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PWL, Issue #021-Magnesium Corrosion, Process Comparison, Aluminum-Lithium, Basic Steel facts, Fumes May 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.
You are urged to pass-along this publication to your
Date: May 2005 - Practical Welding Letter - Issue No. 21
---------------------------TABLE of CONTENTS------------------------
1 - Introduction 2 - Article: Magnesium corrosion 3 - How to do it well: Process Comparison 4 - Filler Metals for Aluminum-Lithium Alloys 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article: Basic facts on Steel 8 - Site Updating 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: On Welding Fumes 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction Welcome to this new issue (# 21) of Practical Welding Letter. A Guide has been set up for our readers who might be interested in looking back to an article seen long ago, and lost in the heap. To find past articles you can click on Welding Topics. We open our presentation with some facts on Magnesium corrosion. As more and more light items are made with alloys of this material, it may be useful to know the basics of attack forms and of protection therefrom. In the next section we present the procedure used for selecting the most convenient welding process for any job. For actually proceeding one has to detail all the basic data and parameters for the job and sketch the realization with any possible process. Only after that, cost estimate can be determined and then compared to find the most economic way of performing the job. For the department of Filler Metals we consider this time Aluminum-Lithium alloys, a relatively new group of materials having attractive properties for aerospace structures. Weldability, as usual, must be taken into account. A reminder of basic facts regarding Steel follows. An explanation is given for the most amazing feature of steels in general, namely the versatility, or the adaptability of particular compositions of steel to vastly different properties, performing successfully for the most diverse requirements. For our Site Update, we present a new page on Welding Software. An important technology like welding cannot remain extraneous to the efforts spent in building theoretical models and practical programs to help in realizing economies in process development and in routine production. See the article further down. In the Contributions section we talk of welding Fumes and of their dangers to Health. Responsible attention should be devoted to the means available to minimize overexposure of welders to hazardous work environments. Other departments follow in place where they can be expected. Let us remind you that you are urged, as always, to click here to send us by e-mail Your Questions and Feedback. Would you be so kind to forward this Letter to some friends of yours, encouraging them to subscribe? Thanks.
2 - Article: Magnesium corrosion Magnesium and Magnesium Alloys are structural materials preferred for their outstandingly low specific weight, making them the metals of choice for light weight items to be hand held (i.e. cameras, luggage, ladders) or for machine elements operating at variable high speed, to reduce inertia. Magnesium alloys are available as wrought shapes, extrusions, forgings and castings. Different alloys were developed to display a wide range of mechanical properties from which the most suitable is to be selected for every application. The concern of corrosion protection is widely diffused. Therefore it may be useful to summarize some of the most important facts influencing magnesium attack. Depending on the environment the corrosion of magnesium can be well confined within acceptable design limits. When unalloyed magnesium is exposed to air at room temperature, a gray oxide forms on its surface. Moisture converts this oxide to magnesium hydroxide, which is stable in the basic range of pH values, but it is not in the neutral or acid ranges. The alloys of magnesium that include from 2 to 10% aluminum, with minor additions of zinc and manganese are quite popular for general use and their room temperature mechanical properties are effective up to 95 to 120 0C (200 to 250 0F). Their designations begin with the letter "A". A second group consists of magnesium alloyed with various elements (rare earths, zinc, thorium, and silver) but no aluminum. Zirconium is present in tiny amounts to provide a fine grain structure and improved mechanical properties. These alloys behave much better at slightly higher temperature but are more expensive. Magnesium is anodic to all other structural metals and therefore galvanic interactions are a serious concern. Tiny amounts of impurities of iron, nickel, copper or cobalt in magnesium alloys effectively increase their corrosion susceptibility. Therefore, to obtain good corrosion resistance these elements must be limited to low levels, or their activity must be modified using manganese or zinc. Corrosion resistance is usually evaluated with the standard 5% salt spray test ASTM B117 (aka fog test). Welds on Mg-Al-Zn alloys should be aged or should be solution treated and aged to improve resistance in corrosive environments and to reduce the risk of failure by stress-corrosion cracking (SCC). Contamination is a common cause of corrosion resulting in general pitting attack. Sometimes flux inclusions residues from melting practice, in conditions of elevated moisture, may develop localized attack on otherwise machined and clean surfaces. Chromic acid pickling followed by chemical treatment and surface sealing can reduce the problem of inclusions in finished castings. The most common problem but also the one more easily controlled by design is that of galvanic corrosion due to intimate contact with other metals (fasteners and assemblies). The solution has to be sought by providing effective electrical insulation between the dissimilar metals.
3 - How to do it well: Process comparison Q: A butt weld has to be performed on short lengths of sheet metal. It could be done by Plasma Arc Welding, by Electron Beam or by Laser Beam Welding: which process should be preferred? A: Given the availability of different processes, once the quality requirements are satisfied, the most economic one should be selected. The economy of performance, expressed as cost per weld, has to be calculated by taking into account all the expenses for equipment, consumables, workforce, handling etc. A first evaluation can be done by comparing advantages and disadvantages of each process in turn. If manual and automatic operation can be performed, the cost of both should be estimated. Plasma Arc Welding (PAW): Advantages of PAW as compared to GTAW:
Disadvantages:
Electron Beam Welding (EBW) Advantages:
Disadvantages:
Laser Beam Welding (LBW) Advantages:
Disadvantages:
4 - Filler Metals for Aluminum-Lithium Alloys Lithium, a highly reactive material, is the lowest density, low melting point metal element. Aluminum-Lithium alloys are a recently developed group of high-performance wrought aluminum alloys intended for demanding uses. Designers of aerospace structures can use the higher elastic modulus, lower density, and, in some cases, higher strength and fracture toughness of aluminum-lithium alloys to reduce structural weight. The advantages of aluminum-lithium alloys over conventional aluminum alloys include relatively low densities, high elastic modulus, excellent fatigue and cryogenic strength and toughness properties, and superior fatigue crack growth resistance. The use of aluminum-lithium alloys does not require changes in manufacturing equipment, design methods, or fabrication techniques. In particular, aluminum-lithium alloys have been found to be readily weldable. Some alloys, presenting good weldability, high strength, and accompanying high fracture toughness, have been considered as candidates for demanding welded propellant tank applications. Weight savings gained from using thinner sections of a higher strength alloy permit a larger launch payload, and can also be achieved by taking advantage of the high specific modulus of aluminum-lithium alloys for use in stiffened or compression-loaded structures. The main concerns for the weldability of aluminum-lithium alloys are for porosity and hot crack susceptibility. However with proper selection of conditions and with some development work, these alloys can be easily welded in practice. In order to control porosity it is recommended to dry machine remove a certain amount (usually around 0.25 mm = 0.01 in.) of material before fusion welding. Shielding gases should also be very dry and with low oxygen (less than 300 ppm). Hot cracking occurs when low-melting, eutectic liquid films are separating at the trailing edge of the weld pool during solidification. The hot cracking susceptibility of any aluminum alloy in general, appears to be related to its copper and magnesium contents and may involve an aluminum-copper-magnesium eutectic. There is a relationship between copper and magnesium content and weldability, known from a time were lithium additions were not yet performed. It is found that aluminum-lithium alloys with high copper and low magnesium contents (e.g., alloys 2094 and 2090) have good weldability. On the other hand, alloy 2091, an aluminum-lithium alloy with intermediate amounts of both copper and magnesium, is less weldable. Alloy 2319 has been typically used to join the high-copper alloys (e.g., alloys 2090, 2094, 2095, and 2195). Low-copper, high-magnesium alloys (e.g., alloy 8090) call for the use of a high-magnesium filler metal such as alloy 5356. Alloys that are difficult to weld (e.g., alloy 2091) require the use of an aluminum-silicon filler metal such as alloy 4043. It must be stressed however that although the above deals mainly with Gas Tungsten Arc Welding, modern practice would prefer to specify, whenever possible, the use of Friction Stir Welding without filler metal instead of any other fusion process.
5 - Online Press: recent Welding related Articles From AWS MPI (Magnetic Particles Inspection) Guide Current Control in MPI From TWI From NIST
6 - Terms and Definitions Reminder Air Carbon Arc Cutting is a process in which metals to be cut are melted by the heat of the arc from a carbon electrode and the molten metal is blown away by a blast of compressed air. Cold Working consists in deforming metal plastically under conditions of temperature and strain rate that induce strain hardening and avoid recrystallization. The procedure involves generally an increase of tensile strength and a reduction of ductility. Usually, but not necessarily, it is performed at room temperature. In a Creep Rupture test, conducted at a specified constant temperature and under a constant tensile stress, the time to rupture is measured. For acceptance it should exceed the minimum time to rupture given by the relevant specification. The progressive specimen deformation is not measured. This is different from a Creep Test where the deformation rate is measured by a recording extensometer. Fatigue is the phenomenon leading to fracture under repeated or fluctuating stresses having a maximum value much less than the ultimate tensile strength of the material. Fatigue failure generally occurs at loads that, if applied statically, would not produce visible effects. Fatigue damage is progressive, beginning as minute cracks that grow under the action of the fluctuating stress. If cracks develop unchecked, fatigue fractures occur of a sudden, as soon as the remaining uncracked section can no more stand the normal working stress. Fumes are a mixture of very fine particles suspended in air. One or more of the many substances that can be present in welding fumes, such as arsenic, asbestos, beryllium, cadmium, carbon monoxide, chromium, cobalt, copper, fluorine compounds, lead, manganese, nickel, nitrogen oxides, ozone, phosgene, selenium, silica, and zinc can be extremely toxic. See hereafter in section 13. A Linear Indication is any thin discontinuity having one dimension at least three times long as the other, and interpreted as such by non destructive inspection. It can be extremely dangerous, easily developing into cracks. Therefore linear indications are examined with attention and may be subject to rejection unless they can be eliminated by local grinding or similar process. Precoating is the process of applying a protecting coating on the surface of a raw material before welding or otherwise fabricating it into useful items. Surface or interface Expulsion in resistance spot welding is a condition resulting from overheating when the pressure applied to the electrodes is insufficient. It appears as a visual surface defect showing traces of expulsed metal. It indicates poor quality welds.
7 - Article: Basic facts on Steel Steel is a general name covering a very large assortment of different metals defined as alloys of iron and carbon and other elements as required. The most impressive characteristic of steel, as a comprehensive category of materials, is its outstanding versatility. By this we mean the capability to display and enhance specific properties of the alloy by two control factors: composition and thermo-mechanical treatments. Iron ores are quite an abundant source of materials used for ironmaking. The modern practice of iron and steelmaking has evolved over several centuries. The primary product is called pig iron, which is used to manufacture steel by further refining. In recent years as a consequence of global developments, a production overcapacity has been established worldwide, with consequent price reductions. The economic pressure has pushed older and least productive activities out of the market, contributing to the introduction and development of modern techniques for competitive production. The element iron displays some peculiar characteristics that provide its exceptional usefulness. In solid form it presents two different crystallographic structures depending on temperature. The transition from one form to the other can be controlled also by the time allowed for the transformation. Crystallographic forms are the base for differentiating micro structures, presenting different properties. The element carbon, essential in iron and steel metallurgy, has the property to modify structure and properties obtained by thermal treatments, depending on the percentage present in any alloy. Other elements further contribute to improve the basic behavior for special purposes and produce specific micro structures of the required alloys, with properties suitable for different applications. The exact composition of different types contributes to their capability to react to heat treatment for developing adequate mechanical properties useful for specific applications. Ductile and deformable steel sheet may be used for deep drawn cups, while hard steel sheet may be used for springs or blades. Many other raw materials forms are available, like bars, wires and special profiles. Manufacturing technologies like forging, forming, drawing, machining, welding and a large selection of heat treatments and of finishing processes contribute to the wide versatility of this category of materials. Special classes of steels including high proportions of other metals in alloy form are useful for special purposes like cryogenic (low temperature), electric and magnetic applications, heat and corrosion resistance, for building tools for any kind of work, hearth moving and oil drilling equipment among others. Steel is used for high rise buildings, for industrial construction, for bridges, for ships. Although steel supremacy has been challenged in recent times by other new materials in exacting niche applications, its superior position in the industrial word cannot be overcome.
8 - Site Updating The new Page of the Month recently added to our website concerns Welding Software. Much progress was done in the past years towards development and application of dedicated computer programs to ease and speed up welding administration, management and production. Welding Software is by now a necessary toolkit for streamlining performance, for easy document writing and managing, for record keeping of Welding workforce information and for weld operations planning and management. Much work is going on worldwide on programs capable of preparing optimized welding procedures. Except for a few processes and configurations where the software is already running, there is still much to do in this field. It is suggested that even the simplest welding shop could profit by entering the computer era even if only for recording on a table the consumables used and their applications, or for calculating their needed quantity for a job. A Note concerning both the new Software page described above and a technique mentioned in the past issue of PWL #20 (3 - How to do it well: Straightening with an Oxyacetylene Torch Flame) was published recently. See: It reports that the Navy Joining Center (NJC) is actively pursuing Thermal Forming to induce curvatures in steel plates by selectively heating and cooling. Due to the dwindling availability of skilled labor force, the task will be performed by developing a Path Planning Software to automate the process. To read the new page on our site click on Welding Software. We recommend to our readers to stay informed on the newest pages of the site by bookmarking and visiting regularly the Site Map.
9 - Short Items 9.1 - Metal Matrix Composites (MMC) are materials consisting of a matrix phase which is usually a low-density nonferrous alloy and a finely mixed reinforcement phase consists of high-performance carbon, metallic, or ceramic additions. They have potential for a wide variety of applications: they are capable of sustaining higher temperatures than regular metal parts, and can be adapted to provide improved strength, stiffness, thermal conductivity, abrasion resistance, creep resistance, or dimensional stability. Most of the commercial work on MMCs was concentrated on aluminum as the matrix metal because of useful mechanical properties combined with light weight and corrosion resistance. Aluminum MMCs are preferred because of their greater ease of manufacture, lower production costs, and relatively isotropic (equal in all directions) properties. In MMCs, mechanical properties depend on the amount, size, shape and distribution of the dispersed reinforcement, apart from the mechanical properties of the matrix material, and on the nature of the interface. Another popular matrix material is titanium, selected because of its good specific strength at both room and moderately elevated temperature and its excellent corrosion resistance. Different manufacturing technologies are used to produce MMCs for special applications. 9.2 - Metal Injection Molding (MIM) is a powder metallurgy process in which very fine metal powders are mixed with suitable binders and then injected into much the same molds as used for plastic injection molding, to obtain useful shapes. The binders are then removed and the green artifacts are sintered to full density in a furnace, producing very good mechanical properties. The technology provides near net shapes in a cost effective way. Small and intricate parts (weighing less than 100 g) of special alloys of elevate melting point (which makes die casting impractical) are particularly suited to be manufactured by MIM, that is exceptionally advantageous as an alternative manufacturing route if machining is more expensive. 9.3 - Patenting is a heat treatment applied to medium or high carbon steel wire before or between drawing passes. This process consists of heating the wire by passing it through tubes in a furnace to a temperature above the transformation range at about 9700C. This high temperature treatment produces uniform austenite of large grain size. The subsequent cooling to a temperature below Ae1 in air or molten lead is rapid since the sections treated are generally small, so that the resulting structure consists of very fine pearlite. Grain size is reduced after a few drawing passes. Patented wire is able to withstand reductions of area up to 90%, providing exceptional strength explained on the basis of the reduced ferrite cells and the alignment of cementite in fibres. 9.4 - A Refractometer is a hand-held simple instrument that measures the refracting index of a solution to determine the concentration of solute. A calibration chart relates the instrument reading to concentration. The refractive index varies with fluid composition and concentration. Therefore, the refractive index must be calibrated for each fluid and each concentration. Industrial optical refractometers that employ an arbitrary scale may also be calibrated. Generally, the use of a refractometer for measuring concentration in water emulsions and in oil based fluids for cutting, machining, quenching etc. is adequate during the initial preparation of the fluid. However, it frequently becomes inadequate as contaminants in the fluid accumulate. 9.5 - Stresscoat is a commercial product available for performing qualitative experimental stress analysis. It consists in a special lacquer that is to be deposited by spraying on the cleaned surface of a mechanical element. After drying, once the part is stressed to a fraction of its load capacity, the brittle coating will fracture in response to the surface strain of the material beneath it. The pattern of cracks developed on its surface indicates the direction and magnitude of stress within the elastic limit of the base material. This application provides a graphic picture of the distribution, location, sequence and magnitude of tensile strains. Coating cracks are perpendicular to the principal tensile strains. Special techniques and calibration permit to obtain a quantitative appreciation of the stress magnitude. The use of strain gages applied in the locations indicated by the qualitative test, permit further more precise analysis. 9.6 - Tellurium Copper is a free cutting alloy designated C14500. Its composition is 99.40 copper and 0.50 tellurium and its relative machinability rating is reported as 80 (free-cutting brass = 100). It may be used where electrical conductivity is a desirable property and extensive machining is required. This metal is however one of the most difficult to extrude copper alloys. 10 - Explorations: beyond the Welder Scientific American http://www.sciam.com/ CERN and LHC (Large Hadron Collider) Brains in Silicon Planetary Discoveries Table top Fusion
11 - Contribution: Controlling Welding Fumes Welding fumes are essentially air borne microscopic particles generated by the welding processes. Since fume primarily comes from the electrode, it consists of oxides of its metals, alloys and flux compounds. However fume constituents can come also from the base material if it contains alloying elements or it is covered by coating or plating. All the personnel involved in welding operations must be protected from overexposure to fumes and gases. It is therefore proper to install adequate ventilation, aspiration or personal protection as required to avoid exposing people to unnecessary hazards. Ventilation is essential to fume and gas control in the welding shop. It is meant to reduce drastically the concentration of dangerous particles that could otherwise accumulate and remain suspended in air for long times. Local exhaust ventilation, which removes the fumes and gases at their source, is the most effective method. This can be provided by a partial enclosure, such as a ventilated work bench, or by fixed or moveable hoods as close to the point of welding as possible. General ventilation uses roof vents, open doors and windows, roof fans, or floor fans to move air through the entire work area. This is not as effective as local exhaust ventilation. General ventilation is often helpful, however, when used to supplement local ventilation. Gun mounted fume extraction equipment, used with self shielded (that is without auxiliary inert gas) flux cored arc welding, eliminates most of the fumes at the point of production. Independently of the fume control system employed, its efficiency should be monitored at regular intervals. Ventilation systems should be cleaned and maintained periodically. It is specially important that self employed welders, who may tend to overlook the implications on their own health of poor or inadequate precautions, deal adequately with this subject even with the help of experts if necessary.
12 - Testimonials From: anbalagan 'anbalagan@kpios.com' Very useful updates and back to basics. Keep it up! R. Anbalagan, QA Manager
On Sat Apr 02 08:33:31 2005, the following was submitted First Name: Paul Elia, 13 - Correspondence: a few Comments As every reader knows, both in my Practical Welding Letters and in my Welding Advisers website I encourage my audience to address me questions. And answering to those letters is for me, usually, quite an enjoyable activity. It seems, however, that more than once the inquirers did not look hard enough, if at all, to find out if by any chance the answer to the question was already available in the Site. So that, in those cases, my answer is reduced to: "You may find what you look for in my page..." I have nothing against that, although it is like squandering my assets. In any case all readers with minimum of care could use one or both of the following means: Either use the search box appearing in almost all the pages, by printing in it the keyword describing what is looked for. The list of pages popping up might contain the page with the answer, or at least clarify the issue. Or just look at the Site Map, essentially an index of subjects. One reader, incidentally, read something interesting in the Introduction of one of the past issues of PWL. Without even looking at the item reported at length in the copy, he hastened to ask for details. The details happened to be included in the article itself, only to be found and read... Obviously these observation have no chance of being seen by new visitors rushing to ask what they need. But all readers who see these lines are urged to think a little before asking (which they probably do already). Another thing is that inquirers asking too general questions cannot reasonably expect to be given a thorough answer within the frame of a short message. This might be long as a chapter of an Encyclopedia, and obviously has no place in correspondence. For any questions on these subjects write us by e-mail. Click here to send us Your Questions and Feedback.
14 - Bulletin Board 14.1 - 7th Trends in Welding Research Conference 14.2 - 2005 National Space and Missile Materials Symposium 14.3 - For those few of my readers who may be somewhat disaffected with their present daily job, and for the retired ones like myself, who may find a new and exciting part time activity, I would like to recommend a short exploration of the page were I recounted my experience with building my Welding Advisers site. It was for me a very positive experience especially as I had no previous familiarity with this kind of occupation. Therefore I am convinced that anyone can do the same and better, with proper guidance and help. There is nothing to risk, and a lot to be gained. To read my story click on How I built this Site. You may then follow the links...
See you next time Copyright (c) 2005, by Elia E. Levi and welding-advisers.com, all rights reserved -
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