Effect of Bonding Temperature on Microstructure Evolution during TLP Bonding of a Ni3Al based Superalloy IC10

Transient liquid phase (TLP) bonding of Ni3Al-based superalloy IC10 was carried out using the interlayer based on the base metal which added B and Hf as the melting point depressant elements. The effect of bonding temperature (1250 – 1270 °C) on the microstructure evolution of bonding joints were investigated. Microstructure of bonding joint composed of isothermally solidification zone (ISZ) formed γ’ phase and athermally solidified zone (ASZ) which consists of newly formed γ+γ’ reticular eutectic among with borides and carbides. Boride precipitates are not formed in diffusion affected zone (DAZ) and the boundary between ASZ and ISZ become not obvious. Isothermally solidification rate decreases as the increase of the bonding temperature.

Wide Gap Brazing of IN 738 With Boron Free Ni-Co-Zr-Hf-Cr-Ti-Al Braze Alloy

In this study, a new boron and silicon free braze alloy, based on Ni-Co-Zr-Hf-Cr-Ti-Al, was used to repair IN 738 superalloy employing a wide gap brazing (WGB) process under two process conditions. It was found that using a combination of Hf and Zr primarily as melting point depressants the amount of each melting point depressant (MPD) could effectively be reduced while still achieving a relatively low liquidus. During WGB process, the new braze alloy was able to bond IN 738 filler powder particles to each other and to the substrate cast IN 738 and achieve defect free joint. The microstructure analysis showed the presence of Zr-containing phase(s) remained in WGB joint, however, the hardness of the Zr-containing phase(s) was similar to that of superalloy substrate. Extended brazing cycle had limited effect on reducing the Zr-containing phase(s); rather it encouraged the formation of larger eutectic γ/γ′ phases.

Vacuum Brazing Porous W and Mo Using Ti-Ni-Nb Braze Alloys

A novel approach of brazing porous W and Mo using three clad Ti-Ni-Nb foils has been performed in the experiment. Clad Ti-Ni-Nb filler foils are featured with low brazing temperature of below 1350°C. Both W and Mo are completely soluble with β-Ti and Nb, and the Ni addition into the braze alloy is served as a melting point depressant (MPD). Decreased brazing temperature and/or time are necessary in order to minimize infiltration of the molten braze into the porous W substrate. According to the experimental results, Ti-Ni-Nb ternary alloys are promising filler metals in low-temperature brazing porous W and Mo.

Liquation Cracking in Heat Affected Zone in Ni Superalloy Welds

Precipitation hardened nickel-based superalloys are widely used in aero and industrial gas turbine engines due to their excellent high temperature strength and remarkable hot corrosion resistance. A drawback of many of these alloys is that they are very difficult to weld due to their high susceptibility to heat affected zone (HAZ) cracking, both during welding and post weld heat treatments (PWHT). Weld cracking in many of these alloys has been attributed mostly to constitutional liquatioin of grain boundary NbC precipitates. however, HAZ cracking has been observed in carbon-free superalloys as well, Therefore, research was initiated to examine grain boundary liquation and cracking in HAZs in a variety of Ni-based superalloys. It was found that intergranular cracking of grain boundaries involved liquation of several other phases, in addition to NbC, that were present in pre-weld microstructure of the alloy. These even included the primary strengthening phase,γ’, in a very widely used superalloy, Inconel 738. In addition, segregation of melting point depressant element B was also observed at gain boundaries in other superalloys, which also caused grain boundaries in HAZ to liquate and resulting in their cracking. An overview of microstructural aspects of different liquation phenomena involved and characteristics of the liquid film contributing to the HAZ microfissuring of nickel superalloys will be discussed in this presentation

Use of Computational Thermodynamics in Rapid Prototyping and Infiltrating Steel Parts

ABSTRACTThe direct manufacture of metal parts by rapid prototyping often involves building a porous skeleton from a metal powder. In this work, a method termed Homogeneous Steel Infiltration has been developed for infiltrating steel skeletons to make conventional tool steel alloys. The method uses a gated infiltration route that uses as the infiltrant a steel alloy with a lower melting point than the base powder. The infiltrant liquid may use carbon and/or silicon as a melting point depressant. Premature freeze-off of the steel infiltrant is avoided by operating at a temperature where some liquid is stable at chemical equilibrium. The compositions of the skeleton and infiltrant and the infiltration temperature are selected by using computational thermodynamics. Examples of successful infiltrations using D2 and A3 tool steels as target compositions are shown. The thermodynamic design method enables suitable parameters to make other tool steels, some stainless steels and manganese steels.