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PWL, Issue #018-Plasma Arc Weld, Step Brazing, Flux Cored Wires, Magnetic Pulse Weld, Business Plans February 01, 2005 |
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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.
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Date: February 2005 - Practical Welding Letter - Issue No. 18
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1 - Introduction 2 - Article: Plasma Arc Welding 3 - How to do it well: Brazing in Steps 4 - Filler Metals: Flux Cored Wires 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article: Magnetic Pulse Welding 8 - Site Updating 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Business Plan 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction This second issue of Practical Welding Letter for the Year 2005 (No. 18 in the total count) opens with an article that summarizes the main features of Plasma Arc Welding. It is its versatility that should appeal for performing successfully some demanding applications where it offers distinctive advantages over other processes. Brazing in Steps is a useful technique for assembling complex brazements with multiple joints. Instead of performing all of them at the same time in a suitable protective atmosphere furnace, the technique suggests to split the process in a few stages, with different filler metals carefully selected. A few indications are provided on Flux Cored filler wires. They may provide economic advantages if correctly applied for suitable welding jobs. A process slowly gaining acceptance for mass production, especially in the automotive industry, is called Magnetic Pulse Welding. Although known for decades it was not massively adopted until recently, when more experience and confidence has been developed. A short description of Site Update is presented, following what amounts to be a certain interest expressed by readers. In the Contribution section we address a specific problem raised by a reader with a practical aim. We hope to be helping him and other readers interested in the same subject. Regular departments appear as they should. We would have loved to address the readers to an article of ours which appeared in print, but unfortunately it is not available online. As usually we would appreciate your wishes, comments and feedback. Please write us by e-mail, click here.
2 - Article: Plasma Arc Welding The special Torch design, similar to but different from that of Gas Tungsten Arc Welding, provides a constricted orifice through which the characteristic columnar arc jet is produced, called plasma. When the arc is contained within the torch, between the tungsten electrode and the copper nozzle, it is called a non transferred arc. In this case the distance between the torch and the workpiece is not critical for maintaining the arc. If the workpiece is part of the electrical circuit the arc is transferred. High Frequency is used to ignite a pilot arc within the torch (non transferred), that is then automatically transferred to the workpiece for welding. The Plasma Arc can be used for surface melting or with the keyhole technique capable of melting through the joint in all its thickness. Three levels of current are commonly referred to:
PAW is useful for thin joints in that the current can be reduced to very low values without compromising arc stability. The most important advantage in using PAW is that the keyhole technique permits penetrating the full thickness of a joint in a single pass, and with a large tolerance for the standoff distance. The limitations are economic, due to more expensive equipment and more accurate torch maintenance when compared to regular GTAW, but this is largely overcome if applications are selected enabling exploitation of the more productive capabilities of the process. Power sources are of the constant current (drooping) type. They supply direct current, normally with electrode negative but they may have the capability to provide variable polarity and pulsing, which may widen the selection of acceptable parameters for successful welding. They may also have devices for controlling current up-slope and down-slope. PAW is used to weld stainless steel, aluminum alloys (where the cathodic cleaning action of the electrode positive portion of the current cycle breaks up tenacious surface oxide film), but also titanium, copper and nickel alloys, zirconium and tantalum. It can also weld common steels but then the economics of the process have to be verified. Although welding with the keyhole technique is autogenous, if beveling is adopted then filler metal as for GTAW should be added. Two streams of gas are directed through the torch, the external one is for shielding. The innermost produces the plasma column. Argon is usually employed although mixtures can be used for special effects. To summarize the Plasma Arc Welding technique provides unique capabilities, not easily found in other welding processes, that are successfully exploited if the application is suitable. 3 - How to do it well: Brazing in Steps Q: A complex assembly to be furnace brazed requires complicated fixturing. How could the preparation be simplified? A: By breaking down the assembly in a logical sequence one can divide the brazing operation in two or more simpler stages. One has to select appropriate filler metals in progressively descending order of brazing temperature. In this way the subsequent furnace brazing temperatures do not impair previous brazements. A secondary gain to be considered is the longer time available for assembling after cleaning operations. This time is limited because harmful oxides form on metals even at room temperature, disturbing wetting and brazing. Performing the brazement in steps allows preparation in shorter times, contributing to the brazing success. Even brazing repair, should it be necessary, is easier in partial assemblies. Another way of simplifying fixtures consists in designing self fixturing (or self-jigging) provisions, that is built in temporary means (clamping, crimping, expanding, press fitting etc.) of keeping parts in place until brazing is completed. Brazing filler metal is usually preplaced in the joint or near to it. A capillary clearance is always required.
4 - Filler Metal: Flux Cored Wires. Electrodes used with Flux Cored Arc Welding (FCAW) represent a natural evolution of the common electrode stick, used in Shielded Metal Arc Welding (SMAW), whose most obvious drawback is the need to interrupt the arc for clamping a new electrode when the one in use is consumed up to its butt. It would be nice, it was thought, to feed a long wire trough the torch like it is done with Gas Metal Arc Welding (GMAW) but without the additional complexity of shielding gas supply. Along these lines probably the concept evolved. The shielding material had to be put inside the wire, to leave the external surface clear for the sliding electrical contact and to provide a protection for the brittle flux. A longer stickout (length from the contact to the work) is a special need of this method, as compared to GMAW, to which process it is otherwise similar. It is not surprising therefore that the two processes FCAW and GMAW are the most cost effective for a large segment of common applications and that they together account for the majority of all welding. Removal of the solid slag covering the weld bead is still required for FCAW, possibly in an easier way, but it becomes feasible to perform welding in the open, without much concern for air drafts removing the shielding gas (a real challenge with GMAW) or perturbing its regular flow. Sometimes shielding gas (carbon dioxide with or without argon) is however provided with a special torch, as an additional precaution or as needed to meet code requirements. Auxiliary gas is also required with electrodes intended to be used for welding high strength structural or alloy steels. In practice the two methods are used, the first one being indicated as FCAW-S where S stands for self shielded (without additional gas). The self shielding method produces less penetration than that using auxiliary gas, a characteristic used to advantage when performing welding on joints with poor fit-up. Materials welded by the Flux Cored Arc method are mostly steels, carbon and low alloy, high strength and high temperature resistant steels, and also stainless steels and nickel alloys. The essential limitation is the availability of suitable wire. This welding method is applicable to manual, semi-automatic and automatic welding procedures, in all positions, especially with thin diameter wire. Minimum thickness of sheet material weldable is 1.6 mm (1/16"). Types of Flux Cored steel wires are classified in AWS A5.29, "Specification for Low Alloy Steel Electrodes for Flux Cored Arc Welding." AWS A5.22, "Specification for Flux Cored Corrosion Resisting Chromium and Chromium-Nickel Steel Electrodes." Click here to Order. However many more types were developed by manufacturers and not yet included in standards. The class includes the letter E for electrode, a two figures number, the first indicating the minimum tensile strength in as-welded condition, expressed in tens of ksi (kilopound = 1000 pounds per square inch), usually 6 or 7 (for 60000 or 70000 psi), the second the recommended position, 0 for flat and horizontal, 1 for all positions. The letter T indicates a tubular electrode and then, separated by a dash, a number for type, which means composition, use of auxiliary gas and an indication for use. The complete class symbol may thus look as E60T-1 or E71T-1. Types -1, -2 and -5 are designated for use with auxiliary gas. Other types are generally self shielded or not specified. Additional indications refer to polarity, to single pass or multipass deposition, to requirements for impact properties if specified and to special characteristics if included. Different types of fluxes are used, not readily pointed to in specifications: as usual different types may produce acceptable results in a well defined situation, the actual selection being based on economic or practical considerations. It should be noted that the FCAW has the potential of producing more fumes and dust particles when compared with GMAW, so that due care must be applied to provide suitable aeration or fume control means. In a typical application for welding of tubes in fixed position, the first pass may be performed with solid GMAW wire E70S-3 and Argon + 15-25% CO2, followed by fill passes with E71T-1 with the same gas. Before deciding on the final selection of the most cost effective filler wire for any given job one should always run a thorough application program to develop appropriate weld parameters and check the results by destructively testing actual specimens, that is by qualifying the welding procedure. 5 - Online Press: recent Welding related Articles Crawl-Type Robot Tackles Difficult Jobs Trends in Aluminum Resistance Spot Welding More about MIG Welding Arc Weldability of Microalloyed Steels Software Support for Plate Welders Note: Our Article on Welding Positioners "In A Position To Weld", from The Fabricator magazine, January 2005 issue is unfortunately unavailable online. A no cost subscription is however available to qualifying individuals. See it at
6 - Terms and Definitions Reminder Cold Welding is a solid state process in which pressure is used at room temperature to produce joining of metals with substantial deformation at the weld. If occurring unintentionally between contacting elements it may be cause for failure. Drag in metal cutting is the distance between the exit spot of the cutting flame and the projection of the entry spot on the exit surface. Fisheye is a weld defect, found on the fracture surface of a weld in steel, consisting in a pore or inclusion surrounded by a round, bright area. Flux Cored Electrode is a composite filler metal electrode consisting of a metal hollow shape containing suitable materials and possibly alloying elements to provide shielding atmosphere, deoxidation, composition control, arc stabilization, and slag formation. External gas shielding may be used or not. Lap Joint is a joint between overlapping components, used for brazing, spot welding etc. Also for welding in certain cases. See Short Item 9.6 in PWL issue 08 of April 2004. Click here to read. Sensitization in austenitic stainless steels, caused by welding, is the precipitation of chromium carbides, on exposure to temperatures of about 600 to 900 0C (about 1100 to 1650 0F). Grain boundaries depleted of chromium are susceptible to preferential attack (intergranular corrosion). Step Furnace Brazing is performed in sequence, starting with a subassembly at the highest brazing temperature, followed by other stages with lower brazing temperature filler metal. Adopted to simplify operations and fixturing. String Bead is a straight weld bead deposited without noticeable weaving or transverse oscillation.
7 - Article: Magnetic Pulse Welding Cold welding is identified by the complete and intimate metallurgical coalescence (union) of two similar or dissimilar metals without fusion and essentially without any applied heat. An example of such a process called Explosive Welding was presented as item 2 in the May 2004 issue 09 of Practical Welding Letter. Click here to read it. The Magnetic Pulse Welding process is similar to the above example in the kind of metallurgical bond it produces and in its results, but it is quite different both in the source of energy and in the type of joints that can be achieved. This process, performed in total absence of fusion, is suitable for cold welding together electrically conductive metals, either identical or dissimilar, in a lap joint where a tubular exterior element surrounds an inner one, either tubular or solid, with a measurable gap in between before welding. The physical principles, that are known for quite some time, were originally applied with success to shape parts by means of magnetic pulse forming. An inductor coil, positioned around the outer part, is energized from a capacitor discharge with an elevated current supplied for a very short time. Eddy currents generated in the components produce forces that accelerate the outer sleeve at high speed against the inner part, producing a solid state welding between their contact surfaces. No heat is produced in the process, preserving the properties developed in the elements before welding. The energy required is much less than what is required for other welding processes where much of it is dissipated around as heat. The process is suitable for mass production of tubular components to be joined at high speed with a very short welding cycle. It is applicable to various similar or dissimilar metal pressure tight joints and, with correct design, it can save weight of components and process time. In a variant application, the inner part need not be metallic, in which case not a weld would be obtained but a sort of shrink wrap of the outer metal strongly gripping the core element. Magnetic Pulse Welding can replace brazing and certain types of friction welding of tubular concentric components. In recent times car manufacturers are exploring the economic advantages of applying this process to mass production of modified component joints.
8 - Site Updating The new Page of the Month was established to help interested readers find past articles, arranged by Title, published in all the previous issues of Practical Welding Letters. We hope this page will be used as a handy tool to organize the already substantial information made available to our readers. To reach the new page click here. We prepared a long list of reference books and of standards. We publish those in the different pages of the site as pertinent to the subjects treated. We prepared also specific links to a well known provider of technical publications, who will be able to accept your orders online through a secure connection. As this work of research and preparation is quite demanding, we are going to update the different pages in due time, as soon as possible. Besides the single pages, all the information will be collected also in the page of Welding Books that can be reached by clicking here. 9 - Short Items 9.1 - Electrochemical Machining (ECM) is controlled metal removal by anodic dissolution. Direct current passes through flowing film of conductive solution which separates the workpiece from the electrode tool. The workpiece is the anode, and the tool is the cathode. With movable conforming tools it permits easy forming of complex shapes (i.e. dies, molds, turbine blades etc.). 9.2 - Electromagnetic Forming also known as Magnetic Pulse Forming: a process for forming metal by the direct application of an intense, transient magnetic field. The workpiece is formed by the passage of a pulse of electric current through a forming coil. The force of repulsion accelerates the workpiece at high speed against a die or a mandrel where it is formed in the impact. 9.3 - Machinability is the art and science studying the relative ease or difficulty of machining a metal by chip removal. It depends in general on the mechanical properties of the material studied, on the rate of work hardening presented by the metal upon chip forming and on the presence or lack of hard phases like carbides in the matrix. Machinability rating intends to grade different metals in order of progressive difficulty of machining. 9.4 - Microalloyed Steels contain minute amounts of selected elements like Niobium, Vanadium, Titanium and Molybdenum. Definite microstructures with controlled grain sizes and distribution are obtainable by well controlled mechanical and thermal treatments to produce elevated mechanical properties not readily achieved with more classic low alloy steels. 9.5 - Thread Rolling is a manufacturing procedure designed to obtain high integrity threads in quality fasteners without metal removal. Instead the thread surface is formed by rolling on the blank the impression of hardened steel dies. The rolling dies displace the surface metal of the blank to form the thread shape. By this method the material is deformed plastically and retains favorable compressive stresses resulting stronger and harder, to display higher fatigue strength. 9.6 - Tribology is the science and technology of fretting surfaces in relative motion and of the practices recommended to decrease friction and wear by selecting suitable materials and lubricants.
10 - Explorations: beyond the Welder Mars Science Laboratory Changes in the Arctic Winters warm up A Fuel Gauge for Spacecraft Researchers Discover Fat Gene
11 Contribution - Business Plan One of our readers who wishes to start a small welding shop asked us to provide information on the business aspects of day to day operation. In particular he was interested in instructions for the preparation of a business plan. As the answers to this question might possibly help also other readers who confront the same problems, we report hereafter a few links to some relevant resources. http://www.sba.gov/starting_business/planning/basic.html http://smallbusiness.yahoo.com/resources/business_plans.html An article written especially for online business, but rich of information for all kinds of entrepreneurial activities, is available online at: http://www.marketingtips.com/newsletters/?article=art_november02b If you have questions that may interest a larger audience, ask them by e-mail. Click here.
12 - Testimonials From: "masoud jorat" 'mjorat@spc-ir.com'
I give you my best wishes for success, peace, health and happiness in the new year. During the last year I and my colleagues used your interesting and useful articles and hope to receive your next issues during this new year. [...] Masoud Jorat From: JMKPRORACING@aol.com Thank you for the subscription for your welding articles.. I find them very helpful and informational... [...]
13 - Correspondence: a few Comments We started a new year of Correspondence. There are no surprises, people continue to ask what bothers them, without giving much of a thought to the form or the approach of their request. It is rare, but it happens sometimes, that a reader introduces him/herself, states the problem politely and asks for information. Most of times the same person will thank for what is provided. Frequently people do not sign, or do so using a different name from that appearing in the e-mail address. I cannot understand why. Sometimes people ask for the sort of information available in printed manuals or books. It costs me time and effort to sort out the relevant references I provide: I would do so much more willingly if I knew that I am helping the readers in reaching their goals, but most of times I am left in the dark. I have the suspect that any information costing money is much less interesting: it must depend on what the reader is going to do with it. So, if you are reading me, please note that I am ready to help individuals (not companies) especially if they address me politely, clearly define their purpose and finally if they comment on the use they put my answers to. 14 - Bulletin Board 14.1 - A multidisciplinary meeting on Materials Science and Technology is announced for September 25-28, 2005 at the David L. Lawrence Convention Center, Pittsburgh, Pa. With more than 500 exhibitors under one roof it promises to be a most important event for anyone involved in Metallurgy, Materials, Technology, Manufacturing. For more information visit the site at: www.matscitech.org 14.2 - Our Web Host, Sitesell, provides us with all the tools needed to build and maintain our successful site. We are constantly amazed by the quality of their help, by the new features they add and by the genuine interest they have in the results achieved by their customers. It is quite probable that even occasional readers who did not consider building an Internet Site, could profit from their advice. You are urged to look into the information available, much of it at no cost, by browsing through the pages in the Welding Advisers Site referring specifically to Sitesell services. In particular you are invited to explore the page called Resources (from the Site Map) and that of Downloads, reachable from the NavBar in every page. Good luck, best wishes, see you next time...
Copyright (c) 2005, by Elia E. Levi and welding-advisers.com, all rights reserved
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Copyright (c) 2005, by Elia E. Levi and welding-advisers.com, all rights reserved |
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