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PWL #022-Resistance Welder Select.,Dissimilar Spot Welding,Thermal Spray Fillers,Titanium Microstruc June 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: June 2005 - Practical Welding Letter - Issue No. 22
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1 - Introduction 2 - Article: How to select your Resistance Welder 3 - How to do it well: Spot Welding Dissimilar Materials 4 - Filler Metals for Thermal Spray 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article: Titanium Microstructure 8 - Site Updating 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Biocompatible Materials 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction Our present Issue of Practical Welding Letter begins by giving some hints on how to select a Resistance Welder. It is obvious that before entering the selection phase one should have a good idea of what can we expect from such a piece of equipment, essentially how we plan to implement our manufacturing. If we are not sure with the characteristics of our future productions we would better ask. If needed, Manufacturers of Equipment can review our design of items to be welded and suggest modifications even before they propose their recommended machines. If we already have a Resistance Welder, can we weld dissimilar materials combinations? It may be possible, but we should limit expectations because the result may be brittle nuggets, that may or may not meet requirements. If we are confronted with the selection of filler materials for Thermal Spray, we should start with the required properties of the planned coating. Within that frame we might have a few choices. We should look for accumulated experience where it may be available. Was a similar application done before? With what results? Our main article reviews the basics of Titanium microstructures and properties that are modified by welding and heat treatment. If more information is needed, it may be available, but we should know at least which questions to ask. For the Page of the Month in our Site Update we present a review of important characteristics of Precious Metals. Anybody interested in these topics will find a comprehensive starting point with basic information and links for further learning. In the Contributions Section we deal with Biocompatible Materials, a rapidly developing branch of Materials Engineering. Having explored new applications of traditional materials, more and more specialized variations are created for better confronting the unique challenges posed by the human body environment. Some references are indicated that include more information on the subject. Other sections follow in their traditional order. Please use our form for sending us Your Questions and Feedback. If you like what you read here, would you mind forwarding a copy of this Letter to a few of your friends, with an invitation to subscribe? Thanks.
2 - Article: How to select your Resistance Welder In our website page on Resistance Welding Equipment, we pointed out that the selection of your equipment could be done by letting a few equipment manufacturers work for you after presenting them with your requirements. Your problem is to specify them in an environment of uncertainty, because you cannot know for sure your future needs. Anyhow you should have formulated at least your plans for your present manufacturing programs. Most important are the materials, if plain or plated, the ranges of thicknesses and combinations thereof, shape and dimensions of the items, to make sure they will be accommodated by the machine throat. The type of work will suggest if a fixed machine is what you need or if a portable welding gun is more appropriate. You should also decide if the machine will be fed by hand, by one or more operators, or if the projected production volume warrants some form of automatic or robotic operation. In this last case, special small and powerful transformers can be integrally mounted on the welding gun, the so called transformer-weldgun or transgun assembly. Power supply, in connection with materials and thickness, is selected from a range of types optimized for specific types of applications. Energy conversion can be performed with a number of different technologies (Capacitor discharge, single phase and three-phase AC, DC, Inverter based high frequency DC power sources). Studies show that for a given nugget diameter, thin gauge sheet metal can be welded using a lower secondary current with (three phase) Inverter DC versus conventional (single phase) AC resistance welding, with all other variables unchanged. The quality you want to guarantee to your production will dictate the level of built in adaptive control, with electronics, sensors and actuators under central command by affordable microprocessor based hardware and software. Some instruments allow welders to monitor welding processes, take corrective actions in real time to reduce the effects of process variations as the welds are being made, and archive the results. You should strive to minimize the cost per weld, by taking into account all the elements concurring in building up the expenses along the lifetime of the equipment. And you should have full control of the level of scrap production. A few accessories may help you monitor the adequate operating condition of your equipment. A force transducer, separate from the welding machine, will confirm periodically that the force applied is indeed within acceptable tolerance limits. Inexpensive tensile devices will let you determine destructively the strength of lap spot welded coupons. Recorders will let you review the actual performance of the process along time and alert you of any developing problems. Dedicated non destructive test equipment will guarantee for you the correct quality level according to your production commitments. See the fifth reference in Section 5 hereafter.
3 - How to do it well: Spot Welding Dissimilar Materials Q: Can dissimilar materials be spot welded together? A: The short answer is yes, but it may be neither simple nor advisable in certain cases if brittle structures are produced by the dissimilar metals mixing in the molten nugget. In particular austenitic stainless steel and carbon steel are not usually spot welded together because the resulting nugget structure risks to be hard and brittle, although it could be studied and modified using the Schaeffler diagram and specially conceived heat treating cycles. Materials having widely different properties require that a heat balance be achieved by compensation. The more conductive material, electrically and thermally, must be heated more as it provides less resistive heat, and the heat is lost more easily by conduction. A common technique uses an electrode of smaller face diameter and higher resistivity facing the more conductive material, or by inserting a foil of poorly conductive material between them. Concerning the number of sheets weldable with a single nugget, normal practice suggests not to exceed three layers, although four sheets are occasionally spot welded together. In any case the ratio of the thickest to the thinnest sheet should not exceed three.
4 - Filler Materials for Thermal Spray The large selection of thermal spray materials, their different forms and conditions and the variety of possible processes require sorting out the best ones recommended for any given purpose. One should see which of the processes is readily available, and then determine if its use will be appropriate for the required characteristics of the deposit. One of the most common uses is for rebuilding and reconditioning a worn out item. The purpose is achieved by selecting a material that will easily bond to the base metal, that will be easily machined to the required dimensions and finish, and that will perform satisfactorily for adequate time. It need not be identical to the original base material. The main materials available in wire form for flame spray are pure metals (Aluminum, Copper, Molybdenum, Tin, Zinc) or alloys based on Copper, Iron, Nickel. These provide sometimes a bond layer for permitting good adhesion for other less bondable overlays. The same materials and many more are available in powder form for plasma spray. This form permits blending of intermetallics and other phases that cannot be processed as wires. Powders permit maximum flexibility of the obtainable coating properties. By filling the two dispensers of the equipment provided with this facility with two different powders, one can grade continuously, if needed, the amounts of the components. This means starting with 100% of powder A and 0% of powder B for the first layer. Then further layers will be obtained by changing continuously the proportions of the powders. Finally one can end the process with 100% of powder B and 0% of powder A. This method has the potential to provide for the best particular properties of the resulting coating. One should define the most important characteristics that the coating is to provide, as thermal barrier, or resistance to wear, to abrasion, to erosion, to heat, to corrosion etc. Different systems may be considered for the same purpose, and only actual experience, both with application and performance, will determine the most useful and economic selection. Specialized literature is available from the manufacturers, with guidelines for selection of materials according to the characteristics of the coating sought for specific applications. Besides metals and alloys, also ceramics or carbides can be plasma sprayed, to confer the best surface qualities to resist abrasive wear, oxidation, abrasion and high temperature corrosion as required. Many of the proprietary products meet special requirements of specifications, from central agencies or from OEM (Original Equipment Manufacturers). Also the resulting coatings must meet specific standards of adhesion (bond), of hardness, of distribution of phases etc., as required for demanding applications. In the second item of "5 - Online Press" hereafter, and in Section 9.3 further down a new and promising development called Cold Spray is presented.
5 - Online Press: recent Welding related Articles Our Article on Fatigue Failures (not exclusively related to Welding, except if the failed item was welded) will be available online June 14 at From AWS Welding Journal Solid Wire vs. Metal Cored Wire Online Filler Metal Help From AWS Inspection Trends From NIST
6 - Terms and Definitions Reminder Alkaline Cleaner is a material including alkali hydroxides and alkaline salts (mainly of sodium and potassium) as borates, carbonates, phosphates, or silicates. The cleaning action, consisting in the removal of solid soils, may be enhanced by the addition of surface active agents and special solvents, and followed by mechanical brushing. Alloy is a metallic substance being composed of two or more chemical elements of which at least one is a metal. The verb may indicate the act of adding a determinate metal to a molten composition for the purpose of obtaining a particular blend exhibiting required properties. Anisotropy is the characteristic of exhibiting different values of a property in different directions relative to a fixed reference frame in the material. For example certain tensile properties of a given material may be different along the rolling direction, transverse to the rolling direction, often called long transverse, or along the short transverse, through the thickness direction. Barkhausen effect is the sequence of abrupt changes in magnetic induction occurring when the magnetizing force acting on a ferromagnetic specimen is varied. This effect is used to test for surface defects (see 9.1 further down). Chemical Vapor Deposition (CVD) is a surface conditioning process, similar to gas carburizing and carbonitriding, whereby a reactant atmosphere gas is fed into a processing chamber where it decomposes at the surface of the workpiece. A material is thus provided for absorption by the workpiece, for diffusion into it, or for accumulation thereon. Flattening Test is an acceptance quality test for tubing in which a specimen is flattened to a specified height between parallel plates. No cracks should appear on the locations of maximum stress. Sintering is a process occurring under heat, consisting in the bonding of adjacent surfaces of particles in a mass of compacted powder. Sintering strengthens a powder mass and normally produces a density increase and, in powdered metals, recrystallization. Transformation Temperature is that at which a change in phase occurs, causing a structural modification. This term is sometimes used to mean the limiting temperature of a transformation range. These changes, except formation of martensite, occur at lower temperatures during cooling than during heating, and depend on the rate of temperature change. In certain cases the changes may be suppressed generating metastable structures.
7 - Article: Titanium Microstructure A short introduction on the different microstructures characterizing different titanium alloys has been presented in our page on Titanium Welding. Here we would like to provide some more insight into the basic metallographic features of Titanium Alloys in order to explain more thoroughly essential aspects of their behavior during welding and heat treating. The good strength, low density (4.5 g/cm3, or 0.16 lb/in3), favorable specific strength (strength per unit weight) at certain intermediate temperatures, relatively high melting point (1668 0C, or 3034 0F), excellent corrosion resistance, and good heat-transfer properties of titanium and its alloys have made them attractive to structural designers for use in aerospace and chemical industries where the unique properties can justify the high cost. The maximum useful temperature range for structural applications is 425 to 580 0C (800 to 1100 0F), depending on alloy and condition. Titanium can be used for cryogenic applications because it has no dangerous ductile-brittle transition temperature (DBTT). Titanium alloys are available as castings, Powder Metallurgy (P/M) products, and in all wrought forms: plate, sheet, tube, forging, bar, and wire. Unalloyed titanium is usually selected for its excellent corrosion resistance, especially in applications where high strength is not required. Unalloyed Titanium exists in two crystallographic forms: the room-temperature hcp (hexagonal close-packed) alpha-phase transforms to the bcc (body-centered cubic) beta-phase at 883 0C (1620 0F). This transformation temperature can be raised or lowered depending on the type and amount of impurities or alloying additions. The microstructure of unalloyed titanium at room temperature is typically a 100% alpha crystal structure. As amounts of impurity elements, primarily iron, increase, small but raising amounts of beta are observed metallographically, usually at alpha grain boundaries. The relatively low strength level of the commercially pure grades of titanium is improved with the addition of alloying elements and the application of thermomechanical processing during fabrication into final products. The alloying elements used in titanium are classified by their individual effects on the phase diagram: whether they stabilize the alpha or the beta titanium phase. The influence of alloying elements is summarized as follows:
The chemical composition and the processing temperatures determine the microstructural constitution of the alloys, which are classified according to the most abundant phase in the alloy, and consequently the mechanical properties. Alpha alloys are predominantly alpha, usually with minor amounts of beta present. Alpha-beta alloys contain both phases, with more beta than the alpha alloys. Beta alloys have sufficient beta stabilizer content that the alloys can be solution treated, water quenched, and retain 100% beta structure. The commercial alloys are normally metastable and are aged to precipitate alpha to increase strength. The microstructure will thus consist of alpha in a beta matrix. Heating titanium in air at high temperature results not only in oxidation but also in solid-solution hardening of the surface as a result of diffusion of oxygen and nitrogen. A surface hardened zone of "alpha-case" is formed. This layer must be removed, before placing a part in service, by machining, chemical milling, or other means, because its presence reduces fatigue strength and ductility. Ti-6Al-4V is the most widely used titanium alloy, accounting for about 45% of total titanium production. Unalloyed grades include about 30% of production, and all other alloys combined the remaining 25%. Selection of an unalloyed grade of titanium, an alpha or near-alpha alloy, an alpha-beta alloy, or a beta alloy depends on desired mechanical properties, service requirements, cost considerations, and other factors that enter into any material selection process. Alpha alloys that contain aluminum, tin, and/or zirconium are resistant to creep at high temperature. Extra-low-interstitial (ELI) alpha alloys retain ductility and toughness at cryogenic temperatures. Alpha alloys have good weldability because they are insensitive to heat treatment. They are annealed or recrystallized to remove residual stresses induced by cold working. Alpha-beta alloys contain one or more alpha stabilizers or alpha-soluble elements plus one or more beta stabilizers. These alloys retain more beta phase after solution treatment than do near alpha alloys, the specific amount depending on the quantity of beta stabilizers present and on heat treatment. Alpha-beta alloys can be strengthened by solution treating and aging. Solution treating usually is done at a high temperature in the two-phase alpha-beta field and is followed by quenching in water, oil, or other quenchant. Solution treatment is followed by aging, normally at 480 to 650 0C (900 to 1200 0F), to precipitate alpha and produce a fine mixture of alpha and beta in the retained or transformed beta phase. Solution treating and aging can increase the strength of alpha-beta alloys 30 to 50%, or more, over the annealed or overaged condition. Response to solution treating and aging depends on section size. Alloy composition, solution temperature, and aging conditions must be carefully selected and balanced to produce the desired mechanical properties in the final product. Beta alloys are richer in beta stabilizers and leaner in alpha stabilizers than alpha-beta alloys. They are characterized by high hardenability, with beta phase completely retained on air cooling of thin sections or water quenching of thick sections. After solution treating, beta alloys are aged at temperatures of 450 to 650 0C (850 to 1200 0F) to partially transform the beta phase to alpha. Although tensile ductility is lower, the fracture toughness of an aged beta alloy generally is higher than that of an aged alpha-beta alloy of comparable yield strength. In the solution-treated condition (100% retained beta), beta alloys have good ductility and toughness, relatively low strength, and excellent formability. Solution-treated beta alloys begin to precipitate alpha phase at slightly elevated temperatures and thus are unsuitable for elevated-temperature service without prior stabilization or overaging treatment. Beta alloys (at least commercial beta alloys) are metastable, because cold work at ambient temperature can induce a martensitic transformation, or heating to a slightly elevated temperature can cause partial transformation to alpha or other transformation products. The principal advantages of beta alloys are that they have high hardenability, excellent forgeability, and good cold formability in the solution-treated condition.
8 - Site Updating Our latest Page of the Month deals with Precious Metals. A common use of these materials, from the prehistory of humankind, is for jewelry and artwork. Creative people may find it a rewarding activity to express their talents and workmanship in producing aesthetic objects of ornament. Dentistry makes a wide diffuse usage of Precious Metals, profiting from the oxidation and corrosion resistance of these materials, one of the first successfully employed exploitations of bio compatibility. But there is much more than that in technological applications of precious materials for solving complex performance problems, making the best use of their unique characteristics not otherwise available. In all these cases price is not an obstacle as the advantages of use are evident. To reach the new page click on Precious Metals. To review what is new in the website, readers are invited to visit periodically the Site Map.
9 - Short Items 9.1 - Atomic Force Microscopes (AFM) operate by measuring forces between a tip and a sample. To acquire an image the microscope utilizes a sharp probe moving over the surface of a sample in a raster scan while measuring the local atomic topography. The resulting image is composed by many rows or lines of information placed one above the other and reproduced in an image similar to an actual photograph. Unlike traditional microscopes, scanned probe systems do not use lenses, so the size of the probe rather than diffraction effects generally limit their resolution. In contact mode, the instrument lightly touches the sample with the tip of the probe at the end of a leaf spring or cantilever, shaped like a diving board. As a raster scan drags the tip over the sample, a detection apparatus measures the vertical deflection of the cantilever, which indicates the local sample height. AFM resembles the record player as well as the stylus profilometer but important refinements provide its usability. In particular the cantilever deflection is measured by a sensitive laser beam that magnifies many times the tiny movement of the probe. 9.2 - Barkhausen Noise Analysis (BNA) can be used as a nondestructive technique to detect dangerous conditions like Abusive Grinding, which is produced on parts as a result of overheating when the process runs out of control. In these conditions burns and minute cracks may occur on the ground surface. Surface defects are particularly dangerous as they may occur in regions of maximum stress and can originate fatigue fractures. The search for grinding defects is generally performed on regular steels by a detailed procedure called "Nital Etch" which involves attacking the surface with a solution of nitric acid in alcohol to bring to evidence the said visual defects. Experience and understanding are required from the inspector to evaluate correctly the defective condition. For automating the search and for integrating the control in the grinding line, with the advantage of immediate alerting that grinding is out of control, one can use the Barkhausen Noise Analysis method that is based on inductive measurement of a noise-like signal, generated when magnetic field is applied to a ferromagnetic sample. BNA is applicable also to magnetic stainless steels which are not attacked by "Nital Etch". BNA as implemented with commercially available instruments, permits the control (in magnetic materials) of surface defects derived from abusive grinding, of heat treatment defects, the measurement of residual stresses and hardness and evaluation of microstructure. 9.3 - Cold Spray is a further development that considerably extends the successes of "Spray" material applications. As is widely known, the Thermal Spray technology includes a group of diversified processes that provide adherent coating layers of various kinds and types upon the most different base materials. All of these processes are characterized by the use of heat in the form of a flame, a plasma or an explosive jet developing very high temperatures. A new process is now introduced which provides important advantages, especially for the application of sensitive materials that would be altered by heat. The essential feature is provided by a high pressure, high velocity gas jet that imparts high speed to minute particles injected into its stream. The powder particles impact upon the surface substrate to be coated and stick there after deforming plastically. Cold spray is a solid state process that introduces favorable compressive stresses resulting in a microstructure of dense, pure, thick and well bonded coatings with density close to theoretical values. Besides the advantages of permitting the application of unusually sensitive materials and also of producing free preforms by computer controlled local deposition, the process seems to be able to provide also reductions in cost and lead time. In summary this is a process whose interesting developments should be closely watched. An Article on Cold Spray Technology was published in the March 2005 issue of Advanced Materials and Processes, an ASM International publication. As we were almost ready to publish this issue of PWL, we found an article on this subject in the current issue of AWS Welding Journal. The link is reported above in Section 5. This is one more proof that Cold Spray is a Hot subject... 9.4 - Engine Endurance Test is the proof of the pudding for engines and other complex pieces of machinery with demanding reliability and safety requirements. It consists in running the engine at predetermined performance levels in series of cycles designed to stress all the structures near the maximum of their capability for extended periods of time. The regime is conceived as equivalent to a much longer running test at a more relaxed pace. At the end of endurance, as it emerged hopefully in good condition (without failures) after having reached all the design objectives in the operational envelope, the engine is disassembled and all the elements are subjected to thorough examinations. If everything is satisfactory the engine model is certified by the competent authority for its intended use (civil or military) with the restrictions that may be applied, depending on the circumstances. 9.5 - Silicon Carbide (SiC) is an advanced ceramic compound being developed as a promising material for demanding tasks in severely stressed components, besides its uses as a semiconductor for electronic hardware. Many research programs were initiated for producing high temperature gas turbine components. They were intended to replace nickel superalloy turbine blades or nozzle vanes. However, none of these projects resulted in production quantities, mainly because of its low impact resistance and its low fracture toughness. Until effective ways are developed to improve toughness by using efficient reinforcements or through advancements in nanotechnology, incremental improvements provide niche applications in industrial and land based engines. 9.6 - Spin Test is the application of high speed rotation upon fabricated items like turbine wheels, for examining the behavior of materials and processes at stresses approaching their strength limits. Testing is generally performed in specially built equipment at room temperature, in a vacuum to eliminate air resistance but also under heating, cooling or cryogenic conditions as required, in a lined pit capable of containing the explosion of fragments following burst failures. Measurable radial growth may occur in parts severely stressed by centrifugal forces. Fatigue testing and crack detection can be performed on spin tested rotors. Spin testing is an essential stage in the development of high speed rotating equipment.
10 - Explorations: beyond the Welder Beryllium Space Telescope Surf the Ant Technology Forecasts Articles on Climate Change Underwater Volcano
11 - Contribution: Biocompatible Materials Different materials have been studied and used for replacement and repair of damaged human tissues. Metals, ceramics and polymers are the subject of severe scrutiny until approval for usage is granted by the regulatory authorities. The tooling used to make aggressive medical procedures is now manufactured from special materials tailored to meet the most demanding requirements. A recent article on Alloys for Surgical Instruments was published in the April 2005 issue of Advanced Materials and Processes. An expression of increased interest in finding solutions acceptable to the medical community can be found in the initiative of ASM International to publish a periodical informative supplement, called MPMD (Materials and Processes for Medical Devices). This quarterly publication, dedicated to report on studies and innovations in biomaterials, is hosted into the widely known and valued Advanced Materials and Processes. Specific Conferences are being organized and even focused courses on Metallurgy for the Medical Device Engineer are offered in many locations. Visit The Handbook of Materials for Medical Devices is available through AMS International. The first Chapter download is available at The Handbook provides an in-depth review of the properties, processing, and selection of materials used in the environment of the human body, an environment that is surprisingly hostile and aggressive. Among the application areas described are orthopedics (hips, knees, and spinal and fracture fixation), cardiology (stents, heart valves, pacemakers), surgical instruments, and restorative dentistry. Materials discussed include metals and alloys, ceramics, glasses, and glass-ceramics, polymeric materials, composites, coatings, and adhesives and cements.
12 - Testimonials From: XCEL333@aol.com Thank you for responding so quickly to my questions. Joseph Tafoya
From: Rhody Triblets 'rtriblets@sbcglobal.net' Design for welding: Letter to an engineer. An article from Dear Elia, This article really opened my eyes. I work as a process developer for a small precision welding facility. I am not a weld engineer but often have to play one at my job. [...] Can you recommend how I should handle this? I really enjoy your work. Best regards, Rhody Triblets
13 - Correspondence: a few Comments 13.1 - The most important change in the correspondence with us is a new system put in place by our Service Provider with the purpose of eliminating or at least limiting drastically the amount of not wanted and not requested junk mail. Everybody willing to correspond with us for the first time, will be able to do so only by using our form available on our Website for sending us Questions and Feedback. If the message is legitimate and if a reply is sent, the correspondent's address will be put on a Whitelist, permitting further communications in both directions. Any message from any other not-Whitelisted address will bounce back to the sender. All readers who were corresponding in the past are urged to use the above form at least once, certainly if their new message bounces back. The new system is running in just now and unforeseen difficulties may occur. Please be patient if a contact does not succeed and try again through the form. 13.2 - Please be again advised that Attachments will not be opened, unless it is verified and sure beyond any doubt that they were sent by friendly correspondents. We recently received a message from a known address, but our defensive software blocked it before opening: it appears that some aggressive and nasty person substituted his name with one that we know and trust only to get in and break havoc in our computer.
14 - Bulletin Board See hereafter two preliminary notices of future events. 14.1 - 5th Int'l Forum on Aluminum Ships 14.2 - AWS Welding Show & FABTECH Int'l 2005 14.3 - Our trusted Internet Service Provider has just announced the release of the Full Beta issue of a complex integrated auxiliary Facility developed to help users to Brainstorm and Research concepts before building Websites. This new and useful tool, which is added to the already large selection of different aids, comes at no additional cost. Click on the SBI Reference Center.
See you next time Copyright (c) 2005, by Elia E. Levi and welding-advisers.com, all rights reserved -
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