Sharif Digital Repository

Metallurgical initial defects, stress concentration due to change in geometry and induced tensile residual stress during welding create an extraordinary high possibility of fatigue cracks nucleation and propagation in the weld toe region. Despite this, the ever increasing use of welding in offshore, aerospace and automotive industries, as the most applied mechanical joint, has occasioned a very extensive attempt to achieve an in-depth understanding of failure mechanisms and fatigue life improvement techniques of welded joints. Generally, these techniques could be classified into two broad categories. The first consists of methods that modify the geometry and the other is related to the... 

Superalloys are widely used in aerospace and power industries due to the good performance at high temperatures for examples gas turbines, steam turbines and aircraft components. In addition to the turbines cobalt-base superalloys are used in medical implants such as knee and hip joints. The main strengths of the superalloys are heat-resistant, retaining their high mechanical and chemical properties at high temperatures, high melting temperatures, high corrosion resistance, resistance to thermal fatigue, thermal shock, creep, and erosion. Because of difficult to machine properties of superalloys, grinding is one of the effective methods for machining superalloys. The surface integrity of the... 

In conventional grinding of hard to cut materials such as Ti6Al4V alloys, surface burning, redeposition and adhesion of chips to wheel and workpeice occur visibly unless it is carried out at low speeds and with high volume of cutting fluid. Ultrasonic assisted grinding (UAG) is an efficient machining process for hard-to-cut materials with poor machinability. UAG improves the machinability of such materials by changing the kinematics of the process. In this research, the effect of imposition of ultrasonic vibration on the grinding of Ti6Al4V is studied. Longitudinal vibration at ultrasonic frequency range (20 kHz) is applied on the workpiece and machining forces and surface roughness are... 

The issues relevant to lubricants in industry, construction and production processes such as machining and grinding have been recently taken into serious consideration. Unlike the positive effects of applying lubricants on machining forces, chip formation, thermal defects which lead to force reduction, lower power consumption and surface quality improvement, its conventional usage has adverse effects such as environmental problems, high cost of high volume lubricant application as well as recycling problems. Therefore, attention was drawn toward chip removal under dry or near dry conditions and as thermal defects such as surface burning is important particularly in grinding, grinding under... 

Fused silica is an important industrial ceramics with low machinability due to high brittleness, low hardness and high chemical reactivity. Traditional machining of this ceramic is very inefficient. Ultrasonic vibrations assisted grinding of this ceramic could create pieces with high accuracy and good surface quality. The main objective of this thesis is to investigative Ultrasonic vibrations assisted grinding of fused silica. Ultrasonic vibrations with a frequency of 20 kHz and amplitude in the range of 10-40 micron is applied to workpiece during grinding process. It is shown that in the ultrasonic assisted grinding, machining forces decreases by up to 45% and surface roughness reduces by...