Sharif Digital Repository

Duplex stainless steels and advanced high strength steels are both known as new materials in automobile industry applications. Process of resistance spot welding is the main process in welding of auto-body structures. Hence investigating the welding metallurgy and weldability of similar and dissimilar joints of duplex stainless steels and advanced high strength steel are crucial. The purpose of this study is to analyse the relationship between welding process and geometric characteristic of weld nugget, phase transformations of Weld nugget and heat affected zone, mechanical properties (i.e force bearing capacity and energy absorption capability) and failure mode (interfatial or pullout). in... 

In this thesis, macrostructural evolutions in dissimilar TLP bonding of as-cast nickel based supperalloy IN792 to wrought HX was investigated. In the first step of this study solidification behavior of Ni-Cr-Fe-Si-B (MBF-15) interlayer in presence of HX as base metal at 1120 °C was surveyed. In second step, bonding of HX/HX, IN792/IN792 and HX/IN792 systems with MBF-15 interlayer at 1120 °C and different times were studied and isothermal solidification time and factors that affect this stage were discussed. In third step, synchronization of dissimilar bonding with standard heat treatment cycle of IN792 was investigated and effect of this cycle on aging response of different zones, isothermal... 

Resistance spot welding is one of the fabrication processes of gas turbines. Due to few published works about using RSW for joining participation hardened nickel base super alloys like Nimonic C263 and solid solution hardened ones like Hastelloy X, this work tries to investigate the microstructure and mechanical properties of RSW joints of these super alloys. In phase one, effect of welding current on structural properties of welds, hardness profile from the center of the weld to the edge of it, failure mod and failure energy were investigated. In phase two, effect of standard heat treatment of Nimonic C263 and the initial microstructure of the base metal on the micro-structure and... 

With the development of electrical engineering and the electrical industry, the use of copper and its alloys has grown tremendously. On the other hand, with development and modern industry, it will be difficult to meet the requirements such as high performance, cost reduction and weight reduction of the structure if only one metal material is used in the production of the desired component or structure. Aluminum can be used in combination with copper because of its properties such as high thermal and electrical conductivity. Al/Cu bonding is used in industries such as microelectronics, automobiles and batteries. This study has investigated parametrically the feasibility of uneven bonding of... 

Automotive steels are dominant material for the manufacturing of automotive structures and components. Since an automotive body is mostly assembled by spot welding, spot weld failure in different loading conditions has a great influence on the crashworthiness of vehicle. Therefore, investigation of microstructure and failure behavior of resistance spot welded automotive steels is an important issue. The first part of the research is dedicated to microstructural evolution and fusion zone hardness of spot welded automotive steels based on optical and SEM micrographs and hardness measurements. In the FZ of automotive steels, except for austenitic steels, a mainly martensitic microstructure was... 

With the expansion of industry, steel to copper joints applications in heat and electricity transmission equipment have increased. Steel has better mechanical properties and weldability and lower prices, and copper has better thermal and electrical conductivity properties; because of that, this dissimilar joint fits needs such as cost reduction and high efficiency. This study has parametrically investigated the feasibility of dissimilar welding of these metals using the resistance spot welding process.This study conducts three phases. In the first phase, low carbon steel and copper were directly welded, and it was found that the mechanical properties of the joint were affected by physical... 

Successfully bonding nickel-base alloys to titanium-base alloys has proven to be one of the most challenging modern joints. Traditional welding methods for dissimilar titanium/nickel joints often lead to cracking in the fusion and heat-affected(HAZ)zones; due to the formation of brittle intermetallic compounds with low fracture toughnesses. Additionally, the significant difference in thermal expansion coefficients between titanium and nickel introduces significant thermal stress during bonding process, exacerbating the current issue. Creating successful dissimilar Ti/Ni joints requires overcoming the formation of continuous brittle intermetallic compounds (IMCs). Brazing stands out as a... 

Selecting materials is a key stage for designing vehicles. Today, advanced high-strength steels and stainless steels are attractive by the automotive industry. In this study, the improvement of mechanical properties of the martensitic stainless steels (MSS420 with annealed condition) and advanced high strength martensitic steels (MS1400 with quenched condition), which have a different response to mechanical tests, were investigated. The welding of MSS420 steel was carried out in three phases. In the first phase, the MSS420 double pulse welding with variable second pulse times (0.2s-1.8s) with a constant secondary pulse current of 4 kA and in the second phase a double pulse welding with... 

The nickel-based superalloys play a key role in the development of aircraft and power-generation turbines due to their exceptional properties of high temperature strength, toughness, crack growth resistance and resistance to degradation corrosive and oxidisation environment.In this thesis, the role of base metal composition in isothermal solidification time during diffusion brazing of Nickel-based super alloys was investigated. To achieve this aim, four different super alloys Hynes230, Hastalloy X, Nimonic 263 and Monel were used in similar joining systems. Systems of H230/H230, HX/HX, C263/C263, and Monel/Monel were joined by BNi2 interlayer at 1323K and for different times, until the... 

Use of materials with both high strength and good formability in car structure is a serious approach in the automotive industry to reduce weight as well as maintain the security of occupants. Stainless steels such as duplex steels are one of the most attractive candidates. Thus, metallurgical and mechanical behavior during resistance spot is important. The aim of this work is to study the microstructure-property relationships in resistance spot welding of duplex stainless steel. Austenite-ferrite phase balance and weld nugget size are two key factors determining the mechanical properties of these steels. High cooling rate of resistance spot welding can have a significant impact on the... 

The new generation of advanced high-strength steels use in the automotive industry cause resistance spot welding study and their failure characteristics during static and dynamic loading are important. In this study, the relationship among microstructure, failure mode and mechanical properties are investigate during similar and dissimilar DP980 and MS1400 jointing advanced high-strength steels resistance spot welding. To search for micro and macro structural changes, the optical microscope, stereographic microscope and scanning electron microscope were used. Micro-hardness test, tensile shear test and Cross tension test to evaluate the mechanical properties were conducted. As well as... 

High-strength aluminum alloys are widely used in various industries; however, their usage is limited to solid-state welding or mechanical joints due to their low weldability. The main metallurgical challenge of alloy 7075 welding is hot cracking. Thus, the mentioned alloy is introduced as a "non-weldable alloys" category. In this study, the PMZ was investigated, and it was shown that precipitations play an essential role in intensifying the phenomenon of partial melting. On the other hand, the result of the mechanisms activated in the PMZ, called the "L" region, occurs in the grain boundaries during welding. Welded plate with FM1050 causes liquation cracking, while Welded plate with FM4043... 

Considering the wide applications of 304 stainless steel in various industries and the existing literature on transient liquid phase bonding of stainless steels with a variety of interlayers, the present work aimed to reduce the bonding time by using a two step bonding method. Faster bonding reduces both energy consumption during bonding and the base metal grain growth. It also leads to a lower reduction in the base metal and joint strength.Due to the melting point of the interlayer, different durations and a temperature of 1200°C were selected for the first step. This temperature was maintained for a specified period of time before it sharply dropped to and remained at 1180°C (the... 

Titanium alloy and aluminum alloy 6061-T6 were successfully joined by resistance spot welding. The microstructure and mechanical properties of the lap joint were investigated. The microstructure of joints was inspected by OM, XRD, and SEM. The results reveal that the reaction between liquid aluminum and solid titanium were caused welding-brazing joints, which restricts the formation of IMCs. The microstructure in the aluminum side is a type of transition structure, which is considered to be the same microstructure that connects the columnar dendrite and Equiaxed dendrite in similar aluminum alloy RSW. The welding current has the most significant influence upon the tensile shear and cross... 

Magnesium alloys, as the lightest structural alloys having a density of about one-fourth that of steels, offer significant potential for improving the energy efficiency of various transportation systems. Welding is a critical enabling manufacturing technology; therefore, the widespread application of a material is highly influenced by their weldability. Liquation and Liquation cracking are crucial weldability issues for fusion welding of cast AZ91 Mg alloy. The constitutional liquation of the coarse eutectic Mg17Al12 intermetallic phase is responsible for the occurrence of liquation cracking during fusion welding of this alloy. Here, friction stir processing (FSP) treatment, a successful... 

Fe-Cr-C hardfacing alloy is the most popular alloy used in severe abrasive condition such as mining and cement industries. One of the limitation of these hardfacing alloys is their low toghness, which reduce the application of these alloys under impact wear conditions. The purpose of this thesis is to investigate the effect of microstructure on the wear and toughness properties of AISI420 hardfaced martensitic stainless steel alloy. For this purpose, after deposition of the hardfaced alloy on a steel substrate, various heat treatments such as quench, quench temper, quench and partitioning and austenite reversion are designed to change the microstructure of the hardfaced alloy from fully... 

In this work, friction stir welding of aluminum to magnesium sheets using alumina and zinc powders was investigated. A preliminary investigation has been carried out on the joining of aluminum 5052 to AZ61 magnesium alloys and finding the optimum welding parameters. The optimum parameters were found to be with the shoulder diameter, pin diameter, and pin height of 14 mm, 5 mm, and 2.7 mm, respectively, with the travel speed of 60 mm/min and rotating speed of 500 rpm. As regards the main purpose of the using zin and alumina powders are controlling of Al-Mg IMCs formation and benefiting from pinning effect of grain boundaries, hence by adopting preliminary investigations and fixing pervious... 

Ferritic stainless steels, despite having a good combination of mechanical properties and corrosion properties and low price, are less used than austenitic stainless steels due to their low weldability. The main challenges of welding ferritic stainless steels are coarse grain formation in the molten area as well as grain growth in the heat affected zone. In this research, in the first phase, an attempt is made to change the shape and size of the grains in the melted zone by changing the GTAW fusion welding parameters, including welding current and welding speed. In the second phase, an attempt is made to prevent the formation of columnar grains by adding micron particles to the molten pool... 

The brazing process is the preferred method for bonding and repairing high temperature resistant superalloys. In this process, filler metals containing MPD elements such as B and Si are used. The presence of these elements causes the formation of brittle and eutectic phases such as borides and silicides in the joint area. These phases reduce the mechanical properties and re-melting temperature of the joint. By increasing the temperature and time of bonding and using the concept of isothermal solidification, the formation of boride and silicide phases can be prevented. In wide gap joints, the time to complete the isothermal solidification is very long, and applying such temperature and time...