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.
Thursday, April 29, 2010
How to repair diffusion pump
We were four Ph.D students and we had one Electron beam welding (EBW). Hence we had to book the machine in advance.In case the machine goes out of order when you are working it is your responsibility to put the the machine back in to working. You cant walk away if the machine is down.
Vacuum in the welding chamber is created by one mechanical pump and one diffusion pump. First the mechanical is switched on and after the desired vacuum is achieved in the chamber, the mechanical pump is isolated by closing a valve and then a valve is opened to connect the diffusion pump. If by chance the diffusion pump is brought in to operation with out isolating the mechanical pump, air will rush in to the diffusion and the oil in the diffusion pump will burn and the diffusion pump stops operation. Then you have to clean the diffusion pump and put it back in to operation.
One day while working in the machine I opened the wrong valve and the diffusion pump conked off. I shut off the machine and was waiting for some worker to come and dismantle the pump and repair it.Prof. Olshanskii happened to come to the laboratory and saw me not working. I told him that the diffusion pump is not working. "Repair it". I told him I don't know how to repair it. He shouted"Then learn it". He called the foreman of laboratory and asked him to give me a set of tools. He then told me to dismantle the pump and see what has gone wrong.He saw me struggling to open the heavy bolts he provided me a hefty worker,Kolya, to dismantle the pump. When the pump was dismantled I found that the internal shrouds in the pump was coated with a thick layer of burnt oil. The shrouds were made of aluminium and cleaning process involved using emery papers to remove the muck accumulated on the shrouds. It is slow and back breaking.Here Kolya helped me. After cleaning every shroud, they have to be degreased for which industrial alcohol was used. By the time cleaning was over it was late in the night and Kolya agreed to fix the diffusion pump in position. I left for my hostel. Next morning I was hauled up before the Foreman of the laboratory because Kolya drank the rest of the alcohols and had passed out in the laboratory. It seems that it was my responsibility to have used the entire alcohol for cleaning and not leave with the worker! This experience gave me confidence that I can repair any diffusion pump.
Vacuum in the welding chamber is created by one mechanical pump and one diffusion pump. First the mechanical is switched on and after the desired vacuum is achieved in the chamber, the mechanical pump is isolated by closing a valve and then a valve is opened to connect the diffusion pump. If by chance the diffusion pump is brought in to operation with out isolating the mechanical pump, air will rush in to the diffusion and the oil in the diffusion pump will burn and the diffusion pump stops operation. Then you have to clean the diffusion pump and put it back in to operation.
One day while working in the machine I opened the wrong valve and the diffusion pump conked off. I shut off the machine and was waiting for some worker to come and dismantle the pump and repair it.Prof. Olshanskii happened to come to the laboratory and saw me not working. I told him that the diffusion pump is not working. "Repair it". I told him I don't know how to repair it. He shouted"Then learn it". He called the foreman of laboratory and asked him to give me a set of tools. He then told me to dismantle the pump and see what has gone wrong.He saw me struggling to open the heavy bolts he provided me a hefty worker,Kolya, to dismantle the pump. When the pump was dismantled I found that the internal shrouds in the pump was coated with a thick layer of burnt oil. The shrouds were made of aluminium and cleaning process involved using emery papers to remove the muck accumulated on the shrouds. It is slow and back breaking.Here Kolya helped me. After cleaning every shroud, they have to be degreased for which industrial alcohol was used. By the time cleaning was over it was late in the night and Kolya agreed to fix the diffusion pump in position. I left for my hostel. Next morning I was hauled up before the Foreman of the laboratory because Kolya drank the rest of the alcohols and had passed out in the laboratory. It seems that it was my responsibility to have used the entire alcohol for cleaning and not leave with the worker! This experience gave me confidence that I can repair any diffusion pump.
Monday, March 29, 2010
How I learnt to operate an Electron Beam Welding Mchine
I am not going to write about what is an electron beam welding machine about which anyone can read in a standard welding text book. I am going to write how students were trained in Moscow Power Institute.
There were two electron beam welding (EBW) machines in the Department of Technology of metals where I joined for my Ph.D program under Prof. N.A.Olshanskii. One was operating at high voltage and the other was operating at a low voltage. The one operating at a high voltage was built by Prof. Olshanskii and the other was a factory built machine. New students were asked to start with the machine built by Prof.Olshanskii and after mastering the principles of operation of EBW they were allowed to go on the factory built machine. There was a reason behind this exercise. The machine with which we started to learn operation of EBW used to go out of order often as it was very old. The cathode which was a filament made of
tungsten used to burn up fast either due to our wrong handling of the machine or due to its life being over. When that happened we were supposed to take out the electron gun and replace the tungsten filament. During this process we learnt how the electron gun looks, how to make the tungsten filament, how to mount the filament and how to align the filament properly etc. All EBW machines are connected to a fore vacuum pump which is a mechanical pump and an oil diffusion pump. Whenever these pumps went out of order we were supposed to strip them open, rectify the mistakes and put them back in to operation. These exercises gave the new entrants enough confidence to handle any EBW machine. When you made mistake no one asked any questions but you were only required to set the mistake alright.This was totally different from the approach of Indian institutions.After my quota of mistakes, when Prof. had the confidence that I can handle factory built machine he allowed me to go on that machine.
There is a particular procedure for opening and closing of various valves to create vacuum. In today's machines everything is automated. In 1968 all these operation were manual. Fist you have to switch on the electric heater to heat the oil in the diffusion pump and after some time you switch on the fore vacuum pump and when the fore vacuum has reached a predetermined level you close the valves of fore vacuum pump and open the valves connected to the diffusion pump. If valves connected to the diffusion pump is opened when the valves connected to the fore vacuum pump is not closed air enters diffusion pump and oil burns and vacuum is lost.Then you have to switch off the machine, clean up the mess and put the machine back in to operation.
I will write in next blog what happened when I burnt the oil in diffusion pump.
There were two electron beam welding (EBW) machines in the Department of Technology of metals where I joined for my Ph.D program under Prof. N.A.Olshanskii. One was operating at high voltage and the other was operating at a low voltage. The one operating at a high voltage was built by Prof. Olshanskii and the other was a factory built machine. New students were asked to start with the machine built by Prof.Olshanskii and after mastering the principles of operation of EBW they were allowed to go on the factory built machine. There was a reason behind this exercise. The machine with which we started to learn operation of EBW used to go out of order often as it was very old. The cathode which was a filament made of
tungsten used to burn up fast either due to our wrong handling of the machine or due to its life being over. When that happened we were supposed to take out the electron gun and replace the tungsten filament. During this process we learnt how the electron gun looks, how to make the tungsten filament, how to mount the filament and how to align the filament properly etc. All EBW machines are connected to a fore vacuum pump which is a mechanical pump and an oil diffusion pump. Whenever these pumps went out of order we were supposed to strip them open, rectify the mistakes and put them back in to operation. These exercises gave the new entrants enough confidence to handle any EBW machine. When you made mistake no one asked any questions but you were only required to set the mistake alright.This was totally different from the approach of Indian institutions.After my quota of mistakes, when Prof. had the confidence that I can handle factory built machine he allowed me to go on that machine.
There is a particular procedure for opening and closing of various valves to create vacuum. In today's machines everything is automated. In 1968 all these operation were manual. Fist you have to switch on the electric heater to heat the oil in the diffusion pump and after some time you switch on the fore vacuum pump and when the fore vacuum has reached a predetermined level you close the valves of fore vacuum pump and open the valves connected to the diffusion pump. If valves connected to the diffusion pump is opened when the valves connected to the fore vacuum pump is not closed air enters diffusion pump and oil burns and vacuum is lost.Then you have to switch off the machine, clean up the mess and put the machine back in to operation.
I will write in next blog what happened when I burnt the oil in diffusion pump.
Sunday, January 3, 2010
Moscow Power Engineering Institute
I was selected by Government of India in 1968 on a USSR scholarship to do my Ph.D in Moscow. After reaching Moscow I was admitted to Moscow Power Engineering Institute.
Moscow Power Engineering Institute was set up in in 1930 to solve technological problems associated with power generation, distribution, control etc. The Institute was so huge that in 1970's the student strength was around 24000. The institute was training students from undergraduate level to post graduate level including Ph.D's and D.Sc's. There are 70 Departments in MPEI, 550 laboratories for students and more than 100 research laboratories. More than 500 Ph.D students out of which 100 foreign students are studying at any given moment. The faculty consists of over 1700 professors, assistants professors and lecturers. Many of the professors are Academicians.Selection to Undergraduate programme was similar to our IIT entrance exams. Very tough and students from all over USSR wrote the exams. If the students are not selected they used to cry their heart out. The admission to Postgraduation depended upon the performance in the Undergraduate programme and the the desire of the Professor to accept a Graduate student. All Postgraduate student got a scholarship of old 200 Roubles. In addition students are allowed to coach undergraduate students for which they were paid another 100 Roubles per month.
My Professor was Alexander Nikiolaevich Olshanskii.
All welding processes are based on heating the metal to be joined to its melting temperature and prevent the molten metal from contamination by oxygen and nitrogen in the atmosphere. For example, in manual metal arc welding, heat is generated by striking an arc between the base metal and the stick electrode. To prevent contamination the stick electrode is coated with chemicals which produce enough gas such as carbon di oxide, carbonmonoxide and other reducing gases. In addition a slag is deposited to blanket the molten metal to prevent contamination from atmospheric air. Some times alloying elements are added in the coating to improve the mechanical properties of the weld metal. Similarly in TIG welding and MIG welding Argon is used to shield the molten metal. The whole idea of using the gas is to displace the Air in the vicinity of weld. Thus whatever may be the heat source it is essential to ensure that weld metal is not contaminated by the atmospheric air.
There is another method of preventing contaminatin of the weld metal-by creating vacuum around the place where welding is going on.When you evacuate the place where welding is going on you have eliminated all gases from the vicinity of the weld. The part to be welded is kept in a chamber which is evacuated and welding is done as usual. This was in 1960's.When Prof. Olshanskii proposed this new process his peers laughed at him and asked him whether he plans to go to space to carry out welding. But undaunted he worked out the process of welding in vacuum for which he got his Ds.c. He had the last laugh when soviet astronauts did weld in space and brought back welded specimens for evaluation.
After successfully establishing welding in vacuum he conceived a new process of Electron Beam Welding about which I will write in the next blog.
Moscow Power Engineering Institute was set up in in 1930 to solve technological problems associated with power generation, distribution, control etc. The Institute was so huge that in 1970's the student strength was around 24000. The institute was training students from undergraduate level to post graduate level including Ph.D's and D.Sc's. There are 70 Departments in MPEI, 550 laboratories for students and more than 100 research laboratories. More than 500 Ph.D students out of which 100 foreign students are studying at any given moment. The faculty consists of over 1700 professors, assistants professors and lecturers. Many of the professors are Academicians.Selection to Undergraduate programme was similar to our IIT entrance exams. Very tough and students from all over USSR wrote the exams. If the students are not selected they used to cry their heart out. The admission to Postgraduation depended upon the performance in the Undergraduate programme and the the desire of the Professor to accept a Graduate student. All Postgraduate student got a scholarship of old 200 Roubles. In addition students are allowed to coach undergraduate students for which they were paid another 100 Roubles per month.
My Professor was Alexander Nikiolaevich Olshanskii.
All welding processes are based on heating the metal to be joined to its melting temperature and prevent the molten metal from contamination by oxygen and nitrogen in the atmosphere. For example, in manual metal arc welding, heat is generated by striking an arc between the base metal and the stick electrode. To prevent contamination the stick electrode is coated with chemicals which produce enough gas such as carbon di oxide, carbonmonoxide and other reducing gases. In addition a slag is deposited to blanket the molten metal to prevent contamination from atmospheric air. Some times alloying elements are added in the coating to improve the mechanical properties of the weld metal. Similarly in TIG welding and MIG welding Argon is used to shield the molten metal. The whole idea of using the gas is to displace the Air in the vicinity of weld. Thus whatever may be the heat source it is essential to ensure that weld metal is not contaminated by the atmospheric air.
There is another method of preventing contaminatin of the weld metal-by creating vacuum around the place where welding is going on.When you evacuate the place where welding is going on you have eliminated all gases from the vicinity of the weld. The part to be welded is kept in a chamber which is evacuated and welding is done as usual. This was in 1960's.When Prof. Olshanskii proposed this new process his peers laughed at him and asked him whether he plans to go to space to carry out welding. But undaunted he worked out the process of welding in vacuum for which he got his Ds.c. He had the last laugh when soviet astronauts did weld in space and brought back welded specimens for evaluation.
After successfully establishing welding in vacuum he conceived a new process of Electron Beam Welding about which I will write in the next blog.
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