Sharif Digital Repository

The paper aims at understanding solidification phenomena and solid state precipitations during diffusion brazing of Hastelloy X nickel base superalloy using a ternary Ni–4.5Si–3.2 B (in wt-%) filler metal. The joint is featured by the formation of Ni-rich solid solution in an isothermal solidification zone, borides/silicide formation during eutectic-type solidification in an athermal solidification zone, on-cooling precipitation of fine nickel silicides in the joint centerline, in situ precipitation of Mo–Cr-rich borides in the diffusion affected zone and grain growth in the heat affected zone. The implication of the phase transformations on the joint properties is discussed. It is shown... 

This paper investigates the metallurgical and mechanical response of 2304 duplex stainless steel-as an interesting candidate for automotive body-in-white applications-to resistance spot welding. The results showed the high cooling rate associated with the resistance spot welding process suppressed the postsolidification ferrite-austenite transformation leading to improper ferrite-austenite phase balance with a reduced volume fraction of austenite and consequent precipitation of chromium-rich nitrides. The effects of welding current, as the key parameter determining the weld heat input, on the austenite volume fraction and precipitation are discussed. The minimum fusion zone hardness... 

This paper addresses the microstructure-properties relationship during resistance spot welding of martensitic stainless steel (MSS). The effect of the initial base metal microstructure (ferritic microstructure in annealed condition vs. partially tempered martensitic in quench and tempered condition) on the weld microstructure evolution is highlighted. Regardless of the initial base metal microstructure, the fusion zone exhibited a predominantly martensitic structure plus some δ-ferrite. The heat affected zone (HAZ) in the quenched and partially tempered (Q-PT) sheet was featured by formation of martensite and carbide in upper-critical zone and tempering of martensite in sub-critical zone.... 

The paper addresses the process–microstructure–performance relationships in resistance spot welding of a martensitic advanced high strength steel. Significant softening was observed in the heat affected zone (HAZ) due to allotriomorphic ferrite formation in the inter-critical HAZ and tempering of martensite in sub-critical HAZ (SCHAZ), with the latter plays more important role in mechanical properties of the spot welds. The strain concentration associated with the HAZ softening promotes initiation of pullout failure from the soft SCHAZ. While, the peak load in the interfacial failure mode is governed by the fusion zone size, that of the pullout mode is significantly affected by the HAZ... 

The paper aims at understanding solidification phenomena and solid state precipitations during diffusion brazing of Hastelloy X nickel base superalloy using a ternary Ni–4.5Si–3.2 B (in wt-%) filler metal. The joint is featured by the formation of Ni-rich solid solution in an isothermal solidification zone, borides/silicide formation during eutectic-type solidification in an athermal solidification zone, on-cooling precipitation of fine nickel silicides in the joint centerline, in situ precipitation of Mo–Cr-rich borides in the diffusion affected zone and grain growth in the heat affected zone. The implication of the phase transformations on the joint properties is discussed. It is shown... 

This paper introduces a novel facile method, called projection friction stir spot welding, to produce a keyhole-free friction stir welds based on a pinless tool method involving using a specially designed projection on the surface of the backing anvil. The projection plays two key roles contributing to the bonding mechanism and the joint strength: (i) encouraging the material flow perpendicular to the joint interface and (ii) bending the joint interface at the edge of the projection. The process enables pathway to produce keyhole-free welds with superior mechanical performance in steel sheets compared to the other spot welding techniques. © 2018, © 2018 Institute of Materials, Minerals and... 

The microstructure of dissimilar transient liquid phase bond between Hastelloy X and IN792 nickel-based superalloys is featured by lack of sufficient formation of γ′ precipitates in the bond-centerline and extensive in-situ precipitation of boride second phases in the diffusion affected zones (DAZ). This paper investigates the impact of two thermal processing strategies, using standard solution treatment and aging of IN792 (STA strategy) and using solution treatment of IN792 followed by a post-bond heat treatment utilizing solution treatment of Hastelloy X and aging treatment of IN792 (SPTA strategy) on the joint microstructure and mechanical properties. The boride precipitates in the DAZ of... 

Martensitic stainless steel (MSS) welds are notorious for their susceptibility to low-energy failure due to the formation of brittle martensitic structure in the fusion zone. The unique approach to enhance the mechanical properties of MSS resistance spot welds during both the tensile-shear and the cross-tension loading is to improve the fracture toughness of the fusion zone. In the present study, the effect of double-pulse welding on the microstructure-mechanical properties relationship of the AISI420 MSS resistance spot welds is investigated. Depending on the second pulse current level, various metallurgical phenomena was observed including (i) rapid tempering of martensite in the fusion... 

This paper addresses the phase transformations and mechanical performance of dissimilar resistance spot welds between 2304 duplex stainless steel (DSS) and 1.2-GPa martensitic (MS) advanced high-strength steel (AHSS). The solidification mode and transformation path of the fusion zone (FZ) were analyzed. The key metallurgical feature of the FZ was the formation of a duplex microstructure consisting of delta ferrite and martensite. The FZ size at the sheet/sheet interface was the critical factor controlling the fraction of pullout failure during the partial thickness-partial pullout mode, the load-bearing capacity, and the energy absorption capability of the DSS/MS dissimilar resistance spot... 

Fundamental understanding of the intermetallic phase formation is the key for enhancing the robustness and reliability of the brazed joints. The paper addresses the phase transformations during brazing of the Hastelloy X nickel-base superalloy using the quinary Ni–13Cr–4.5Si–4.2Fe–2.8B (wt-%) braze alloy. The mechanisms of intermetallic formation via solidification and solid-state precipitation are discussed. The athermal solidification zone (ASZ) is featured by the formation of brittle and hard borides and boro-silicides that are formed via eutectic reactions. However, in contrast to other commercial B-bearing Ni-based filler alloys, it was identified that the presence of a high-volume... 

This investigation aims at understanding the underlying fundamentals of the isothermal solidification phenomenon during the transient liquid phase (TLP) bonding process. The isothermal solidification is governed by solid-state diffusion of the melting point depressant (MPD) into the base material, which, in turn, is controlled by both kinetic and thermodynamic parameters; however, the latter factor is generally ignored. In this work, the competition between kinetics and thermodynamics of diffusion were considered in TLP bonding of a nickel alloy, Monel 400, using two distinct filler metals including pure copper (Cu) and Ni-Si-B filler metal. The joint generated by Ni-Si-B filler metal... 

Martensitic stainless steel (MSS) resistance spot welds are prone to quasi-cleavage interfacial failure with very low energy absorption capability due to formation of hard and brittle carbon and chromium rich martensite in the fusion zone (FZ). In this work, a new pathway is proposed to enhance strength/toughness of the MSS resistance spot welds based on modification of the FZ composition/microstructure via introducing a nickel interlayer. This altered the FZ microstructure from dual phase microstructure of martensite and δ-ferrite to austenitic microstructure with finely dispersed ultra-fine chromium rich carbides. Formation of a tough predominately austenitic microstructure in the FZ... 

This paper introduces a novel facile method, called projection friction stir spot welding, to produce a keyhole-free friction stir welds based on a pinless tool method involving using a specially designed projection on the surface of the backing anvil. The projection plays two key roles contributing to the bonding mechanism and the joint strength: (i) encouraging the material flow perpendicular to the joint interface and (ii) bending the joint interface at the edge of the projection. The process enables pathway to produce keyhole-free welds with superior mechanical performance in steel sheets compared to the other spot welding techniques. © 2018, © 2018 Institute of Materials, Minerals and... 

This paper addresses the factors affecting the mechanical properties of dissimilar resistance spot welds between Nimonic 263 and Hastelloy X nickel-based superalloys. The fusion zone (FZ) of the dissimilar weld displayed higher hardness values compared to the base metals which can be attributed to (i) the formation of Mo-rich carbide in the interdendritic zone due to positive segregation behavior of Mo and C during non-equilibrium solidification of the weld, (ii) the enhanced solid solution strengthening due to mixing of the base metals, and (iii) the formation of ultra-fine dendritic structure in the FZ due to ultra-fast cooling rate of the resistance spot welding. Formation of... 

This paper addresses the metallurgical and mechanical response of Nimonic 263 nickel-based superalloy to resistance spot welding. Solidification structure of the fusion zone is described in terms of dendrite arm spacing, segregation behaviour and secondary carbide formation in the inter-dendritic area. The heat affected zone is featured by negligible grain growth and constitutional liquation of primary (Ti, Mo)C carbide present in the initial microstructure of the base metal. Mechanical behaviour of the welds was characterised by interfacial to pullout failure mode transition, peak load and energy absorption of the joints during the tensile-shear loading. The fusion zone size and the... 

Isothermal solidification stage during transient liquid phase bonding is the key to preclude intermetallic phase formation during solidification of the liquid phase. The rate of isothermal solidification is controlled by solid-state diffusion. Therefore, the bonding time required to complete isothermal solidification is generally long. This paper reports a very short isothermal solidification time for transient liquid phase bonding of IN617 solid solution nickel-based superalloy using a boron-containing filler metal. A eutectic-free bond with limited grain growth in the base material with high shear strength is achievable by utilizing a short (i.e., 5 minutes) thermal bonding strategy. The... 

Magnesium alloys provide critical opportunities for energy and lightweight-materials challenges. The alloy design approach based on utilizing the second phase strengthening mechanism plays an essential role in producing low-density high strength Mg-based alloys. However, the second phase in the microstructure (e.g., Mg17Al12 phase in Mg–Al–Zn alloys) can deleteriously affect the weldability of the metallic materials by promoting the liquation and liquation cracking during fusion welding. The lack of sufficient weldability of Mg-based alloys is a crucial barrier to their potential use in safety-critical applications. In this paper, it is shown that the severe plastic deformation induced by... 

AZ91 alloy, the most widely used Mg casting alloy, exhibits low strength/ductility and weak energy absorption, which is a function of its large grain size and the presence of a coarse and continuous network of β-Mg17Al12 intermetallic compounds. This work demonstrated that friction stir processing (FSP) enables enhancement of strength and energy absorption capability of AZ91 alloy. The influence of FSP treatment on various potential strengthening mechanisms, including grain boundary, solid solution, and sub-micron particle strengthening mechanisms, was studied. It is identified that the grain boundary strengthening plays a significant contribution to the strength of the FSP treated AZ91. FSP... 

In the present work, failure analysis of an automatic coupler in railway carriages which failed in service and caused derailment, was carried out. An evaluation of the failed coupler was undertaken to assess its integrity, including visual examination, photo documentation, fractography, chemical analysis, hardness measurement, tensile and charpy impact testing and microstructural characterization. Results show that some casting defects (e.g. mis-run, cold lap, hot crack and inclusion) resulted in poor mechanical properties. Mis-runs were also responsible for fatigue crack nucleation from two different locations. Beach marks can be observed on the fracture surface. According to the results of...