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.     

Optimised running system design for bottom filled aluminium alloy 2L99 investment castings

Abstract

This research has evaluated the effect of surface turbulence introduced during the filling of moulds on the reliability of Al-7Si-Mg alloy (2L99)investment castings. Four different running systems were designed to provide different amounts of surface turbulence: top filled, uncontrolled bottom filled, and controlled bottom filled with and without a filter. Computer modelling has been used to simulate the filling of the different designs and the results have been compared with the actual flow behaviour established using real time X-ray radiography. Castings have been produced using the different running systems, tested in four point bending, and the results analysed using the Weibull statistical technique. It has been shown that top filling produces significant turbulence, which results in the least reliable castings (Weibull modulus of 25.5). There was no significant difference between this and the Weibull modulus of 27 found for uncontrolled bottom filled systems. This indicates that a poorly controlled bottom filled system is no better than a top filled system. Both were inferior to the controlled bottom filled systems without and with filters, which had Weibull moduli of 38 and 54 respectively. SEM examination and oxygen analysis of representative fracture surfaces has provided supporting evidence for the important role that oxide films played in reducing properties.     

Weld brazing of copper thick plates

Abstract

The weld brazing of 6 mm thick copper plates without preheating was investigated. The weld metal of weld brazing is composed of α-Cu solid solution and Cu–Ag–P eutectic structure, and the α-Cu solid solution in the weld metal of weld brazing is larger than that in the brazing weld. The average hardness values within the weld are 60 HV (50 g) lower for weld brazing than for brazing. In all weld brazing specimens tested, the failure was located in the heat affected zone with a tensile strength slightly lower than the base metal but similar to the arc welding joints.     

In situ thermal studies and post-weld mechanical properties of friction stir welds in age hardenable aluminium alloys

Abstract

The 'quench sensitivity' of various friction stir welded (FSW) aluminium alloys were investigated. Cooling rates of FSW were altered by utilising various active (weld side) and passive (anvil side) cooling/heating conditions. Media used included a chilled water mist (active cooling) and hot air (active heating) immediately behind the tool, and water chilled (passive cooling) or heated (passive heating) anvil. Microhardness and transverse tensile properties were used to assess the natural aging response of friction stir welds (FSWs) in 2195-T8 and 7075-T7351 subject to these conditions. Test results indicated that the quench sensitive alloy 7075 exhibits a more rapid natural aging response and improved mechanical properties while the less quench sensitive 2195 exhibits little or no change in mechanical properties. In the present investigation FSWs in alloy 7075 under active and passive cooling conditions exhibited an increase of approximately 10% in tensile properties over conventional FSWs.     

Properties and fracture processes of Si3N4/ Si3N4 joints brazed using Cu–Zn–Ti filler alloy

Abstract

Si₃N₄ ceramic was jointed to itself using a filler alloy of Cu-Zn-Ti at 1123-1323 K for 0.3-2.7 ks. Ti content in the Cu-Zn-Ti filler alloy was varied from 5 to 20 at.-%. The effect of brazing parameters, such as brazing temperature, holding time and Ti content, on the mechanical properties and facture processes of the Si₃N₄/Si₃N₄ joint were investigated. The results indicated that the increased brazing temperature, holding time and Ti content increase the thickness of the interfacial reaction zone in the Si₃N₄/filler alloy, and the size and amount of the reaction phases in the filler alloy. Their increases lead to increasing shear strength of the joint. The fracture behaviour of the Si₃N₄/Si₃N₄ joint greatly depends on the microstructure of the joint. A suitable thick reaction zone with reaction phases yields the high strength of the Si₃N₄/ Si₃N₄ joint.     

Laser brazing of alloy 600 with precious filler metals

Abstract

The diode laser brazing of Ni base heat resistant alloy with precious filler metals has been conducted using the tandem beam for preheating and brazing. A couple of 1 mm thick plates of alloy 600 (Inconel 600) were butt brazed using Au–18Ni, Ag–10Pd and Ag–21Cu–25Pd filler metals of 0·5 mm diameter with a brazing flux. Sound butt joints which were free from brazing defects such as porosity and lack of penetration could be obtained at brazing clearances of 0·1–1·5 mm. The tensile strength of the braze joint produced using Ag–Pd filler metal increased with decreasing brazing clearance and reached ～70% of the base metal strength at a brazing clearance of 0·1 mm while those obtained by using Au–Ni and Ag–Cu–Pd filler metals were comparable with the base metal strength at any clearances between 0·1 and 1·5 mm. The laser brazing technique could be successfully applied to the brazing of Ni base superalloy to attain a joint with high performance and reliability.     

Microstructure evolution and mechanical properties of nickel based honeycomb sandwich

Abstract

A nickel based superalloy honeycomb sandwich was manufactured by high temperature brazing. The microstructure evolution and the out of plate mechanical properties were investigated for honeycomb sandwiches aged at 1000&degC in the present paper. The maximum tensile stress was 28·5 MPa and the compressive yield strength was 29·6 MPa for the original specimens. These parameters decreased to 22·9 and 20·5 MPa for specimens aged for 2 h, to 18·2 and 12·2 MPa for specimens aged for 5 h and to 20·2 and 14·3 MPa for specimens aged for 10 h respectively. With increasing aging time the tensile elongation decreased, the intermetallic compounds and the eutectic structure in the brazing region disappeared, and the solid solution approaching the matrix gradually increased.     

Microstructure and mechanical performance of brazed joints of Cf/SiC composite and Ti alloy using Ag–Cu–Ti–W

Abstract

C_f/SiC composite was brazed to Ti alloy using interlayer of Ag–Cu–Ti–W mixed powder. The effects of W content and brazing parameters on the microstructure and properties of the brazed joints were investigated. The results show that W grains mainly distribute in Ag phase in the brazing layer and provide the effects of reinforcement and lowering residual thermal stress on the joint. The room temperature and 500°C shear strengths of the joints performed at 500°C for 30 min with Ag–Cu–Ti–50W (vol.-%) are remarkably higher than the optimal strengths of the joints brazed with Ag–Cu–Ti.     

Capillary filling process during ultrasonically brazing of aluminium matrix composites

Abstract

Alumina borate whisker reinforced aluminium metal matrix composites were ultrasonically brazed in air. The liquid filler metal (FM) of a Zn–Al eutectic alloy could get into the clearance between composite couples by the action of ultrasonic capillarity. The velocity of the FM getting into the clearance decreases with increasing clearance width. The time exhausted to remove completely the oxide film on the surface increases with increasing clearance width. The oxide film could be removed completely and the metallurgical bonding formed under the action of ultrasonic vibration for sufficient time. The shear strength of the bond could reach up to 145 MPa (the nominal strength of the Zn–Al filler alloy).     

Effect of high energy ball milling on solid state reactions in Al–25 at.-%Ni powders

Abstract

The effect of high energy ball milling on the solid state reactions between aluminium and nickel in Al–25 at.-%Ni powders has been investigated using scanning electron microscopy, thermal analysis techniques, and X-ray diffractometry. It has been observed that the microstructure of the powder particles evolves in three stages: stage I is the formation of entrapped nickel particles in the aluminium matrix structure; stage II is the formation of an Al–Ni multilayered structure; and stage III is the formation of Al₃Ni single phase. The temperature required to activate the reaction between aluminium and nickel during heating decreases by more than 200 K as the powder particle microstructure evolves from the entrapped particle structure to the multilayered structure, and then it decreases gradually with decreasing nickel layer thickness. The nucleation and lateral growth of Al₃Ni phase at the Al/Ni interfaces occurs at much lower temperatures than those required for the transverse growth of Al₃Ni. The fraction of Al₃Ni formed through nucleation and lateral growth at the interface is almost linearly proportional to the interfacial area. The activation energy for nucleation and lateral growth of Al₃Ni at the Al/Ni interfaces is independent of nickel layer thickness, but the activation energy for transverse growth of Al₃Ni decreases substantially with decreasing nickel layer thickness. The latter is attributed to the observation that the nickel layers are thinned by plastic deformation and thus contain an increasingly higher density of dislocations.