Vacuum Diffusion Research Articles

Precipitation reconstruction of a diffusion brazed austenite joint with Ni-filler

Abstract The paper will explain the formation and effects of brittle precipitates in form of borides and silicides during high-temperature vacuum diffusion brazing on the example of a nickel-based filler metal from the NiCrSiB system. This filler metal foil was used to braze butt joints of the metastable austenite AISI 304L. Energy and wavelength dispersive X-ray spectroscopy was applied to identify the precipitates. Using a focused ion beam in scanning electron microscopy, the brazed joint was removed layer by layer over a large area to gain insights into the 3D structure of the precipitates. Subsequently, a methodology will be presented on how to reconstruct a 3D model from the image data using deep learning-based image segmentation. The final model unravels the complex morphology of these precipitates and thus contributes to a better understanding of the solidification and precipitation mechanisms in diffusion brazing.

Bonding mechanism of TC4 titanium alloy/T2 copper vacuum diffusion bonded joint with nickel as transition interlayer

Vacuum diffusion bonding of TC4 titanium alloy (TC4) to T2 copper (T2) using nickel foil as transition interlayer was explored. Ti3Ni, Ti2Ni, TiNi, AlNi2Ti and TiNi3 phases arose at the TC4/Ni bonded interface, and Cu-Ni solid solution appeared in the Ni/T2 interface. Thereinto, AlNi2Ti was a kind of discontinuous nano precipitated phase, which distributed between TiNi and TiNi3 phases. The crystallographic orientations of Ti2Ni, TiNi, AlNi2Ti and TiNi3 phases were (201), (020), (11¯1) and (031), respectively. The interplanar spacing of (031), (11¯1) and (020) was correspondingly d(031) = 0.144 nm, d11¯1 = 0.275 nm and d(020) = 0.137 nm. The lattice mismatch between TiNi3 and TiNi was calculated to be 2.5 %, with low strain energy. The order of effective formation enthalpies for TiNi3, TiNi and Ti2Ni phases formed between titanium and nickel was ∆H′NiTi2 > ∆H′NiTi > ∆H′Ni3Ti. The growth activation energy of TiNi3, TiNi and Ti2Ni phases was correspondingly 35.8 kJ/mol, 180.6 kJ/mol and 347.1 kJ/mol. When welding at 880 °C for 60 min, the highest shear strength of the joints could achieve 150 MPa. The joints fractured along the Ni/T2 interface, the fracture surface of joint was composed of elongated dimples and cellular pits, presenting a shear ductile fracture mode. FCC-Cu, FCC-Ni and (Ni, Cu)ss phases were detected on TC4 and T2 fracture surfaces by XRD. The interdiffusion coefficient ratio of (Ni in Cu)/(Cu in Ni) and (Ni in Ti)/(Ti in Ni) decreased gradually with increasing temperature.

Toughening of a vacuum diffusion-bonded heterostructured AZ31/WE94/AZ31 alloy by interfacial reinforcement

Heterostructured materials have recently attracted extensive attentions from the materials community due to their superior combination of strength and ductility as compared to the conventional homogeneous counterparts. In the present study, the laminate AZ31/WE94/AZ31 Mg alloy was produced by vacuum diffusion bonding. The post annealing was carried out to improve the mechanical properties of the heterostructured Mg alloy. It is found the elemental diffusion and formation of interfacial phases enhance the interface bonding strength, thereby improving the comprehensive mechanical properties. The laminate sample annealed at 470 °C for 12 h shows a superior mechanical properties with a yield strength, ultimate tensile strength, and elongation of 158 MPa, 212 MPa and 12.9%, respectively. The higher annealing temperature leads to the growth of interfacial phase, which provides preferential nucleation sites for crack initiation and weakens the tensile properties.

Microstructure Evolution of Vacuum Diffusion-Bonded 304 Stainless Steel/20 Carbon Steel Bimetallic Interface after Solution Treatment.

In this study, a 304 stainless steel (304 SS)/20 carbon steel (20 CS) bimetal was prepared by vacuum diffusion bonding, with 20 CS as the substrate and 304 SS as the cladding layer, and the interfacial microstructure and bonding strength before and after solution treatment were studied. The 304 SS and The 20 CS formed a strong metallurgical bond after being held at 1380 °C for 60 min without defects such as unbonded regions. Diffusion of Cr and Ni atoms from the 304 SS to the 20 CS occurred, and the C atoms diffused from the 20 CS to the 304 SS, forming a carburized region. A pearlitic region with an average width of approximately 20 μm was formed on the 20 CS side. After solution treatment, austenitization was formed in the carburized region, which was accompanied by the formation of twin crystals. The interfacial bonding strength of the bimetal was measured to be 485 MPa, which increased to 547 MPa after the solution treatment.

A novel high entropy composite interlayer for diffusion bonding of TC4 titanium alloy to 316L stainless steel

Titanium/steel hybrid structure exhibit broad prospects in the nuclear industry and aerospace applications. Achieving high-performance titanium/steel bonding is crucial. Herein, the novel AlCoCrNiCuAg/Cu high entropy composite interlayer was designed for vacuum diffusion bonding of TC4 alloy to 316L stainless steel. The effects of bonding temperature, holding time and assembly sequence on the interfacial microstructure and mechanical properties were investigated, and the fracture behaviors were researched. Typical microstructure of the joint bonded at 1010 °C for 90 min via AlCoCrNiCuAg/Cu sequence was TC4/β-Ti/β-Ti+TiFe/TiFe2+χ+σ/α-Fe+χ+σ/316L. With bonding temperature increased, due to the dispersed distribution of the Cuss phase in the Ti-Fe phases, the highest shear strength of 222.8 MPa was achieved at 1010 °C/90 min. The cleavage characteristics were confirmed in the fracture surfaces, suggesting the brittle fracture. The main cracks were located at the junction of the TiFe2+χ+σ and the β-Ti+TiFe layers, further confirmed that the interface appeared immense strain, accelerating the joint fracture.

Influence of Al foil interlayer on performance of vacuum diffusion bonding joint of 6061 aluminium alloy

Influence of Al foil interlayer on performance of vacuum diffusion bonding joint of 6061 aluminium alloy

Preparation of TiN compound layer by intermittent vacuum diffusion nitriding on Ti6Al4V titanium alloy

An intermittent vacuum diffusion nitriding (IVDN) treatment was used for the preparation of a TiN compound layer on the surface of the Ti6Al4V titanium alloy. The underlying microstructure evolution and the properties were thoroughly investigated. The IVDN process was performed alternately by using intermittent nitriding and vacuum diffusion. The TiN compound layer was formed on the surface discontinuously, layer by layer. As a result, the TiN compound layer was formed easily on the surface of titanium alloy compared to the vacuum nitriding. Dislocations were also generated in the adjacent α-Ti grains of the TiN compound layer. Interestingly, the induced dislocation defects possessed high free energy and contributed to the generation of the TiN grain nucleus, which facilitated the diffusion of the N atoms towards the inner substrate. The acquired results disclosed the intrinsic reason for the diffusion behaviour of N atoms. The Ti6Al4V titanium alloy was nitrided by using IVDN treatment at the temperature of 700 °C for 10 h. The surface microhardness reached 900–950 HV and presented continuous gradient change. In addition, the IVDN sample exhibited enhanced wear resistance properties than the un-nitrided sample.

Effect of vacuum diffusion bonding on the mechanical and conductive properties of bonded bulk copper single crystals

Recrystallization-free atomic-level (AL) bonding between bulk copper single crystals was achieved by combining chemical mechanical polishing (CMP) and vacuum diffusion bonding. Under various bonding conditions, the bonded copper specimens can be classified into three categories: AL-bonded, HH-bonded, and L-bonded specimens. AL-bonded copper single crystals were obtained by performing bonding under 20 MPa at 600 °C for 60 min, achieving the mechanical and electrical properties that are closest to those of the parent copper single crystal. The AL-bonded interface is characterized by an asymmetrically tilted sub-grain boundary with a 2° misorientation and consists of two sets of edge dislocations with mutually perpendicular Burgers vectors. The formation of the AL-bonded interface depends on two critical factors: the creation of ultra-flat bonded surfaces and performing diffusion bonding within the elastic deformation range. Achieving an ultra-flat surface that eliminates residual stress allows for bonding under relatively low pressure. With a moderate bonding temperature and holding time, it is possible to simultaneously avoid the formation of voids and recrystallizations at the bonding interface, thereby achieving a high-quality AL-bonded interface between bulk copper single crystals.

Vacuum diffusion bonding of TC4 titanium alloy to T2 copper with VCrNi1.8 eutectic medium entropy alloy interlayer

A new eutectic medium entropy alloy VCrNi1.8 was designed for vacuum diffusion bonding TC4 titanium alloy (TC4) with T2 copper (T2). The influence of welding parameters on the microstructure and shear strength of joints was investigated. β-Ti/Ti3Ni/Ti3AlNi/Ti2Ni/TiNi/TiNi(V, Cr)2/BCC + FCC/Ni(V, Cr)2/Cu phases existed in sequence in the TC4/VCrNi1.8/T2 joint. The crystallographic orientations of Ti3AlNi, Ti2Ni, TiNi and TiNi(V, Cr)2 phases were (200), (020), (011) and (012), respectively. The zone axis of Ti3AlNi, Ti2Ni, TiNi and TiNi(V, Cr)2 phases was correspondingly [002¯], [202], [100] and [4¯00]. The maximum shear strength of joint made at 880 °C for 90 min reached 214 MPa. The growth activation energy of diffusion zones I and II on TC4/VCrNi1.8 side was 230 kJ/mol and 246 kJ/mol, respectively. The interplanar spacing of (200), (020), (011) and (012) was d(200) = 0.656 nm, d(020) = 0.375 nm, d(011) = 0.556 nm and d(012) = 0.225 nm, respectively. The crystal face mismatch of Ti3AlNi/Ti2Ni, Ti2Ni/TiNi and TiNi/TiNi(V, Cr)2 interface was calculated to be 54.5%, 58.3% and 84.8%, respectively, revealing non-coherent with high strain energy. When the bonding temperature increased and holding time prolonged, the joint fractured along the TC4/VCrNi1.8 interface. The fracture morphology of joint presented cleavage fracture characteristics. The average diffusion coefficient of Cu in VCrNi1.8 interlayer at 840 °C, 860 °C, 880 °C, and 900 °C was 0.9 × 10−15 m2/s, 1.2 × 10−15 m2/s,1.6 × 10−15 m2/s, and 2.5 × 10−15 m2/s, respectively.

Microstructure and mechanical properties of copper/al interlayer/graphite joint prepared utilizing “two-stage method” vacuum diffusion bonding

The high property Copper/Graphite joint with Al foil as intermediate layer has been prepared utilizing the “two-stage method” diffusion bonding. At stage I, the Cu–Al diffusion alloying behavior occurs, and when the alloying temperature reaches to 500 °C, the stable Cu9Al4 layer is formed on the surface of Cu substrate. At stage II, Cu (with Cu–Al alloying layer) and Graphite were diffusion bonded, and the microstructure of joint is composed of Cu/Cu9Al4 alloyed layer/Graphite. The C atom diffuses into the Cu9Al4 alloyed layer, which improves the joint property. With the temperature increases from 600 to 900 °C, the diffusion distance of C atom in Cu9Al4 increases from 5.8 μm to 11.4 μm. And the tensile strength of joint increases 25.8 MPa–38.6 MPa. During the diffusion bonding processing, there are three routes for C atom from graphite diffusing in Cu9Al4 alloyed layer, which are OIⅠ→OIⅡ, OIⅠ→TI→OIⅡ and TIⅠ→TIⅡ routes, respectively. The diffusion coefficient of C atoms diffusion along OIⅠ →OIⅡ route is the largest, indicating that OIⅠ→OIⅡ is the preferential diffusion route for C atom diffusing in alloyed layer, and playing the decisive role in the microstructure and properties of joint utilizing two-stage method diffusion bonding.

Multiscale microstructural consideration for high strength diffusion bonding of Ti2AlNb alloy to GH4169 superalloy with (TiZrHfNb)95Al5 interlayer

A high entropy interlayer (TiZrHfNb)95Al5 was successfully designed for vacuum diffusion bonding of GH4169 superalloy to Ti2AlNb alloy. The relationships between grain size, grain boundary, lattice mismatch and microhardness, elastic modulus, shear strength and fracture behaviors were revealed. The typical microstructure was Ti2AlNb/B2/Solid solution/(Ti, Zr, Hf)2(Ni, Nb)+(Ti, Zr, Hf)(Ni, Nb) + Solid solution/(Ti, Zr, Hf)(Ni, Nb) + (Cr, Ni, Fe)ss/(Cr, Ni, Fe)ss/Cr-rich(Cr, Ni, Fe)ss/Ni-rich(Cr, Ni, Fe)ss/GH4169. With the holding time increased, the shear strength overall increased and decreased sharply, and the highest shear strength reached 384 MPa at 980 °C for 90 min. The microstructure had anisotropic crystal orientation, and the joints of Zone Ш for 90 min consisted of 92.08 % HAGBs and 7.92 % LAGBs. The residual strains and stresses decreased when the holding time increased. Meanwhile, recrystallisation appeared in Zone Ш, and the increase in the recrystallisation rate led to grain refinement. The lattice mismatch between (Ti, Zr, Hf)(Ni, Nb) and (Cr, Ni, Fe)ss phases was 25.05 %, indicating that the interface was non-coherent, and (Ti, Zr, Hf)(Ni, Nb)+(Cr, Ni, Fe)ss phases were located at an elastic modulus dramatically changed interface. Thus, the cracks easily formed in this interface and led to joint fracture. Moreover, the fracture mechanism was brittle fracture.

Phase formation and elemental transport in the vacuum diffusion bonding between CoCrFe0.2NiMn and CoCrFe16NiMn multicomponent alloys

To efficiently explore the influence of Fe content on the phase stability of the multicomponent CoCrFexNiMn (hereafter abbreviated as Fex) alloys. A diffusion layer with continuous, concentration gradient of Fe element is created based on diffusion couple technique. The microstructure and element distribution of the diffusion bonded layer between Fe0.2 and Fe16 alloys are analyzed. A new layer of FCC and HCP solid solution phases generates in the diffusion zone, which is consistent with the prediction of parametric approach and phase diagram calculation. As diffusion proceeds, the FCC phase is transformed from the BCC phase with the decreasing of Fe content and the HCP phase might be the transitional phase during this transition. The newly formed FCC and HCP phases are shown specific orientation relationships with the BCC phase. Elemental analysis indicated that the diffusion rate of the alloy components in the FCC phase is Cr > Mn > Co > Ni. The large concentration gradient in the diffuse interface and grain boundaries diffusion of BCC phase promoted the Fe diffusion, causing the BCC phase to transform into FCC phase.

Research on the interfacial microstructure and mechanical properties of glass/Mg joint prepared by anodic bonding and vacuum diffusion bonding

The joining of glass and Mg by a combination of anodic bonding and vacuum diffusion bonding with Al as the interlayer was achieved in one step. The microstructures of glass/Mg dissimilar joints were analyzed using optical microscopy, SEM, and EDS. The results show that the bonding interface was joined well without any voids and cracks. Na+ and Ca2+ depletion regions would be produced in the glass near the Al foil, where the width of the Na+ depletion region was larger than that of the Ca2+ depletion region. In addition, the content of Al element in the depletion regions increased, and the diffusion of Al into the depletion regions was responsible for the successful glass/Al bonding process. Al3Mg2 and Al12Mg17 intermetallic compound (IMC) layers were formed between the Al and Mg. The anodic bonding process and the vacuum diffusion bonding process were independent of each other: and the thickness of the IMC layer increased with the increase of the diffusion bonding time, while the width of the Na+, Ca2+ depletion region remained constant; Meanwhile, the variation of the anodic bonding parameters only affected the width of the depletion layer and had no effect on the thickness of the Al3Mg2 and the Al12Mg17 IMC layer. The fracture position of the sample changed from the Al/Mg bonding interface to the glass/Al bonding interface as the thickness of the IMC layer increased. The maximum tensile strength of 14.6 MPa was obtained by vacuum diffusion bonding at 450 °C for 20 min and anodic bonding under 400 °C and 800 V.

High strength diffusion bonding of Ti2AlNb to GH4169 with (TiZrHfNb)95Al5 high entropy interlayer

A high entropy interlayer (TiZrHfNb)95Al5 was designed for vacuum diffusion bonding of Ti2AlNb to GH4169. The influence of bonding temperature on the interfacial microstructure morphology, mechanical properties and fracture behaviour of the resultant joints was investigated. The evolution of microstructure and fracture mechanism were revealed. The typical interface microstructure of Ti2AlNb/B2/Solid solution/(Ti, Zr, Hf)2(Ni, Nb)+(Ti, Zr, Hf)(Ni, Nb)+(Ti, Zr, Hf)(Ni, Nb)2+Solid solution/(Ti, Zr, Hf)(Ni, Nb)+(Cr, Ni, Fe)ss/(Cr, Ni, Fe)ss/Cr-rich(Cr, Ni, Fe)ss/Ni-rich(Cr, Ni, Fe)ss/GH4169 was formed. As the bonding temperature increased, the thickness of interfacial reaction layer gradually increased, and the shear strength gradually increased until 980 °C, then decreased sharply. The highest shear strength reached 376 MPa at 980 °C for 90 min. The highest microhardness region of 11.79 GPa existed on the GH4169 side, which was mainly composed of (Ti, Zr, Hf)(Ni, Nb) and (Cr, Ni, Fe)ss phases. Meanwhile, the elastic modulus dramatically changed interface formed near this region. The fracture surface morphology indicated typical brittle fracture characteristics such as cleavage steps, transgranular fracture and intergranular fracture. According to the fracture analyses, major cracks initiated in the (Ti, Zr, Hf)(Ni, Nb) phase+(Ti, Zr, Hf)2(Ni, Nb) phase and gradually extended to the (Cr, Ni, Fe)ss phase. Thus, the major crack emerged region was located at the elastic modulus dramatically changed interface.

Effect of al/si multilayer foil on microstructure and mechanical properties of SiCр/Al composites vacuum diffusion bonded joints

The peculiarities of joint formation in vacuum diffusion bonding through an intermediate multilayer foil (MF) of Al/Si produced by electron beam physical vapour deposition (EB-PVD) in vacuum were studied on SiCр/Al aluminium matrix composites with different alloying systems of the aluminium matrix. It is shown that Al/Si interlayer enables making permanent joints without degradation of the composite properties at the temperature of 500°С, which corresponds to the start of intensive plastic deformation of the interlayer at thermomechanical loading. It is found that the microstructure and phase composition of the joint zone (JZ) depend on the interlayer and aluminium matrix of the composite. It is demonstrated that reduction of the bilayer thickness of Al/Si MF and its alloying by manganese ensures intensive mass transfer of the components in the butt joint and equivalent joint formation. The maximum value of shear strength was 105 MPa in the case of an interlayer with a nano-scale period of the layers.

Mathematical modeling and optimization of vacuum diffusion bonding parameters for predicting and enhancing the strength of dissimilar IN-718/MSS-410 joints using RSM for power generation applications

The dissimilar welding of Inconel 718 (IN-718) alloy and AISI 410 martensitic stainless steel (MSS-410) is crucial in advanced gas turbines, and ultra-supercritical power plants to meet the demands of different operating conditions and lower the cost. However, the dissimilar fusion welding of IN-718/MSS-410 is challenging due to the differences in thermal expansion coefficient, physical and mechanical properties of base metals. In this study, the solid-state vacuum diffusion bonding (VDB) technology is employed to develop the dissimilar IN-718/MSS-410 joints. The aim of this study is to find the optimal combination of VDB parameters such as diffusion bonding pressure-DBP (MPa), diffusion bonding temperature-DBT (°C) and diffusion bonding time-DBt (min) for enhancing the strength of IN-718/MSS-410 joints. The response surface methodology (RSM) was integrated for designing the experimental matrix. The strength performance of VDB joints was evaluated by conducting the lap shear strength (LSS) and bonding strength (BS) tests. The mathematical LSS and BS predicting models were established using regression analysis and verified employing the variance analysis. The microstructural features were analyzed using optical and scanning electron microscopy (SEM). The X-ray diffractometer (XRD) was employed to identify the phases evolution in the joint interface. The experimental results revealed that the IN-718/MSS-410 joints diffusion bonded using the DBP of 14 MPa, DBT of 960 °C and DBt of 90 min exhibited the greater LSS of 280 MPa and BS of 373 MPa. The prediction models accurately predicted the LSS and BS of IN-718/MSS-410 joints within 2 % error at 95 % confidence. It is primarily concerned with developing the optimal bonding width with the fewest possible embrittlement implications and better joining interface coalescence. According to variance analysis, the DBt was the most significant parameter influencing the LSS and BS of joints followed by the DBP and DBT.

Effect of Temperature, Vacuum Condition and Surface Roughness on Oxygen Boost Diffusion of Ti–6Al–4V Alloy

Oxygen boost diffusion (OBD) is an effective technology for improving the surface hardness of titanium and its alloys. In this present paper, the effect of temperature, vacuum condition and surface roughness on oxygen boost diffusion of Ti–6Al–4V alloy are studied. Test results show that OBD processing can be achieved at a low temperature and over long times, as well as at a high temperature and over short times. By comparing processing efficiency and mechanical properties, high temperatures and short times are preferred for OBD treatment. The influence of vacuum conditions on oxygen vacuum diffusion is significant. Under low vacuum degree conditions, relatively high oxygen content not only corrodes the OBD layer but also leads to spalling of the outmost surface of the OBD layer and the remaining oxide layer. High surface roughness can induce cracking not only in the oxide layer during the oxidation process but also in the outmost surface of the OBD layer during the vacuum diffusion process.

Underlying mechanism of interfacial evolution in laminate IN625/TC4 composites for enhanced mechanical properties

Compared to the homogeneous materials, laminated heterogeneous materials, in contrast to their homogeneous counterparts, display enhanced performance by effectively balancingcombination of strength and plasticity. However, fabricating such materials is challenging, relying on precise interfacial control. In this work, an IN625/TC4 layered structural material is successfully fabricated by vacuum diffusion bonding process at 900 °C. The IN625/TC4 composite exhibits good combination of strength and ductility due to its layered hetero-structure, is due to mechanical incompatibility between different layers. The diffusion layer's Ni and Ti gradient distribution leads to distinct intermetallic compounds formation, including Ti2Ni, Ni3Ti precipitates, and disordered TiNi entities at the interface. Diffusion-driven interfacial reactions suggest interconnected growth mechanisms of individual intermetallic compounds. Post-bonding annealing treatments reinforced the composite, with annealing temperature affecting tensile strength and ductility. Increased annealing temperature effectively enhanced strength, peaking at 800 °C (857 MPa), but reduced ductility due to additional intermetallic compounds. This study presents a novel structural design for metallic materials to obtain good combination of strength and ductility.

Mechanism of solid-state diffusion reaction in vacuum between metal (Fe, Ni, and Co) and 4H–SiC

Experiments were conducted to investigate the vacuum diffusion reactions between 4H–SiC and various metals (iron (Fe), nickel (Ni), and cobalt (Co)). The effects of temperature, time, and metallic materials on the diffusion reactions were systematically analyzed. Notably, a solid-state diffusion reaction between the metal and 4H–SiC was observed at low temperatures, and the mechanism governing the solid-state interface diffusion reaction between the single-crystal 4H–SiC and metals was elucidated. The results indicate that a chemical reaction occurs between the metal and 4H–SiC during solid-state diffusion, leading to the formation of metal silicides under vacuum conditions. The temperature threshold for the solid-state diffusion reaction between 4H–SiC and metals falls within the range of 550–650 °C. Interestingly, the diffusion reaction proceeds without soaking. Furthermore, the metal type significantly influenced the diffusion rate.

A novel ultra-high vacuum diffusion setup to study Knudsen diffusion

An ultra-high vacuum setup was conceived to study Knudsen diffusion in channels with varied geometrical characteristics that can be modified using 3D printing. This new experimental methodology aids to gain insight into diffusion in nanoporous media.