Void formation in wide gap brazing using prepacks of nickel base braze mixes

Abstract

The various types of void formed in wide gap brazed joints of C1023 nickel base superalloy produced using prepacks of nickel base braze mixes have been investigated and systematically categorised. Of particular interest are interfacial and interstitial voids and unwetted pockets, which are features frequently found in joints brazed using such a technique. Examination of brazed joints produced under a wide range of conditions revealed that during heating to the brazing temperature, the braze mix partially sinters together, causing the prepack to shrink towards the centreline, leaving two channels next to the joint faying surfaces. At the same time, relatively large pockets of free space are created within the partially sintered mass of prepack. At the brazing temperature, the filler metal deposited at the gap mouth becomes molten and this molten filler is drawn into the gap preferentially through the fine capillary paths in the partially sintered mass of prepack. The relatively large pockets of free space in the prepacks as well as the channels adjacent to the joint faying surfaces are bypassed by the molten filler metal owing to their lack of capillarity. As a result, the various brazing defects described above will be formed should solidification occur before these empty spaces are filled. The effects of materials and process variables on the formation of various brazing defects are briefly discussed.

MST/1925     

Nickel base wide gap brazing with preplacement technique Part 2 - Forl11ation mechanisms of macrovoids

Abstract

Microstructural inhomogeneities and variations in the extent of erosion of base metal in nickel base wide gap brazed joints produced by the preplacement technique with braze mixes of different gap filler contents were investigated, from which the flow behaviour of the braze mix constituents and the formation mechanisms of the various types of macrovoid were deduced. The results show that the formation of various types of macrovoid is closely related to the flow behaviour of the constituents of braze mixes during brazing, the latter in turn being strongly influenced by the braze mix ingredients, the brazing temperature, and the gap depth. For a wide gap brazed joint to be free from macrovoids, the braze mix must be sufficiently viscous to bridge the gap faying surfaces and must flow as a whole into the gap. Braze mixes with gap filler contents of 30–40% are ideal for such requirements. With too Iowa gapfiller content, the molten filler would flow preferentially ahead of the mass-of braze mix, leading to the formation of irregularly shaped macrovoids at the tail end of the joint. With too high a gap filler content, the molten filler metal available was insufficient to completely fill the interstices among the gap filler particles, leading to near spherical macrovoids in the braze mix deposit and adjacent joint area. At too Iowa brazing temperature, the braze mix would be too viscous to penetrate into the gap freely and local premature solidification would occur, leading to large, irregularly shaped macrovoids throughout the longitudinal section of the joint.

MST/3132     

Microstructure and strength of TiAl/steel joint induction brazed with Ag-Cu-Ti filler metal

Abstract

Intermetallic TiAl was induction brazed to steel in an induction furnace with Ag-Cu-Ti filler metal at 1143 K for 0·2–2·4 ks. Microstructural analysis indicates that Ti, Al, and C atoms in base metal diffuse to the interface and react strongly with the filler metal during brazing. The interface structure of the joint can be divided into three distinct zones: the reaction layer near TiAl, composed of Cu-Al-Ti compounds and Ag based solid solutions; the central zone of the interface, consisting of Ag based solid solutions in which Ag-Cu eutectic phases are dispersed; a TiC reaction layer adjacent to the steel. The relationship between brazing parameters and tensile strength of the joints is discussed, and the optimum induction brazing parameters obtained. When brazed at 1143 K for 0.9 ks, the tensile strength of the joint is 298 MPa.     

Cu-P-Sn-Ni钎料真空钎焊MGH956合金的研究

为扩展Cu-P基钎料在连接MGH956合金中的应用,采用新型Cu-P-Sn-Ni钎料对MGH956合金在800~890℃进行了真空钎焊,研究了不同钎焊温度和保温时间对焊缝组织及力学性能的影响.结果表明:在所研究的钎焊温度范围内保温5 min均可获得成形效果良好的钎焊接头,其主要由钎缝中心区和界面反应层组成,其中,钎缝中心区由α(Cu)固溶体基体和化合物Cu_3P+(Fe,Ni)_3P+FeCr组成,反应层由α(Fe)固溶体、Fe_3P和Cu_3P组成;随着钎焊温度的升高,反应层厚度逐渐增加,钎缝中心区中的化合物Cu_3P+(Fe,Ni)_3P+FeCr的形态也随之发生明显改变;各钎焊温度下获得的钎焊接头经室温拉伸,断裂均发生在钎缝中心区,断口形貌呈现韧性和脆性的混合断裂特征.830℃钎焊5 min的接头抗拉强度最大,为510.3 MPa,达到了母材抗拉强度的70.9%.     

Interface characteristics and mechanical properties of the induction brazed joint of magnesium alloy AZ31B with an Al-based filler metal

In this paper, wrought magnesium alloy AZ31B sheets were brazed by means of high-frequency induction heating device using a novel Al-based filler metal in argon gas shield condition. The interfacial microstructure, phase constitution and fracture morphology of the brazed joint were studied. The experimental results show that α-Mg solid solution and β-Mg₁₇Al₁₂ phase were formed in brazing region. Moreover, the homogeneous Mg₃₂(Al, Zn)₄₉ phase in the original Al-based filler metal disappeared entirely after the brazing process due to the fierce alloying between the molten filler metal and the base metal during brazing. Test results show that the shear strength of the brazed joint is 45 MPa. The fracture morphology of the brazed joint exhibits intergranular fracture mode and the crack originates from the hard β-Mg₁₇Al₁₂ phase.     

Infrared brazing Inconel 601 and 422 stainless steel using the 70Au-22Ni-8Pd braze alloy

Infrared brazing Inconel 601 and 422 stainless steel using the 70Au-22Ni-8Pd braze alloy is performed in the experiment. The brazed joint is primarily comprised of Au-rich and Ni-rich phases, and there is no interfacial intermetallic compound observed in the joint. The (Ni,Fe)-rich phase is observed at the interface between 422SS and the braze alloy, and the Ni-rich phase is found at the interface between the braze alloy and IN601. With increasing the brazing temperature and/or time, the microstructures of the brazed joint is coarsened. For the infrared brazed joint at 1050^∘C for 180 s shows the highest average shear strength of 362 MPa. In contrast, the shear strength of the infrared brazed joint is higher than that of the furnace brazed specimen due to coarsening of the microstructure in the furnace brazed joint.     

Brazing of 422 stainless steel using the AWS classification BNi-2 Braze alloy

The study of brazing 422 stainless steel (422SS) using the AWS classification BNi-2 braze alloy as the filler metal is evaluated in the study. The BNi-2 braze alloy demonstrates excellent wettability on the 422SS substrate for temperatures exceeding 1025 °C. The brazed joint is primarily comprised of the Ni-rich matrix and chromium boride. Additionally, the B–Cr–Fe precipitates are formed at the interface between the braze and 422SS. Some Kirkendall porosity is also observed in the braze close to the interface, due to nonsymmetrical interdiffusion between the braze and 422SS substrate. Shear strengths of brazed joints are varied from 306 to 481 MPa. The infrared brazed specimen shows the highest shear strength among all brazed specimens. Increasing brazing temperature and/or time result in decreased shear strength of the brazed joint.     

Microstructure of alumina ceramic/Ag–Cu–Ti brazing alloy/Kovar alloy joint

Abstract

The microstructure of the alumina ceramic/Kovar alloy joint brazed with Ag–35·2Cu–1·8Ti (wt-%) was studied. The effects of brazing temperature on the microstructure were also discussed. It was found that the microstructure of the joint brazed at 1173 K for 5 min was TiO + TiNi₃ + TiFe/eutectic Ag–Cu/TiFe₂ + TiNi₃/TiFe₂ + Cu (s.s) +Ag (s.s). When the brazing temperature was >1193 K, there was no TiO formed on the alumina ceramic/brazing alloy interface.     

Growth mechanism of compound at interface of Ti-6Al-4V joint brazed with 72Ag-28Cu filler alloy

Abstract

The growth process of Ti-Cu compound at the interface of a Ti-6Al-4V/72Ag-28Cu (wt-%) joint was analysed using X-ray diffraction, SEM, and energy diffraction spectra. According to the investigated results, when the joint was brazed for a relatively short holding time, atoms of Ti and Cu diffused into the interface would combine into Ti₂Cu by eutectoid reaction during the cooling stage. As the holding time is beyond the critical brazing time, Ti₂Cu compound decomposed owing to a large amount of Ti in the base metal dissolving into the brazing zone and the relatively gentle concentration gradient of Cu, thus resulted in the solid dissolving of Cu into Ti. In this case, the resulting joints exhibited high strengths. On the basis of the analysis mentioned above, a concept 'critical brazing time' was proposed.     

Microstructure and bond strength of Ni-Cr steel/Si3 N4 joint brazed with Ag-Cu-Zr alloy

Abstract

The interfacial microstructure and bond strength were examined for a Ni-Cr steel/Si3 N4 joint brazed using an Ag-Cu eutectic alloy containing 5 wt-%Zr. The reaction of Si₃N₄ with the brazing alloy formed a very thin ZrN layer with a cubic structure (a = 4.577 Å) at the interface. No Zr silicide (Zr₅Si₃) was present at the interface even though its formation is thermodynamically possible. The reaction product did not contain any of the dilute ceramic phases or intermetallics which are commonly seen in other active metal brazing systems. This strongly implies that the elements of the Ni-Cr steel and Ag or Cu in the brazing alloy, did not participate in the interfacial reaction. The shear strength of the joint was strongly dependent on the thickness of the reaction layer and the morphology of CuZr₂ precipitates in the brazement. A joint with a reaction layer thickness of 0.5 μm, which was formed by brazing at 950°C for 30 min, showed the highest fracture shear strength (～ 202 MPa).     

Evaluation of transient liquid phase bonding between nickel-based superalloys

Induction brazing of Inconel 718 to Inconel X-750 using Ni-7Cr-3Fe-3.2B-4.5Si (wt.-%) foil as brazing filler metal was investigated in this paper. Brazing was conducted at the temperature range 1373–1473 K for 0–300 s in a flow argon environment. Both interfacial microstructures and mechanical properties of brazed joints were investigated to evaluate joint quality. The optical and scanning electron microscopic results indicate that good wetting existed between the brazing alloy and both Inconel 718 and Inconel X-750. Microstructures at joint interfaces of all samples show distinct multilayered structures that were mainly formed by isothermal solidification and following solid-state interdiffusion during joining. The diffusion of boron and silicon from brazing filler metal into base metal at the brazing temperature is the main controlling factor pertaining to the microstructural evolution of the joint interface. The element area distribution of Cr, Fe, Si, Ni and Ti was examined by energy dispersive X-ray analysis. It was found that silicon and chromium remain in the center of brazed region and form brittle eutectic phases; boron distribution is uniform across joint area as it readily diffuses from brazing filler metal into base metal. The influence of heating cycle on the microstructures of base material and holding time on the mechanical properties of brazed joint were also investigated.     

箔状非晶态镍基含磷钎焊料的研究

与同类粉状或膏状钎料相比,非晶态箔状镍基含磷高温钎焊料是100%密度金属钎焊料,具有优良的焊接冶金性能及钎焊工艺性能,与大多数铁基合金有满意的相容性及足够的钎焊接头抗剪切强度,是核工业堆内组件及薄壁耐热蜂窝组件的理想钎焊料。     

Ni-Cr-P焊膏钎焊C/C复合材料的组织和性能

用真空熔炼、惰性气体雾化法制备Ni-Cr-P金属粉末,再加入有机黏结剂高速搅拌,制备Ni14Cr10P膏状活性钎料。用制备好的焊膏真空钎焊C/C复合材料,测试钎焊接头的剪切强度,通过OM,SEM,EDS,XRD等对钎焊接头界面组织结构进行分析。结果表明:在钎焊温度1000℃、保温时间0.5 h条件下,获得的接头剪切强度达到28.6 MPa,然后随着钎焊温度上升或保温时间延长,钎焊接头强度下降;通过界面组织结构分析发现焊膏可以增加钎料层与C/C复合材料表面的接触面积,有利于堵塞C/C复合材料表面的孔隙。焊后在界面处形成了交错分布的Cr碳化物相缓冲层,使得界面呈现热膨胀系数梯度增加的结构,有助于缓解热失配,提高C/C复合材料钎焊接头强度。     

New process for brazing ceramics utilizing squeeze casting

A new joining process for ceramics to ceramics and ceramics to metals, SQ brazing, has been developed. This process utilizes squeeze casting; a brazing material is squeezed into the interface channel to be brazed and is solidified under a high pressure. This new process has several advantages, low cost, mass producibility, high interface strength and high reliability, no severe reaction, etc. Alumina to alumina and silicon nitride to silicon nitride brazing with pure aluminium are shown as examples. Alumina containing silica as a sintering additive brazed by a conventional method severely reacted with aluminium braze so that the joint strength was low. After SQ brazing, reaction was moderate and the strength almost reached that of the parent alumina. Silicon nitride could be brazed by SQ brazing. Although the simple SQ brazing could not make a strong interface, pre-oxidization treatment of silicon nitride increased the joint strength beyond 400 M Pa.     

Oxidation behaviour and phase stability at and above 700°C in brazed areas of metal-supported automotive converters with honeycomb structures made of ultra-thin FeCrAlRE materials for diesel applications

Abstract

Metal-supported automotive catalytic converters for diesel applications are made of flat and corrugated thin foils, brazed together at their contacting points. Brazing is done with nickel based alloys. The foil alloy is of type Fe–20Cr–(4–7)Al with the addition of reactive elements RE. In the lower temperature range for diesel applications (500°C to 900°C), attention should be paid to high temperature properties of brazed areas. Unusually large polygonal phases can occur in the original brazed area beside the fine globular phases. Both are of type NiAl. Polygonal phases are less corrosion resistant, resulting in shorter product lifetime. The occurrence of such phases, their stability and oxidation resistance were analysed under isothermal test conditions in ambient air using brazed samples with honeycomb structure.

Diffusion of Ni into ultra-thin foil materials and Fe as well as Al into the Ni-based brazing material (erosion) leads to different compositions in brazed areas. NiAl phases (polygonal or globular) are already present after brazing. They do not change their shape over time. No oxidation of these phases was detected.

Further investigation is focused on the effects of metallurgical and production parameters as well as cyclic load and test atmospheres on the occurrence and oxidation behaviour of the undesired large polygonal phases.     

Microstructural evolution of brazing 422 stainless steel using the BNi-3 braze alloy

The 422 stainless steel (422SS) is one of the typical martensitic stainless steels with both excellent creep strength and corrosion resistance up to 650°C. Its application includes steam turbine blades, high temperatures bolting ... etc. Repair welding of 422SS is one of the most common methods to fix the turbine blade. However, repair brazing of surface shallow cracks, e.g., less than 1 mm in depth, is an alternative way to fix such blades. The microstructural evolution of brazing 422SS with BNi-3 braze alloy using both infrared and furnace brazing was performed in the study. Based on the experimental results, BNi-3 cannot effectively wet 422SS substrate below 1025°C. As the brazing temperature increases above 1050°C, comprehensive wetting can be obtained in 1200 sec. For the infrared brazed specimen with a short brazing time, the cooling path starts from the formation of a BNi₃ phase in the molten braze, subsequently forms a Ni-rich phase, and finally a eutectic phase is solidified from the residual eutectic liquid. The microstructure of the furnace-brazed specimen is similar to that of infrared brazed specimen, but the interfacial reaction zone is significantly increased in furnace brazing. There are Kirkendall voids in the braze close to the interface between BNi-3 and 422SS, and the size of Kirkendall porosity is increased with increment of the brazing time and/or temperature. The homogenization treatment of the brazed joint at 900°C results in growth of both the interfacial reaction zone and porosity.     

Joining of Cf/SiC composite to Ti alloy using composite filler materials

Abstract

C_f/SiC was successfully joined to Ti alloy with Ag–Cu–Ti–W, Ag–Cu–Ti–SiC and Ag–Cu–Ti–TiC mixed powders by some suitable brazing parameters. Microstructure and shear strengths of the preformed joint were investigated. The results showed that the W particulate and reaction products can uniformly distribute in the brazing layer of the performed joint. These composite brazing layers relaxed the thermal stress of the joint effectively. These characteristics were beneficial to the joint, which had shear strengths that were significantly higher than the optimal shear strengths of the joint brazed with pure Ag–Cu–Ti at room temperature and 500°C.     

Study on low temperature brazing of magnesium alloy to aluminum alloy using Sn–xZn solders

In this article, a series of Sn–xZn solders are designed for joining Mg/Al dissimilar metals by low temperature brazing. The effect of Zn content in Sn–Zn solders on microstructure evolution and mechanical properties of the different brazed joints are investigated. The experimental results indicate that Sn–30Zn alloy is identified as the optimized solder. Al–Sn–Zn solid solutions form and disperse in the brazing zone of the Mg/Sn–30Zn/Al brazed joint, decreasing the risk of embrittlement of the brazed joint. The average shear strength of Mg/Sn–30Zn/Al brazed joint can reach 70.73 MPa. The joint fractures in the coarse blocky Mg₂Sn intermetallic phases in the center of the brazing zone.     

SiC颗粒增强铝基复合材料的钎焊性

采用氩气保护炉中钎焊和真空钎焊两种试验方法,对SiCp/101Al复合材料的钎焊性进行研究。结果表明,通过选择合理的钎料和钎剂及采用正确的钎焊工艺参数,可以实现对SiCp/101Al复合材料的钎焊连接。对获得接头进行力学性能测试,表明钎焊接头的剪切强度随钎焊温度的升高而升高,当达到一定值以后,又随着钎焊温度的升高而降低。对接头钎缝区的XRD相结构分析中发现,接头中含有Al-Cu、Al-Si共晶组织相,并且有SiC相存在,说明母材中有部分SiC增强相颗粒过渡到了钎缝之中,有利于提高钎缝接头的力学性能。从钎焊接头的断口扫描照片中可以看出,接头大部分都呈韧性断裂特征,且大多数接头都断裂于靠近钎缝的母材部位,说明钎焊接头的质量较高,钎焊工艺可行。     

Development of novel CsF–RbF–AlF3 flux for brazing aluminum to stainless steel with Zn–Al filler metal

Brazing 6061 Al alloy to 304 stainless steel by flame brazing has been carried out with an improved CsF–RbF–AlF₃ flux which matched Zn–xAl filler metals. The results showed that, the spreading area on stainless steel of Zn–xAl filler metals has been improved with the addition of RbF to CsF–AlF₃ flux. It is found that a Zn-rich phase appeared between the brazing seam and the intermetallic compound (IMC) layer in the joints brazed with Zn–2Al and Zn–5Al filler metals, and the thickness of the IMC layer was approximately 1.76–6.45 μm which increased with the increase of Al added to the filler metals. Moreover, a Fe₄Al₁₃ phase formed in the IMC layer, while a Fe₂Al₅ phase appeared as the second layer in Zn–25Al brazed joint. Neither the Zn-rich phase nor Fe₂Al₅ phase was found in the joint brazed with Zn–15Al filler metal, so that the joint was exhibited the maximum shear strength which was up to 131 MPa. All the lap joints were fractured at the interfacial layer of the brazing seam and stainless steel.