In the present study, similar bonding of UNS32750 super duplex stainless steel was eva‎luated by the transient liquid phase method at a constant temperature of 1200 °C in a vacuum furnace and variable times of 2 and 3 hours and using a copper-nickel-phosphorus composite interlayer with different times of nickel - phosphorus coating on copper. The microstructural investigations of the bonding zone were carried out by means of an optical microscope and a scanning electron microscope. The microstructure of the diffusion affected zone of the sample joined by pure copper interlayer in 2 hours bonding time and the samples joined by copper interlayer with 3 μm of nickel-phosphorus coat in 2 and 3 hours bonding time, was in the form of a ferrite matrix and containing different types austenite included grain boundary austenite, allotriomorphic austenite, Wiedmannstatten austenite and intragranular austenite, which led to the creation of joint with low shear strength. Increasing the bonding time in the joined sample by the pure copper interlayer and increasing the nickel-phosphorus coating time in the interlayer led to the removal of austenite types and the similarity of the morphology of the diffusion affected zone with the base metal, which caused a significant increase in the shear strength of the joint. In the diffusion affected zone of the samples joined by the copper interlayer with 10 and 11 μm of nickel-phosphorus coat, the initial microstructure of the steel was preserved, but during cooling from the bonding temperature, in some of these samples molybdenum and phosphorus rich sigma phases with secondary austenite island was formed, which led to increase the hardness of this zone. Also, in the bonding zone of the sample joined by copper interlayer with 10 μm of nickel-phosphorus coat in 2 hours bonding time, an isothermal solidificatin zone and a non-isothermal solidification zone were formed, which non-isothermal solidification zone includes two different phases of austenitic solid solution and a phosphide phase rich in chromium and molybdenum were in the center line of the joint. The formation of this phosphide phase in the central line of the joint led to a significant increase in hardness in this zone. Also, it was found in fractography analysis that this phosphide phase was broken or torn off during the application of shearing force and led to the creation of holes on the fracture surface and decrease of shear strength of the sample. By increasing the bonding time to 3 hours or the nickel-phosphorus coating time to 45 minutes, the non-isothermal solidification zone was removed and led to an increase in shear strength. The sample joined by the copper interlayer with 11 μm of nickel-phosphorus coat in 2 hours bonding time had the highest shear strength among all the samples joined by the copper-nickel-phosphorus composite interlayer. Increasing the bonding time in this sample from 2 to 3 hours led to the diffussion of phosphorus from the base metal into the bonding zone and remelting of the bonding zone, which led to repeat TLP bonding process and due to insufficient time to complete the isothermal solidification, the bonding zone of this sample consists of two components, the isothermal solidification zone and the non-isothermal solidification zone, and therefore its shear strength decreased. The results of X-ray diffraction analysis showed the presence of phases rich in chromium, molybdenum and phosphorus in the non-isothermal solidification zone and the different chemical composition of austenites created in the bonding zone of different samples. In the fractography analysis of the samples, no trace of cleavage planes and river patterns were observed, which indicates that the fracture behavior of the samples is not brittle, and in the samples with the highest strength, holes were observed on the fracture surface, which indicates the ductile fracture behavior of these samples.

عباس بهرامي , حسام فلاح آراني

محمد رضايت , علي اشرفي