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PWL#039 - HT Welded Aluminum, Out of Pos. Welding, F/M for Repair Alum Alloy Castings, Hyperbaric W. October 30, 2006 |
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| We hope you will find this Letter interesting and useful. Let us know what you think of it. Heat Treating Welded Aluminum, Out of Position Welding, Filler Metal for Repairing Aluminum Alloy Castings, Progress in Dry Hyperbaric Welding, Interview with David J. Keats, Upset welding and more... 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 friends, if you like it, and if you want to help them. If you received this from a friend and if you like what you read, please subscribe free of charge and you will also receive a bonus book on Practical HARDNESS TESTING Made Simple.
1 - Introduction 2 - Article: Heat Treating welded Aluminum 3 - How to do it well: Out of Position Welding 4 - Filler Metal for Repairing Aluminum Alloy Castings 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article: Progress in Dry Hyperbaric Welding 8 - Site Updating: Upset Welding 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Interview with David J. Keats 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction This 39th Issue of Practical Welding Letter for November 2006 opens with an article on Heat Treating Welded Aluminum. We got a hint that the subject is in need of clarification from a question in a Forum showing how far was the author from understanding the problem. A question from a reader on all position electrodes required an answer that we make here available to all. Filler metal for weld repair of aluminum alloy castings is probably a useful reference just in case one needs to do it. Dry Hyperbaric Welding for performing deep under sea work on pipelines, under control from a central station, is a hot subject of development, because of the limits on the depth reachable by welders-divers. We report briefly on important progress and applications. We are going to present in this and in future issues of PWL, direct interviews with welding experts from all over the world. The first one is with David Keats, whose name and activities are probably known to some of our readers. We think you should find interesting points of view reported here. This month Update of our Website deals with a new page on Upset Welding, a technology applied successfully in certain specific cases of end-to-end joining. Other departments show up at their regular places. We hope you will enjoy this issue. Please let us know what you think of it.
2 - Article: Heat Treating Welded Aluminum To anyone familiar with welding and heat treating of steel parts, it may seem that welding and heat treating aluminum alloys can be performed straight away without any problems. Unfortunately it is not so easily done. Let us remind briefly that aluminum alloys are divided into those hardenable by a complex heat treatment and those not hardenable at all by any heat involving methods. The last ones as a group can only be hardened by mechanical deformation in a process called strain- or work-hardening, indicated by a -H temper symbol convention. With reference to welding, the down side of this group of alloys is that any welding, or more exactly any heating, will soften the strain hardened material, bringing the mechanical properties down to the levels of the annealed condition . The complex heat treatment of the aluminum alloys responsive to it, takes advantage of the fact that certain alloying elements have high solubility at elevated temperature, while at room temperature their solubility falls to very low values. This type of treatment is indicated by the -T temper symbol. Heat treatment of responsive aluminum alloys is performed in two stages. In the first one, some of the alloying elements are put in a nearly homogeneous solid solution by exposure to a sufficiently high temperature for the required time. By rapidly quenching, fast enough to retain the solute in solution, one achieves at room temperature a supersaturated solid solution. From this metastable conditions one can reach the highest mechanical properties of the alloy through controlled precipitation of hardening phases. The actual properties will depend on the size and distribution of the precipitated particles, which are established by the precipitation temperature. As if these conditions were not complex enough, one has to pay attention to three most important additional points. First: while solutioning must be as complete as possible, at the highest possible temperature, to achieve optimum properties, one must avoid at any cost to reach the eutectic melting temperature, which is only a few degrees higher than the solutioning temperature. Otherwise an unrecoverable damage is done, as grain boundary melting occurs, whereby the alloy becomes brittle. Second: quenching from the solutioning temperature in cold water must be very rapid, of the order of a few seconds at most. If quenching is delayed, the type of precipitations occurring at high temperature is detrimental to mechanical and corrosion resistance properties. Third: the transition from high heat to room temperature may cause warping and deformation, especially if the parts present significant thickness changes. Some straightening may be done in this soft condition. The material being in a state of residual stresses, machining may cause additional deformation because of uneven cutting through stressed material. The above exposition should convince anyone that if heat treatment is required on aluminum parts, it should be done by experienced personnel with suitable equipment, like it may be found in specialized shops. Repair or modification of a machined casting or addition by welding of a wrought element, with the program of subsequent full heat treatment cycle of solutioning and precipitation may be a questionable proposition, in that it may be exceedingly difficult to maintain shape and dimensions as required and to develop the sought mechanical properties. Furthermore if two different aluminum alloys are welded together, the heat treatment to be applied to the weldment must be compatible with both alloys, and possibly some compromise must be accepted as to the properties obtained. In certain cases one should be ready to explore new approaches involving mechanical joints instead of welding or other solutions requiring different materials. Please let us have your comments on this article, click on Contact Us.
3 - How to do it well: Out of Position Welding Q: What makes an all position welding rod/wire, 'all position'? What is the difference between this type of rod and the others for flat or horizontal only? A: Covered electrodes (SMAW) for flat and horizontal positions are optimized for maximum weld deposition rate. As such they provide a large but still manageable weld pool that does not run out. Out of position electrodes, useful for vertical or overhead welds, are made with modified shielding cover, designed to control the viscosity of the molten metal to make it more sluggish and capable of adhering to the surface even in overhead position, instead of dripping down immediately. The electrode cover for these electrodes contains elements that affect the wetting of the base metal and the viscosity of the molten weld metal. Viscosity can be controlled also, within limits, by mastering the technique and by adjusting the current. Adequate skill is essential to obtain good and repeatable results. Note that vertical and overhead joints are welded with smaller diameter electrodes than would be used in flat position for the same thickness, with corresponding lower deposition rates. It should be noted that the welding position is an important variable that can affect the weld metal quality. For this reason one should select, whenever possible, the flat position.
4 - Filler Metal for repairing Aluminum Alloy Castings Rejected castings in need of correcting surface and dimensional defects are usually salvaged by welding. Inert gas shielded arc welding processes are normally used. The techniques for repairing castings are similar to those applied for welding wrought products. Castings presenting thick surface aluminum oxide film or substantial voids must be treated with special provisions. Welding quality may be poor in porous castings and in regular die castings. (Vacuum die castings have better properties.) Gas Tungsten Arc Welding (GTAW) is normally used for limited areas. Pneumatic chipping or hand tool milling without lubricants are used for removing defects. The area prepared for welding should be smooth and clean, with gradual chamfer to promote sound fusion. Cleaning and degreasing should follow regular practice. Hand brushing with a clean stainless steel brush should be done immediately before welding to remove aluminum oxide. The largest size filler rod is used that will not interfere with the process. Heat treatable alloy castings are often repaired or welded with a filler rod of the same composition as the base metal. If needed, filler material can be cast in rod-shaped molds in the same casting facility. In this case post weld heat treatment will develop the same properties in the weld as in the casting. In most cases however, the filler metal is not heat treatable or only mildly heat treatable. This selection is adopted for ease of welding, if hot short weld cracking is feared. However, to obtain at least some response to post weld heat treatment, one should strive to keep weld composition around a ratio of 70% filler alloy to 30% base alloy. Different filler metals may be adequate for a given application but only one may give the optimal response to specific performance requirements. The following factors must be considered:
The most popular filler metal alloys for casting repair are usually selected from the following list: 2319, Preheating of the casting is not necessary before making a weld repair except in case it is required to reduce the size of gas porosity in the weld. However preheating to 150 to 205 0C (300 to 400 0F) may be necessary to prevent cracking or distortion. Castings that require heat treatment should be heat treated after welding. Welding heat treated castings will reduce locally the tensile properties. Postweld heat treatment, even if limited to aging alone, will restore some of their properties if the proper filler alloy has been used in welding. A most complete list of properties of Aluminum Casting Alloys can be found in the ASM International Metals Handbook Desk Edition, available online to Members. Composition and properties of the filler materials listed above (among others) appear in the ASM Handbook Volume 6 - Welding and Brazing.
Note: an example of repair of a Machined Aluminum Alloy Casting by Electron Beam Welding was presented in Section 3 of Issue 10 of Practical Welding Letter for June 2004. As we inquire on usefulness and interest of this publication for our readers, we would ask you to please let us have your comments on the presentation above by clicking on Contact Us.
5 - Online Press: recent Welding related Articles Our article on Promising Welding Innovations A kind reader sent us the following link: Skilled worker shortage: New program seeks to provide training Fanuc Robotics America starts 'Save Your Factory' effort Further to our new website page (see Section 8) on Upset Welding see:
6 - Terms and Definitions Reminder Arc is a bright discharge of electrical energy bridging the gap between two electrodes. Columnar Structure is one of parallel, elongated grains formed by unidirectional growth in the direction of a temperature gradient existing at the time of solidification, observed in castings and in welded structures. Dip Brazing is a process that provides heat from molten flux or filler metal kept in a suitable tank where items to be brazed are briefly immersed. Downhand Position is a nonstandard term used to describe the flat position where the weld is made from the upper side of a joint kept approximately horizontal. Out-of-position welding indicates a position other than flat or horizontal, i.e. vertical, up or down, and overhead. Shrinkage is the contraction of dimensions following solidification and cooling. Soaking in thermal treatments is the time allowed at a given temperature for equalization and completion of structural transformations. Upset Welding is a resistance welding process where coalescence is produced over the whole area of the faying surfaces, or progressively along a butt joint, while heating is provided by resistance to the flow of electrical energy at the interface of the areas to be welded together, kept in contact under pressure. The process is solid state, without fusion. See Upset Welding. 7 - Article: Progress in Dry Hyperbaric Welding Two recent articles published in the Welding Journal relate on developments in Dry Hyperbaric Welding conducted for construction and maintenance of pipelines in the oil fields of the North Sea. The first article, in the June 2006 issue at page 52, describes the ongoing work, started decades ago. A remotely controlled mechanized complete system for hyperbaric welding of subsea pipelines was initiated in 1984 by Norsk Hydro. The process selected was all position narrow groove GTAW with wire feeder for cold filler welding wire addition. The equipment was assembled and tested in the welding pressure chamber of the Hyperbaric Welding Laboratory. The welding parameters are controlled from the weld station computer. The operator monitors the welding process through video cameras and remotely controls all functions, including position control of the welding torch and welding wire additions. The laboratory is used for training and qualification of welding operators and welding engineers prior to each offshore operation. Welding parameters are adjusted to accommodate variations in joint fit-up, groove dimensions variations and pipe ends alignment, with the purpose of defining the welding procedures to be qualified. Changes in water depth, reflected in the actual gas pressure in the welding chamber, require welding parameter adjustment. Welding in the sea depths is diver assisted but still remotely controlled from the surface support vessel. The system was used for depths up to 600 m under sea level but qualified only down to 360 m. It is reported that in 18 years 71 applications of tie-ins were hyperbaric welded with this system in different diameter pipelines at various depths. The second article, in the October 2006 issue at page 35, after having made reference to the above mentioned work, remarks that with increasing depths the ability of divers to function correctly decreases. Therefore the development of diverless equipment becomes the only option. For fillet welded sleeve repair a fully diverless remote technology was designed and developed using hyperbaric gas metal arc welding (GMAW). Initial operation is planned in the 180 to 370 m depth range, with capability down to 1000 m seawater depth. One of the remote applications developed is to connect branch pipelines into production pipeline system without stopping production. This is achieved by the welding of hot taps, where the branch pipe is welded to the pipeline, and then tapping is performed by hydraulic drilling machines. The branch pipe includes a valve that permits the retraction of the cutting head, used for cutting the hole in the pipe wall. Then the valve is closed, and the cutting head is disconnected and recovered. Consumable development to meet offshore pipeline welding standards was essential to the success of the whole project. Although still not widely used offshore, hyperbaric GMA welding processes have shown in pressure chamber tests that qualification parameters are possibly water depth independent, providing a great advantage versus GTA process. Investigations were conducted on preheat temperatures, on weld metal cooling times and on hydrogen and moisture pickup. To overcome the difficulties and reduce the cost of diver welders used for performing hot tapping a novel design was promoted and tested. It includes a remotely installed mechanical clamp and a hyperbaric GMA welding system remotely operated for performing the seal weld inside the branch pipe. The seal welding qualification program included much research work and succeeded in demonstrating the repeatability of the weld quality obtained, with acceptable mechanical properties. The development opens the way to application of this strategically important new technology with hyperbaric GMAW for retrofitting tee branching to unprepared pipelines at depth unreachable by human divers. Interested readers are urged to seek the original articles as detailed above. Let us have your comments to this article, click on Contact Us.
8 - Site Updating: Upset Welding Our newest Page of this Month presents a very successful if specialized welding process, namely Upset Welding, that was not yet addressed to in our website, up to now. It is useful for joining bars or strips end to end. In some form or other it may be familiar to our readers, even if they do not recognize the name. Among the most important advantages we may note that it is very rapid, that the quality is outstanding, that the equipment is simple and that no skilled operators are needed to run it. Readers will be able to read the new page at Upset Welding. A research article on Upset Welding is referenced to in the last item of Section 5 above. Interested readers are invited to read it. As usual all the website page titles can be seen and reached from the Site Map. Let us have your comments and feedback by clicking on Contact Us. If you wish to see what is new with our website (www.welding-advisers.com), either subscribe to our RSS or see frequently our Welding Blog.
9 - Short Items 9.1 - Eutectic Melting at grain boundaries occurs in localized microscopic areas, in aluminum alloys, at a definite temperature slightly above that of solutioning. The appearance of intimately mixed solid phases that melted during solution treating and resolidified during quenching, is visible upon metallographic examination, and proofs that the eutectic melting temperature was exceeded, making the material brittle and non salvageable. This dangerous condition is not detectable visually or by nondestructive testing. 9.2 - Mechanical Properties of a material are those that reveal its elastic and plastic behavior when force is applied, thereby indicating its suitability for mechanical applications. They are determined experimentally in tests that provide reproducible values of modulus of elasticity, tensile strength, yield strength, elongation, reduction of area, hardness, and fatigue limit. 9.3 - Physical Properties of a material are relatively insensitive to structure and can be measured by suitable instrumentation without the application of force. Among these are density, electrical conductivity, coefficient of thermal expansion, magnetic permeability, and lattice parameter. 9.4 - Sigmajig Test is a cracking test commonly used for screening sheet metals for weldability. The test uses a pre-applied load during welding and determines a threshold stress above which a centerline crack initiates. It is commonly accepted that the higher the threshold stress the more resistant to cracking the alloy will be. Below the alloy's threshold stress the material is not expected to crack in the Sigmajig test. The threshold stress has been shown to be sensitive to variations in alloy composition, welding process and parameters, and is used as a quantitative index for an alloy's susceptibility to cracking. 9.5 - Sigma Phase is a hard, brittle, nonmagnetic intermediate phase that appears after prolonged service at elevated temperatures in certain nickel base alloys. Alloys presenting this kind of instability must be excluded from long time elevated temperature service. 9.6 - Solvus is an equilibrium temperature at which solid phases with various compositions coexist with other solid phases, representing the limits of solid solubility. For determining acceptable forging temperature in titanium alloys the solvus temperature can be determined by Differential Thermal Analysis, a fingerprinting technique that provides information on phase transformations and structural changes that occur in a sample during a heat-up or a cool-down cycle.
10 - Explorations: beyond the Welder Invisibility Edu-Pack Seminars on Materials Information Technology Material Search Engine Moon, Mars and Beyond Discover Tuscany
11 - Contribution: Interview with David J. Keats We are grateful to David J. Keats, who agreed to this interview. He is also the Author of a fundamental Book on Underwater Welding
titled "Underwater Wet Welding - A Welder's Mate" that can be ordered online from our page at PWL: Hi David, Q - What is the main Speciality of your Company? Q - Which products and services do you offer? Products include: Underwater Welding electrodes: Cutting Lances: Welding and Cutting Torches: Safety Isolation Switches: Waterproof 'Auto-Darkening' welding filter/lens/visor Our Services are described in our website. (www.specialwelds.com) Q - Which is your most important and rewarding personal contribution? I always prefer to be in the workshops, getting my hands dirty, than being in the office. I think my most rewarding accomplishment will be to get C.Eng. I've been accepted and passed my interviews, but still have to complete my thesis, which is tough going. My most important personal contribution to the business is that I have both practical 'hands-on' welding skills/knowledge, as well as formal technical/business abilities. Q - What trends do you spot in welding likely to have remarkable development? Q - What advice would you offer to young people just starting in the welding profession? They don't have to take an academic route, staying hands on works just as well, but to maximize the enjoyment and fulfillment from the world of welding, they must possess a high degree of skill and knowledge, then welding will provide them with much more than just a living. PWL: Thank you, David. Note: As we inquire on usefulness and interest of this publication for our readers, we would ask you to please let us have your comments on the interview above by clicking on Contact Us.
12 - Testimonials Oct. 4, 2006 I'm an auto shop teacher in Montreal, Canada. As you surely know, welding is part of any auto mechanics course. I sometimes have students that pick up welding technics very quick, so I resort to your website to get ideas for projects to keep these quick learning students busy. I'm writing you to thank you for your good work and I hope that this site will be up and running for a long time! Keep up the good work! Sam Nour
From: Frederik Ford (e-mail removed for security) Dear Sir,
13 - Correspondence: a few Comments To the last issue Article on Fluidized Bed Furnaces we got the following: Date: 28 Sep 2006, 02:44:59 PM Hi Elia,
On the Low Temperature Carbon Supersaturation Article we got the following: 29 Sept. 2006 From the initial lead your article provided I was able to conduct some research into the process and will research it further. I thank you very much for providing this information and would appreciate any further information you could provide, especially if there is any firm timeline on when this process will be commercially available. Note: Thanks to our Correspondents.
14 - Bulletin Board 14.1 - 32th International Symposium for Testing and Failure Analysis (ISTFA/2006) - www.asminternational.org 14.2 - Friction Stir Welding and Processing IV Symposium and TMS Fall Meeting 14.3 - We are pleased to announce that our Website Host, SiteSell.com has recently released two important Guidebooks addressing the subject of Building Successful Websites on the Internet. They are now available as no cost downloads. (Up to now they were sold at a very modest price). Having built my Website (www.welding-advisers.com) upon retirement with the generous help of SiteSell, I know after a few years that it was for me a lucky decision that keeps me busy and happy to this day. You may click on How I Built this Site. The value of these practical books is amazing and I recommend them to anyone (you, your family, your friends...) just thinking of picking up a new career at retirement or at any other time. Download them now at no cost! 1) - Make Your Site Sell! 2) - Make your Content PreSell! 14.4 - To stay informed on updated and new pages from our site, we would suggest that you either click on the Blog button from our website pages or that you bookmark for frequent visits our Blog at: 14.5 - If you wish to browse through our other website on a completely unrelated subject, click on Home Surveillance Guide.
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