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PWL#012 - Electric Arc, Filler Metal for EBW, Automatic Spotwelding and more... August 01, 2004 |
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| We hope you will find this Letter interesting and useful. Let us know what you think of it. Practical Issues, Creative Solutions
This publication brings to the readers practical answers to welding problems in an informal setting designed to be helpful and informative. We actively seek feedback to make it ever more useful and up to date. We encourage you to comment and to contribute your experience, if you think it may be useful to your fellow readers.
You are urged to pass-along this publication to your
Date: August 2004 - Practical Welding Letter - Issue No. 12
-----------------TABLE of CONTENTS---------------
1 - Introduction 2 - Article: The Electric Arc 3 - How to do it well: keeping Flux in Place 4 - Filler Metal for Electron Beam Welding 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article: Automatic Resistance Welding 8 - Site Updating 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Pictures, Graphics 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction While the number of subscribers continues to grow slowly and steadily, we feel there is no more point in keeping track of the exact total, as it seems that Practical Welding Letter is sufficiently affirmed to go on by itself. We would like to ask those readers who find this publication interesting and useful enough, to take a few minutes to inform just a few of their friends and associates of our Site, and to invite them to browse and to subscribe. Thanks. We present hereafter a short article on the basic properties of the Electric Arc which is so useful in welding. Understanding the properties should make it easier to change some of the parameters when needed to obtain better results. Do you know which simple setup may be used to provide flux bed backing for Submerged Arc Welding of groove joints, to eliminate the need of a backing strip while assuring complete root penetration? See how this is done well. Filler metal may be needed sometimes not just to fill up an empty space. This is the case with Electron Beam Welding, which, almost by definition, seems to be meant to dispose of additional material. As it happens it is not so. Quite a few metallurgical difficulties can be overcome by careful Filler material selection: it may be useful to have an idea of how to use this technique. Another article summarizes the features of automatic spot welding and of adaptive controls and monitoring sensors used to guarantee weld quality even at very high production rates. Site Update points to a new page on Weld Cracking. This is a most important subject as cracks tend to grow under stress and may endanger the stability of welded structures. The link is available further down. Other Departments are presented as usual, with more Terms and Definitions and with new Short Items to enlarge the scope of interest and to suggest new avenues to follow, for curiosity or for filling a need with new and creative solutions. In any case you are always welcome to let us have your feedback.
2 Article: The Electric Arc The Electric Arc is used in several welding processes due to its advantages as compared to alternative energy sources. The arc for welding is a continuous electrical current discharge occurring in a thermally ionized gaseous medium called a plasma column. It can be viewed as a high temperature gaseous conductor capable of transforming electrical energy into heat. The arc heat can be conveniently concentrated and controlled to deliver its energy precisely where required for fusion welding of metals. Plasma is the energetic state of matter composed or nearly equal proportions of negative electrons and positively charged gas ions. The plasma carries the arc current. Electrons flow from the cathode, the negative terminal, towards the anode, the positive terminal or pole. Positively charged ions (gas atoms stripped of some of their electrons) make their way (at much slower speed than electrons) in the opposite direction, from the anode towards the cathode. Mixed in the plasma are also molten metal, slags, neutral molecules of gases and vapors. The temperature of the plasma is very hot, estimated between 5000 and 50000 degrees Kelvin (or centigrades starting from Absolute Zero). The modes of energy delivery are partitioned among radiation, convection, conduction and diffusion. Heat is not constant neither in place nor in time, and depends obviously on the current level, on the type of gas, on the type of electrodes and on the presence of elements which influence the ionization potential. The voltage drop along the plasma column is not constant: near the terminals it depends also on their shape, material and weld parameters. Therefore the arc is complex and much variable. Most of the heat concentrates at the anode side. Therefore when using Gas Tungsten Arc Welding with direct current, electrode negative, the workpiece (anode) receives, for a given current level, the highest portion of heat ensuring maximum penetration of a narrow weld bead into the base metal. The heat required to form the weld pool is principally derived from the transfer of the kinetic energy of the electrons as they are absorbed into the surface. By switching polarity of the direct current (now electrode positive) the tungsten electrode would become the anode, sustaining most of the heat. In these conditions, preserving the same current level as before, the thin electrode that was used in the previous situation would be no more suitable to bear all the heat (it would readily melt). Therefore the same current as before would require changing the electrode to a larger diameter, better capable of standing the heat. The size change would modify the shape of the plasma column that would provide a larger bead of less penetrating weld. Using alternating current instead, would provide a somewhat intermediate penetration in the base metal, with the added benefit of disruption of surface oxides on the workpiece, during the electrode positive part of the cycle. This cathodic cleaning action is essential for welding aluminum and magnesium. In simple terms, current density may be thought as the current per unit area passing through a given electrode size. One should consider the influence of this important parameter on penetration, weld size and shape and on weld deposition rate. In general it is good practice to use the smallest electrode capable of carrying the required amperage.
3 - How to do it well: Keeping Flux in Place Q: How can one keep Submerged Arc Welding Flux in place underneath a joint? A: While performing Submerged Arc Welding of a groove joint from above, in flat position, backing flux is needed also at the underside. To keep it in contact with the surface and to avoid contamination, one can press the flux against the back of the joint by the use of an air inflated hose laying at the bottom of a metal channel full of flux, on top of which the joint is placed. By this method the underside of the root pass remains clean and sound so that little grinding is needed before depositing the next bead from the opposite side of the joint.
4 - Filler metal for Electron Beam Welding. In normal practice Electron Beam Welding is performed without Filler Metal because joint configuration is usually such that additional material is simply not needed. In some cases filler material is provided in the form of an extended edge, for at least one of the elements. In this article we wish to present the considerations that suggest how to deal with the exceptions. It happens sometimes that performing Electron Beam Welding in a properly designed and prepared joint results in cracking or unacceptable porosity for reasons due to the kind or condition of the base metal. This may occur if the metal is either hard and stiff or if it includes gases that are liberated and trapped in the weld. In these cases, by cautiously selecting a different material to be placed as a fine shim between the surfaces to be welded together, one can often improve the situation to the point of obtaining weld metal that is less brittle than would be obtained by welding the base metal without a filler. Heat treatable aluminum alloys like 6061 or 6063 may need a filler metal to avoid cracking while being Electron Beam welded. Other examples include the provision of a more ductile filler material like nickel or a nickel rich alloy, for welding tool steels or high alloy steels, or introducing an oxygen scavenging material like a titanium foil for absorbing the gas, should it be liberated from the base metal (i.e. rimmed steel) while welding. Also using a filler metal that contains a deoxidizer such as aluminum, manganese, or silicon may help to minimize porosity. Copper alloys that do not contain residual deoxidizers may produce porous welds if a suitable filler with deoxidizers or a nickel foil is not used. Another technique used sometimes to rebuild an edge, uses a filler wire supply device, of the type used for Gas Tungsten Arc Welding. Such a wire feeder should permit fine adjustment of attitude, proximity, timing and speed from the outside of the Electron Beam vacuum chamber. One of the applications most interesting economically is rebuilding of knife edges on labyrinth seals of gas turbine engines. As these are expensive items that need to be replaced when damaged or worn out, there is much incentive in repairing them for further use. The technology of these repairs, available for all the materials currently employed, has superseded previous methods which resulted more expensive and less reliable.
5 - Online Press: recent Welding related Articles Welding Root Beads in P91 Welding Miniature Devices In-running nip point hazards Articles on Welding from Encyclopaedia Britannica at New Ideas for Aluminum Parts
6 - Terms and Definitions Reminder Backgouging consists in removing the surface of a root weld and base metal from the opposite side of a partially welded joint to prepare clean and sound material for obtaining complete fusion and penetration of the subsequent weld from that side. Bond strength of a Thermal or Plasma spray layer, deposited on a suitable specimen, is determined by adhesive bonding its surface to a similar test piece, and then pulling the assembly in a tensile testing machine. A good bond should exceed the minimum specified (for material and process) and rupture in the thickness (not at the interface). Consumable insert is a specially shaped filler wire to be preplaced in a joint and later fused completely to provide the root pass. Current density is the current flowing to or from a unit area of an electrode surface, expressed by the number of Amperes per unit area. Fillet weld is that deposited in the corner formed by two surfaces at right angles. Flux is an auxiliary material used to remove oxides (in brazing) or to protect the surface being welded from contamination (in Submerged Arc Welding). Liquation is the partial melting of an alloy due to local compositional differences because of segregation or inclusions. Weld face is the open surface of a weld bead from the welding side.
7 - Article: Automatic Resistance Welding Resistance Welding Machines, and especially Spot Welders, are among those most commonly equipped for some form of automatic operation. This concept is an extension of the observation that a properly controlled and maintained spot welding machine does not need, to produce useful work, a skilled operator, his/her job being limited to the correct positioning of the elements to be welded between the electrodes and starting the cycle. If the size of the production run is such that an automatic positioner and initiator for a dedicated standard machine could economically do the job without employing human operators (except for supervision and for maintenance), then chances are that the expense for such a device could easily be recouped in a short time, opening the way to more economic production. Next step is the construction of dedicated machines, for high rate production of multiple spot welding, in high volume applications. These are built around some special fabricated item, like part of an automotive cabin or of an appliance. Such machines are integrated in lines designed to work without human assistance. This approach requires the rebuilding of the machine or of the tooling any time the design of the welded item is modified. A different approach involves the use of standard portable spot welding clamp machines, moved around by a standard robot whose movements can be easily reprogrammed as needed. This adaptive concept, possibly useful for smaller production lots, conserves the machines and the robotic positioners for multiple use, and permits economic change over to new parts and requirements by simple rewriting of software. Controls are much more important in automated processes than in manual operation, because human judgment can detect and correct faulty situations as they appear. Repetitive processes not sufficiently controlled risk to affect large number of defective items before supervision is alerted. Additional monitors can keep record of all the actual parameters having been used and of some useful quality data that may be employed to reduce the need to actually test the performance of the spot welds. Monitoring usually addresses one or more of the following parameters: current, power or electrical resistance, equipment status and condition, production output for signaling the need for electrode dressing. Microprocessor adaptive controls, when employed, not only present correct schedules for each weld or group of welds, but also provide adjustment of parameters in real time to ensure consistent spot weld quality. The use of special sensors can provide indication of long time trends in current change, indicating the need for some maintenance, if automatic correction does not prove sufficient. Ultrasonic transducers embedded into the electrodes can monitor weld size and soundness in real time. Alternatively acoustic emission sensors can provide weld quality signatures. Correct use of automation and robotics to resistance welding is successfully applicable to high production projects. Implementation will not only increase production rate and reduce unit cost but will keep and improve quality control while reducing the need for actual tests.
8 - Site Updating This month a new page on Weld Cracking was added to our Site, to provide information on the types of cracks that can develop during or after welding. It is important to remember that any kind of cracks can become dangerous as they have the tendency to propagate under the influence of stresses. To see the new page on Weld Cracking click here. We think that our Site Map should provide the whole picture for useful navigation. However, as a service to those readers who may need to inquire about other subjects, a new search window has been incorporated in some of the pages, to facilitate the satisfaction of their curiosity. We would appreciate knowing if you value and could use this facility and we will be glad to see your comments. Click here.
9 - Short Items 9.1 - Acoustic Emission Inspection consists in the analysis of sounds (stress waves) generated by the release of strain energy within a material as a consequence of structural transformations. It is performed by using sensors for detection and by analyzing frequency and intensity of emissions. During solidification following a welding process, phase transformations and shrinkage strains generate sounds that can be monitored. The formation of cracks, porosity and other defects is accompanied by typical acoustic emissions that can be analyzed in real time. Recorded emissions of questionable recordings can be compared with similar traces obtained with acceptable welds. In particular spot welds can be inspected in process to verify their quality. It was observed by experiment that the shear strength of a spot weld is proportional to the total count of emissions measured during the welding cycle. Continuous quality control can therefore be based on this inspection method. 9.2 - Electrogas Welding is a mechanized arc welding process similar to Gas Metal Arc Welding (see the page on Arc Welding in the Site, click here), in that it uses continuous electrode feeding, but similar to Electroslag welding (see article in PWL Issue 07 of March 2004, click here), in the weld pool containment using water cooled dams or shoes. It is used for thick section welding of joints in vertical position. Electrode wires can be solid, with gas shielding, or flux cored with or without auxiliary gas. The process has been used on steels, titanium and aluminum. 9.3 - Induction Brazing uses local heating to brazing temperature from eddy currents generated in the metal by electric energy supplied from a separate coil. Depending on the frequency of the primary current, the skin effect causes heating of only a surface layer close to the inductor. Flux and brazing filler metal must be preplaced, and the joints should be self-jigged. The process is quite rapid and economic for repetitive work. 9.4 - Investment Casting is a process that uses disposable molds produced by surrounding an expendable pattern (of wax or plastic) with a ceramic slurry that sets at room temperature. The pattern is removed by burning or melting and the mold is cured as required to stand the heat of the molten metal poured in it. This process (known from ancient times for artistic productions) has been brought to an elevated degree of maturity that permitted the fabrication of internal cavities into the castings, and the metallurgically important development of directionally solidified grains and even of single crystals for sizeable objects like high temperature blades for gas turbines. 9.5 - Thermite Welding, also called Aluminothermic Welding, is a process that exploits heat produced by exothermic reactions of blended mixtures of oxides with metallic reducing agents. A typical composition involves finely divided metal oxide and aluminum powder. The reaction produces metal and aluminum oxide, and delivers excess heat. It has to be started by ignition with an external heat source at elevated temperature. The elements to be welded, typically rail sections, are positioned with a gap. A refractory mold is put in place to contain the superheated molten metal generated in the process and made to flow into the gap. Both faces of base metal melt before solidifying with the added metal in a complete joint. 9.6 - Welders Qualification is a process intended to evaluate personal workmanship. A welder or welding operator is requested to demonstrate ability in producing joints that meet prescribed standards. The results are summarized in a formal document, a Certification, issued by a central recognized Authority. The specific details used during qualification establish the range of applicability of the given certification. A new one is to be secured to perform actual work with materials or parameters not covered before. The Specifications or Codes governing the actual structures to be welded,
or the Purchase Order, prescribe by which other document Welders Qualification
must be conducted. A popular such document used to be Foreign similar documents are: Other standards are available as requested by applicable Codes. It is the responsibility of the Contractor to make sure that all welders employed in any given job be holders of valid certificates covering the required materials, processes, positions etc.
10 - Explorations: beyond the Welder An offer for a limited time at Nature, wildlife, travel, adventure and sports photography On Powder Finishing see The sky is the limit How can plastics become conductive?
11 - Contributions: Pictures, Graphics 11.1 - Two readers were so kind to draw my attention to what in their opinion is a serious deficiency both of the Site and of this Practical Welding Letter: it is about the lack of pictures and of graphics to illustrate aspects of the subjects dealt with in the text. I could not agree more than I do on this observation. Unfortunately I am afraid it would entail so much work that I cannot gauge. It would also take more time to load on the readers screens. The real question is how much more useful to the readers would be these publications if enriched with sketches and photos. What do you think? If I perceive a real request I will not have other choice than to comply. Please let me know by e-mail. Click here. 11.2 - Another reader asked if we could provide a buyers guide on auto darkening welding helmets (which we cannot at this time). It could be a good idea if there is a need for such a thing. Are there other subjects you would like to have dealt with? Let us know.
12 - Testimonials From: "Isaac Wong" 'but_if_not@msn.com' Thank you very much for your speedy reply. I would also like to commend your efforts in making such an appealing and informative site. Isaac ---------------- From: "Mark Wigley" 'wigley_mark@hotmail.com' Hi Elia, The support idea sounds awesome and I think it'll "do the trick" as they say! Will keep you informed of my progress. Thanks heaps!! Keep Cool!! Mark
13 - Correspondence: a few Comments 13.1 - I really enjoy answering the questions sent in by readers, and I usually do my best in order to provide useful answers. More often than not however the enquirers provide only the most scant information on the application they deal with. I feel that I cannot come up with useful help without knowing more. Please be more complete in your expositions. 13.2 - The other frustration comes from the fact that most of times there is no followup. I understand that the problem may either be solved or abandoned. It would be exciting to know that my suggestions really helped. Please let me know! 13.3 - I am informed sometimes by our readers that they have just experimented some different approach of technique and ask what I think of it. Without endorsing anything, I would like to encourage all experiments, except for one proviso: that due attention be paid to safety. Please be cautious, do not take unnecessary risks. In case of doubt ask. 13.4 - Nervous subscribers often inquire how they should download the Hardness Book. We send it as an attachment by e-mail, generally with a delay of not more than three days. Only those not receiving it in a few days should let us know, for sending it again. All other readers should wait and relax.
14 - Bulletin Board 14.1 - I will be out of town (hopefully) for a short vacation in the second half of this month. Please be advised that the questions I will find on my desk at my return will be answered with a certain delay. 14.2 - Also the Hardness Book for new subscribers may suffer some delay in being delivered. Please be patient and you will get it. 14.3 - In case anyone of you is wondering if you could use the Internet for more than for finding some information, let me recommend our Site Host, SiteSell.com, for the complete and helpful service they offer. I know them for quite some time: I could not have built my Site without them. As I use daily their facilities and help, I am convinced that they are among the best ones that can be found in the Internet. I believe that exploring their Site is an enriching experience. You can see for yourself, and then come up with some new ideas to investigate further and possibly to try. You can also dowload a lot of information to study at your leisure. Therefore I would like to urge you to follow the following links. And if you have then any questions that I might be able to answer concerning the subjects treated in the SiteSell.com Site, please do not hesitate to ask me by e-mail. Click here. Good Luck. See you next time. Copyright (c) 2004, by Elia E. Levi and welding-advisers.com, all rights reserved
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