Prof. Olshanskii suggested that I should examine what forces were responsible for driving the molten metal from the front of the pool to the back of the pool where the metal solidifies.
To visualise the flow of metal during welding I did some ingenious experiments. In a stainless steel plate of 10mm thick a hole of 2mm dia was drilled. Wires of copper,titanium,tantalum etc were embedded tightly in to these holes.A full penetration Electron beam weld passed over these metals embedded in the stainless steel.The welds were sectioned along the axis of the weld.The specimens were polished and etched to reveal the distribution of these tracer metals. The tracer metals had moved over a distance 6 to 8 mm in a direction opposite to the direction of welding. The concentration of the tracer metals along the depth of the weld and along the length of weld was measured using an electron micro probe analyzer.The maximum concentration was found at the top of the weld.
We were at a loss to explain how the metal could move such long distances considering that the dimensions of the electron beam welds are generally small.We searched entire literature to see whether any body had come across such a phenomenon. We could not find any thing similar to us.We realised that we have stumbled upon a new phenomenon. Finally my professor decided to consult one of the top class Physics Professor who was working in the same institute, that is, Moscow Power Engineering Institute. My professor took me to the Physics Professor and made me explain my experiments and also show him the results obtained till now. The Physics professor thought for some time and told me that I should look for "Thermocapillary Effect" and suggested that I should read a book on physics which was in the library. The meeting ended and we came out. I was disappointed with the meeting and I told my professor so. Then he told me something which I have not forgotten and which is helping me even now. He told me " I talked to several professors and all of them suggested that he is the right person to guide us. You may take a long time to find out the right person to consult. Once you have found him follow his advice implicitly. You jolly well go to library and read that book" I went to the library and read the book suggested by the Physics Professor. Indeed the back word movement of the metal could be explained by thermocapillary effect which will be the subject of next blog.
Tuesday, May 18, 2010
Thursday, May 6, 2010
Dimensional Analysis applied to predict depth of penetration
The second portion of my thesis was to find an equation for predicting depth of penetration for a given welding parameter for a given material.
The formation of weld bead in EBW is controlled by many factors and to predict bead formation by rigorous mathematical analysis was not possible.While width of weld in EBW was not a significant parameter, depth of penetration is. Many research papers published at that time depended on fitting equations to the experimental data. These curves are applicable in limited field. To get a generalaised equation covering all metals and all welding parameter needed a different approach.
I had some experience in Dimensional Analysis and thought of applying this technique to predict depth of penetration.
The various factors that control depth of penetration are: beam current, beam voltage,beam diameter, welding speed and material properties such as thermal conductivity,melting point,specific heat,and latent heat of fusion. An advanced method of Dimensional Analysis was applied to solve the resulting equations. Three non-dimensional parameters were obtained. The first parameter related to the ratio of heat lost by conduction to the beam power, the second, the heat required to melt a given volume to the beam power and the last was the heat required to heat a given volume to the melting temperature to the latent heat of fusion. Plotting these parameters one against the other we got graphs which predicted the depth of penetration.The theoretical depth of penetration closely followed our Experimental results and the data obtained from other workers.These equations can used to predict depth of penetration in EBW for any metal at any given voltage,current and speed.
This was the first time any one had applied dimensional analysis to solve a welding problem.
The formation of weld bead in EBW is controlled by many factors and to predict bead formation by rigorous mathematical analysis was not possible.While width of weld in EBW was not a significant parameter, depth of penetration is. Many research papers published at that time depended on fitting equations to the experimental data. These curves are applicable in limited field. To get a generalaised equation covering all metals and all welding parameter needed a different approach.
I had some experience in Dimensional Analysis and thought of applying this technique to predict depth of penetration.
The various factors that control depth of penetration are: beam current, beam voltage,beam diameter, welding speed and material properties such as thermal conductivity,melting point,specific heat,and latent heat of fusion. An advanced method of Dimensional Analysis was applied to solve the resulting equations. Three non-dimensional parameters were obtained. The first parameter related to the ratio of heat lost by conduction to the beam power, the second, the heat required to melt a given volume to the beam power and the last was the heat required to heat a given volume to the melting temperature to the latent heat of fusion. Plotting these parameters one against the other we got graphs which predicted the depth of penetration.The theoretical depth of penetration closely followed our Experimental results and the data obtained from other workers.These equations can used to predict depth of penetration in EBW for any metal at any given voltage,current and speed.
This was the first time any one had applied dimensional analysis to solve a welding problem.
Tuesday, May 4, 2010
Ph.D programme
After passing the Russian language exam which was conducted six months after joining the course, I was allowed to join the Department of Technology of Metals for my Ph.D programme. Taking in to account my background Prof.Olshanskii suggested that I study the temperature distribution during Electron Beam Welding and correlate the metallurgical changes that take place in the metal with measured temperature distribution. In addition he suggested that I should develop a method of predicting depth of penetration for a given welding parameter.
At first instance the problem appeared to be simple. But experimentally determining the temperature distribution had its own problems. Platinum-Platinum Rhodium thermocouples were used for measuring the temperature. The Heat affected Zone in Electron Beam welds are extremely narrow and to locate the thermocouples precisely in that zone was very tricky. Initial experiments were done to measure the width of the heat affected zone for various weld parameters. The welding chamber was equipped with a precision table which can move in the X and Y direction. The table was adjusted such that the electron beam passed through the centre of the table in the Y-direction. A jig was prepared such that the electron beam passes through the centre of the jig. The jig was bolted to the traverse table. Test specimens were machined such the they fit snugly in to the jig. Holes were drilled in the test plate and thermocouples were embedded to measure the temperature in the heat affected zone.The location of thermocouples was decided by the earlier experiments done to measure the width of the heat affected zone. The thermocouples were taken out of the welding chamber and connected to a high speed strip chart recorder. Welding was started and the temperatures were recorded. These experiments look simple but it was very difficult ensure that the electron beam traverses precisely where you want. Slightest changes in parameters or alignment of the test plate will result in the electron beam going over the thermocouples or going far from the thermocouples. The recorded temperature distribution was compared with the theoretical temperature distribution considering the heat transfer is two dimensional, The specimens were sectioned and were subjected to metallurgical tests.Several test specimens were welded to establish the repeatability. The first portion of the assignment was finished and the next was to find a method of predicting depth of penetration for a given welding parameter.
At first instance the problem appeared to be simple. But experimentally determining the temperature distribution had its own problems. Platinum-Platinum Rhodium thermocouples were used for measuring the temperature. The Heat affected Zone in Electron Beam welds are extremely narrow and to locate the thermocouples precisely in that zone was very tricky. Initial experiments were done to measure the width of the heat affected zone for various weld parameters. The welding chamber was equipped with a precision table which can move in the X and Y direction. The table was adjusted such that the electron beam passed through the centre of the table in the Y-direction. A jig was prepared such that the electron beam passes through the centre of the jig. The jig was bolted to the traverse table. Test specimens were machined such the they fit snugly in to the jig. Holes were drilled in the test plate and thermocouples were embedded to measure the temperature in the heat affected zone.The location of thermocouples was decided by the earlier experiments done to measure the width of the heat affected zone. The thermocouples were taken out of the welding chamber and connected to a high speed strip chart recorder. Welding was started and the temperatures were recorded. These experiments look simple but it was very difficult ensure that the electron beam traverses precisely where you want. Slightest changes in parameters or alignment of the test plate will result in the electron beam going over the thermocouples or going far from the thermocouples. The recorded temperature distribution was compared with the theoretical temperature distribution considering the heat transfer is two dimensional, The specimens were sectioned and were subjected to metallurgical tests.Several test specimens were welded to establish the repeatability. The first portion of the assignment was finished and the next was to find a method of predicting depth of penetration for a given welding parameter.
Sunday, May 2, 2010
Precautions while working on high voltage equipment
Electron Beam Welding(EBW) machines generally operate on accelerating voltages of 60Kv to 150Kv. There is rule in Russia that those who work on such machines must pass an examination. Classes were conducted for all students, teaching us what precautions must be taken to handle the high voltage equipment. We were taught how to check glows and ensure that we use the tools and tackles rated for repairing high voltage equipment.While repairing any electrical system we should switch off the mains and remove the fuses and keep it with you and not leave them at the fuse board. Otherwise some body may see the fuse board open and fuses lying unplugged and may insert the plugs leading to disaster. There were many DO's and DONT's which we had to remember and at the end of the course we had a written exam and viva. I failed the exam even though I thought I did well. I failed for the second time and I went and complained to my Professor. He smiled and said that he had asked the examiner to be tough on me because if I am not thorough and if any accident occurs and I am injured he will end up in Siberia because he did not take good care of a Foreign student! Finally I passed the exam and then i was allowed to operate the high voltage EBW. I am writing this because the training we got in Russia was thorough in all respects.We had the disadvantage of learning every thing in Russian. But at the end I was happy that I came to Russia for my higher studies because I learned a new language and worked under an eminent professor.
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