metals has been conducted using tandem beam which consisted of preheating beam and main brazing beam. The 1mm thick Inconel 600 and A286 alloys were laser-brazed with the 0.5mm diameter Au-18%Ni, Ag-10%Pd and Ag-21%Cu-25%Pd filler metals using a brazing flux. The processing parameters of laser power of tandem beam and brazing clearance were varied. Sound butt joints could be obtained by using tandem beam with laser powers of 200-400W/100-150W even at the narrow gap brazing below 0.3mm where the melted filler metal did not infiltrate completely the joint gap by only using the main brazing beam (single beam). The tensile strength of the brazed joint using Ag-Pd filler metal increased with decreasing brazing clearance and attained about 70% of the base metal strength at brazing clearance of 0.1mm, while those using Au-Ni and Ag-Cu-Pd filler metals were comparable to the base metal strength at any brazing clearance between 0.1-1.5mm. The preheating during the tandem beam brazing resulted in superior brazability at the narrow gap to wide gap brazing because of the improvement in wetting/spreading and erosion esistance of melted filler metal by depressing the extremely high peak temperature and steep temperature distribution in the base metal.

Diode laser brazing of aluminium alloy (A5052) to interstitial free steel (IF steel) or type 304 stainless steel (SUS304) was conducted using aluminium filler metal (BA4047) with Nocolock flux. The processing parameters of laser power, wire feed rate and travel speed were varied. The strength of lap joints of A5052 on steels was evaluated by tensile shear test. The joint strength of A5052/steels was increased with increasing laser power and reached the maximum strength, more than approximately 80% of the A5052 base metal strength, at a laser power of 1300 W. Voids and incomplete penetration of filler metal were observed at the A5052/braze layer interface when the laser power was below 1100 W. The Fe–Al intermetallic compounds were formed at the steel/braze layer interfaces and grew drastically when the laser power exceeded 1300 W. Superior brazability of A5052/steels was found at brazing conditions corresponding to a temperature of filler metal droplet of 1050–1250 K.

The laser brazing of Inconel600 alloy with precious filler metals has been conducted using a diode laser generator with single beam and tandem beams for preheating and brazing. Inconel 600 alloy of 1mm thickness was butt-brazed using Au-18%Ni, Ag-10%Pd and Ag-21%Cu-25%Pd filler metals with diameter of 0.5mm by using brazing flux. We make butt-joint with single beam brazing for various brazing clearance and use tandem beam brazing to improve the wettability of melted filler metal. Experimental results have shown that the sound butt joints which are free from defect such as porosity and lack of penetration could be obtained with brazing clearance of 0.1-1.5mm. The joint strengths of brazed joint which were produced in the optimal condition by using Au-Ni and Ag-Cu-Pd filler metals were comparable to the base metal strength in the joints with the brazing clearance between 0.1-1.5mm, whereas those of the joints which were brazed by using Ag-Pd filler metal increased with decreasing the brazing clearance and attained about 70% of the base metal strength at brazing clearance of 0.1mm.

A new conceptual bonding technique, 'hyper-interfacial bonding' has been proposed as the most effective bonding technique for ultra-fine grained steels. The hyper-interfacial bonding process was characterised by instantaneous surface-melted pressure bonding which involved a series of steps, namely surface heating by high frequency induction, rapid removal of a heating coil and simultaneously pressing specimens together with an oil pressure system. All sequences were typically completed within 1 s under a vacuum/gas atmosphere. Ultra-fine grained high strength steels with an average grain size of 1·0–1·5 μm were used for bonding. The surface of the specimen could be rapidly heated up and melted within about 0·2 s. No voids existed on the bond interface and primary ferritic phase was formed at the bond interface. A bainitic structure and martensite–austenite constituents were confirmed in the heat affected zone (HAZ), while the base metal indicated a fine ferrite–pearlite texture. The hardened area...

Grain growth of prior-austenitic grain in ultra fine-grained (UFG) steels during isothermal heat treatment at temperatures from 1173 to 1473 K was investigated quantitatively. The average grain size of UFG-T and UFG-B steels used was 1.25 μm and 1.21 μm, respectively. Grain growth of UFG steels followed Beck's theory with different time exponents in the early and later stages of grain growth. In the early stage of grain growth, grain growth was depressed by the pinning effect of (Nb, Ti) (C, N) particles in UFG-T steel and the particles composed of AlN and (Nb, Ti) (C, N) in UFG-B steel since the time exponent was 5. The time exponent was changed to 4 in the later stage of grain growth because of coarsening or dissolution of pinning particles. The distribution of prior-austenitic grain size in hyper interfacial bonded joints of UFG steels was estimated from the kinetics of grain growth considering thermal cycle and the transition of time exponent during the bonding process. The calculated grain size in the HAZ of hyper interfacial bonded joints was larger than the measured one. This fact suggested that thermo-mechanical factor would also act on the depression of grain growth during hyper interfacial bonding process.

The microstructure and grain growth behavior were investigated in the hyper interfacial bonded joint of ultra fine-grained steels. The influence of M-A constituents on hardening and/or HAZ softening in hyper-interfacial bonded joint was also examined. Microstructure of the hyper interfacial bonded joint of ultra fine-grained steels was changed melted zone, coarse-grained zone, mixed-grained zone to base metal region in order from bond interface. Martensite and bainite were formed in coarse-grained zone, and ferrite existed at bond interface. M-A constituents and fine carbides were formed in the lower temperature region of HAZ. Grain growth in joint adjacent to bond interface was restrained below 12.0μm. The maximum hardness in the hyper interfacial joint was about Hv380, and HAZ softening was suppressed by M-A constituents formed in AC1-AC3 region. Hyper interfacial bonding process was superior to fusion welding processes such as TIG, LBW and EBW from the viewpoint of the restraint of grain coarsening.

Dissimilar diode-laser brazing of A5052/IF steel and A5052/SUS304 has been conducted using BA4047 filler metal with Nocolock flux (KAlF4+K3AlF6). The processing parameters such as laser power, wire feeding speed and travel speed were varied while defocusing distance and shielding gas (Ar) flow rate were constant. Strength of lap joints of A5052 on steels was evaluated by tensile shear test. Fracture strength of A5052/steels joints was increased with increasing laser power and reached the maximum strength more than about 80% of the A5052 base metal strength at laser power of 1300 W. Fracture position shifted from at the A5052/braze layer interface to at the steels/braze layer interface with increasing laser power. The voids and incomplete penetration of filler metal were observed at the A5052/braze layer interface when the laser power was below 1100 W. The Fe-Al intermetallic compounds were formed at the steels/braze layer interface and grew drastically when the laser power exceeded 1300 W. The superior brazability of A5052/steels was indicated at the brazing conditions corresponded to the temperature of melted droplet of BA4047 filler metal being 1050-1250 K during laser brazing process.

(2001). Mechanism of hot cracking in the heat affected zone of repair welds. Repair weld cracking of service‐exposed HP‐modified heat‐resisting cast alloys (3rd report) Welding International: Vol. 15, No. 9, pp. 699-708.

(2001). Changes in microstructure of HP‐modified, heat‐resisting cast alloys under long‐term aging. Repair weld cracking of service‐exposed, HP‐modified, heat‐resisting cast alloys (2nd report) Welding International: Vol. 15, No. 7, pp. 509-517.

The diffusion-bondability of A6061 aluminum alloy to SUS316 stainless steel has been improved by using surface-activated pre-coating technique for SUS316 stainless steel. The surface-activated treatment for SUS316 was carried out by series of steps as alkaline cleaning → electrolytic cleaning HCl activating → metal-striking prior to diffusion bonding. Diffusion bonding of A6061/SUS316 was conducted at 758 K-823 K for 0.6 ks-7.2 ks applying 9.8 MPa in vacuum. ESCA analyses revealed that the surface oxide film could be removed by surface-activated pre-coating treatment and that striking insert metals would act as a barrier to reoxidation of SUS316. The tensile strength of A6061/SUS316 joints using Ag, Cu and Ni striking insert metals was risen up to about 100 MPa bonded at 758 K for 0.6 ks indicating nil joint strength in direct-bonding situation. The reaction layer growth in A6061/Ag striking/SUS316 joint was followed by the parabolic growth law, and the incubation time for reaction layer growth using Ag striking insert metal was shortened compared with direct-bonding situation.

The hot cracking susceptibilities of HP-modified heat-resisting cast alloys exposed at the elevated temperature for the long term were evaluated by the miniature spot-Varestraint testing. The as-cast, aging and/or used (service exposed) specimens of seven types of the HP-modified alloys were employed for the spot-Varestraint testing. Ductility-dip cracking as well as liquation cracking was occurred in HAZ of each specimen after the spot-Varestraint testing. The ductility-dip cracking susceptibilities of alloys except for HP43AZ much increased in aged and used conditions compared with as-cast condition, while the liquation cracking susceptibilities did not change so much during long term aging. The ductility-dip cracking susceptibility of HP43AZ containing zirconium was remarkably restrained even in used condition. It was deduced that the improvement in the ductility-dip cracking susceptibility of HP43AZ was attributed to reducing the occupied ratio of microconstituents on cellular dendrite boundary by the zirconium addition which possessed refinement effect of dendritic cell.

The effect of bonding misfit on creep rupture properties of TLP-bonded joints of Ni-base single crystal superalloy CMSX-2 were investigated bonded using MBF-80 insert metal. The bonding misfit was defined by ‹100› twist angle (rotating angle) at bonded interface. The post-bond heat treatment consisted of the solution and sequential two-step aging treatments was carried out in the Ar (oxygen partial pressure : 6.7 mPa) or vacuum (oxygen partial pressure : 4 mPa/0.27 mPa) atmosphere prior to creep rupture testing. Creep rupture properties of joints were comparable to those of base metal within the rotating angle being up to 3° while drastically fell down with increasing the rotating angle more than 5°. The boundary energy and oxygen content at bonded interface increased with increasing the rotating angle. Creep rupture properties of joints could be improved with lowering the oxygen partial pressure during post-bond heat treatment attributed to the restraint of grain boundary oxidization of bonded interface. It was deduced that the bonding misfit which promoted the grain boundary oxidization during post-bond heat treatment caused by the increment of boundary energy at bonded interface was deteriorated the creep rupture properties of joints.