STUDIES ON METALLURGICAL AND MECHANICAL BEHAVIOR OF DIFFUSION BONDED STEEL JOINTS

Abstract

Solid state joining processes avoids melting and solidification of metals which in turn leads to superior quality joints in comparison to fusion joining processes. Diffusion bonding is one of the solid state joining processes, which is extensively utilized in manufacturing of precision components without subsequent post processing in electronics, nuclear engineering and aerospace industries. In diffusion bonding, the joint is produced at approximately 60 - 80 % of melting temperature of parent metal under externally applied pressure for a specified period of time. However, the soundness and mechanical performance of diffusion bonds is highly influenced by surface preparation including roughness of mating surfaces. Therefore, tedious and time consuming polishing requirement for preparation of mating surfaces makes the diffusion bonding process less productive and more costly. These shortcomings limit the application of diffusion bonding process. Thus, to increase the productivity and reduce the cost of the diffusion bonding process, one way is to reduce the surface preparation requirements prior to the bonding without any compromise with the quality of developed diffusion bond. Various approaches namely impulse pressure assisted diffusion bonding and prefriction treatment assisted diffusion bonding were explored in the present thesis work. The primary objective of the current work is to improve the process effectiveness by minimizing the surface preparation requirements and diffusion bonding time without compromising the metallurgical and mechanical properties of diffusion bonds. The work was initiated with the impulse pressure assisted diffusion bonding of AISI 304 austenitic stainless steel. Lap joints were developed using 3 mm thick plate having overlapped area of 20 mm × 20 mm for varying bonding temperatures of 850 °C, 900 °C and 950 °C. Tensile shear testing was carried out to assess the load carrying capacity of the diffusion bonds. Afterwards, the impulse pressure assisted diffusion bonding of AISI 409 was done. The effect of input process parameters on tensile shear load carrying capacity of diffusion bond was studied using univariate approach. Optimization of the process parameters such as maximum pulse pressure, pulsation time, pulse duration and surface roughness was done to achieve maximum tensile shear load carrying capacity using Taguchi method. Impulse pressure assisted diffusion bonding of P92 steel was done for different surface roughness conditions. The microstructural study, micro-hardness test, tensile test and toughness tests were performed to study the soundness of the diffusion iv bonds. The effect of post diffusion bond heat treatment on metallurgical and mechanical properties was also investigated. Further, dissimilar diffusion bonding of AISI 316L and P92 steel was performed with and without using copper interlayer and the effect of surface roughness and bonding time on metallurgical and mechanical properties was investigated. The diffusion bonding of AISI 316L and titanium (Ti-6Al-4V) alloy was also performed using impulse pressure assisted diffusion bonding. Effect of bonding temperature on metallurgical and mechanical properties were examined. The feasibility of pre-friction treatment assisted diffusion bonding was studied for diffusion bonding of structural steel. Pre-friction treatment was done on lathe machine with the help of indigenously designed setup and the effect of bonding time on metallurgical and mechanical properties was studied. Further, the dissimilar diffusion bonding of AISI 316L and P92 steel was performed using conventional constant pressure diffusion bonding approach. The AISI 316L and P92 steel rods of square cross section of 25 mm × 25 mm were used for bonding. The integrity of the diffusion bonds was analyzed using microstructure study, hardness, tensile, impact toughness, and creep tests. The microstructure of the diffusion bonds was characterized using FE-SEM and grain sizes were measured. Further, the effect of post diffusion bond heat treatment on microstructure and mechanical properties was also studied. Impulse pressure assisted diffusion bonding successfully produced sound diffusion bonds even at a high surface roughness by deforming the surface asperities during pressure pulsation. Use of pressure pulses reduced the voids at the interface by plastic deformation of the surface asperities. In AISI 304 diffusion bonding, the interface bonding ratio was found to increase with increase in diffusion bonding temperature and a maximum 91.25 % interface bonding ratio was obtained at 950 °C temperature, 15 MPa maximum pulse pressure and 30 min bonding time in vacuum 8 × 10-6 mbar. An increase of 59 % in interface bonding ratio was observed with increase in bonding temperature from 850 °C to 950 °C. In AISI 409 ferritic stainless steel diffusion bonding, at 40 MPa maximum pulse pressure and 18 pressure pulses, 94.6 % interface bonding ratio was obtained with increase of 43.7 % as compared to the diffusion bonds developed at constant pressure. With increase in maximum pulse pressure and decrease in surface roughness, the large voids at the interface disappear and a good joint (free from the voids) was obtained. The pressure pulsation also provides more grain boundary area by refining the grains at the interface. Taguchi optimization of diffusion bonding of AISI 409 showed the optimum level of process parameters for maximum shear load at 40 MPa maximum v pulse pressure, 540 s pulsation time, 30 s pulse duration and 0.02 μm surface roughness. A maximum tensile shear load of 37.79 kN was obtained at these optimum levels with prediction error of 0.26 % only. In diffusion bonding of P92 steel, the microstructure of the diffusion bond interface was found to be similar to the base metal, as new grains were formed across the diffusion bond interface. The P92 steel diffusion bond failed from the base metal far away from the bond line during tensile testing. In diffusion bonding of AISI 316L-P92 steel, the tensile strength of the diffusion bond was found to increase with decrease in surface roughness. A maximum joint efficiency of 93.76 % was obtained for the diffusion bond developed using specimens having roughness 0.52 μm at 1000 °C, 20 min bonding time and 20 MPa maximum pulse pressure. Use of copper interlayer in diffusion bonding of AISI 316LP92 steel, the remnant copper possessed the lowest hardness on micro-hardness profile. The strength of the diffusion bond increases with decrease in surface roughness but the fracture occurred from the remnant copper present at the bond interface. A maximum joint efficiency of 61.2 % was obtained for the diffusion bond prepared using polished specimens and copper interlayer. Pre-friction treatment assisted diffusion bonding eliminates the pre-bonding surface preparation requirements. It produces a layer of plastically deformed grains with several non-equilibrium defects, which recrystallize during diffusion bonding and makes the diffusion faster. In pre-friction treatment of structural steel, the strength of the diffusion bonds increased with increase in diffusion bonding temperature and a maximum 482.5 MPa tensile strength was obtained at 875 °C bonding temperature, 15 MPa maximum pulse pressure and 30 min bonding time in vacuum 8 × 10-6 mbar. An increase of 16.4 % in tensile strength was observed with increase in bonding time from 20 min to 30 min. FE-SEM results showed that for lower bonding time, some weakly bonded regions were present at the interface and these weakly bonded regions were totally eliminated for higher bonding time. Post diffusion bond heat treatment fully recrystallized the grains at the diffusion bond interface by diffusional reversion. After post diffusion bond heat treatment an improvement of 70.33 % in ductility of the diffusion bond was realized corresponding to 30min bonding time due to grain coarsening. Using constant pressure diffusion bonding, defect free sound diffusion bonds of AISI 316L-P92 steel were developed at 20 MPa bonding pressure, 60 min bonding time at a vi varying bonding temperature 950 °C, 1000 °C and 1050 °C. Increase in bonding temperature coarsens the grain structure in AISI 316L side. At bonding temperature 950 °C, the average grain size was measured as 30.64 ± 4.2 μm which further increased to 39.96 ± 4.8 μm as the bonding temperature approached to 1050 °C. The inter-diffusion zone width increases with increase in bonding temperature. The average micro-hardness of the diffusion bond interface was 330 ± 4 HV. However, the average hardness at AISI 316L side was 193.5 ± 5.7 HV and it decreased to 178.4 ± 5.8 HV with increase in bonding temperature to 1050 °C. Tensile strength of the diffusion bond was found to decrease with increase in bonding temperature and failure in all the conditions occurred from the AISI 316L base metal. At 950 °C, 1000 °C and 1050 °C; the tensile strength 675.01 MPa, 653.99 MPa and 615.85 MPa was obtained, respectively. Post diffusion bond heat treatment increases the elongation of the diffusion bond by 20.65 % for the diffusion bond developed at 1050 °C bonding temperature.