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PWL #028-Welding Defects, Joining Silicon Bronze, Brazing in Space, High Tech Ceramics, Positioners December 01, 2005 |
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| We hope you will find this Letter interesting and useful. Let us know what you think of it. Welding Defects, Joining Silicon Bronze, Brazing in Space, High Tech Ceramics, Positioners, Aluminum brazing to Cast Iron 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.
Date: December 2005 - Practical Welding Letter - Issue No. 28
1 - Introduction 2 - Article: Welding Defects 3 - How to do it well: Brazing and Welding Silicon Bronze 4 - Filler Metal for Brazing in Space 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article: High Performance Ceramics 8 - Site Updating: Welding Positioners 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Aluminum brazing to nodular cast iron 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
This new 28th issue of Practical Welding Letter, that closes the year 2005, starts with an Article including some considerations on the definition of weld defects and on their relative importance depending on the intended purpose and service of parts. The need for investigations to point at the real causes of defects, out of a numerous list of possible factors, is stressed as a major effort required towards economic production. In the Section on "How to do it well", joining methods are described, suitable for Silicon Bronze, a versatile material used for artistic castings. The note was written to answer a specific question proposed by one of our readers. A short report follows on an article that appeared in the technical press describing programs for the development of cost effective brazing processes useful for the manufacturing in Space of large dimension truss structures. Much progress was recently achieved in the characterization and manufacture of High Performance Ceramics permitting the realization of implements meeting exacting requirements. The expectations of rapid advancement, that were believed to be easily attainable a few decades ago, are slowly and painfully being won only now through tireless and continuous research work. The new Page of the Month presented in the Site Update is devoted to a review and description of capabilities of welding Positioners, those helpful working aids that promote productivity and quality. The Contribution of this issue stems from a question of one of our readers, originating from an academic research relative to the possibility of brazing aluminum to nodular cast iron. The difficulties are outlined together with a few hints about possible ways of circumventing them in order to realize successful brazed joints. Other departments follow as usual, providing insight and information on more arguments and subjects. Send this letter to your friends who may enjoy it. We welcome your input and comment. Write us by e-mail by clicking on Your Questions and Feedback.
2 - Article: Welding Defects The definition is reviewed in our page to be seen by clicking on Welding Defects. Essentially a discontinuity or flaw is called a defect if it exceeds the acceptance limits established by engineering based on Fitness for Service criteria. Therefore it is improper to upgrade a weldment from one use to another without investigating the stress conditions, the environment and the life expectancy in the new application. The engineering decision is to be left to those having the education and preparation to assume that responsibility upon themselves. Weldments including defects must be either repaired or discarded. The methods used to look for, find, identify and assess the importance of discontinuities susceptible to be defined as defects, are the object of Welding Inspection. Weld defects can originate from many causes. It is important to investigate the origins of recurring weld defects in established production lines, in order to provide for the correction of the conditions generating them. Improper design, deficient working conditions, defective base metal, incorrect preparation, insufficient cleaning, inadequate fixturing, unsuitable welding procedures, faulty equipment, inexperienced welder, substandard consumables and bad housekeeping, insufficient heat treatment are among possible reasons for defective production. Whatever the defects, it is sure that having to repair them, or worse, to reject welded parts, is costly in terms of loss of materials, labor and delivery time. Not to mention Customers' confidence and reliability. Therefore every effort should be used to minimize them. An important aspect of this struggle should be devoted to determine the causes of defects and failures, and in finding ways to avoid them in the first place, by making defect free production. One formal way, adopted in advanced industries, to deal with production failures consists in submitting the research of their causes to a suitable professional team, in charge of investigating the nonconformance and of recommending appropriate ways to avoid its recurrence. Training and refreshing welders' skills, explaining the causes for appearance of cracks or porosity, showing to the workers defective metallographic sections, explaining the importance of adhering to well constructed and successful welding procedures, should all contribute to improve performance. Well conceived bonus programs for defect free accomplishments may play a role in associating the welders to the success of the enterprise. They may therefore find and suggest new ways to improve their results. They may request better working aids or conditions as directly influencing the outcome of their work and better serving their personal interest and that of the firm.
3 - How to do it well: Brazing and Welding Silicon Bronze Silicon Bronzes and Brasses are copper alloys containing Silicon as the main alloying element. Silicon Bronze is ideal for the hobby foundry because the material does not change composition upon remelting many times. Silicon Bronze has excellent chemical resistance. Its composition is approximately 95% Copper, 4% Silicon, and 1% Manganese. Its pouring temperature is 1010 to 1177 0C (1850 to 2150 0F). Pouring characteristics are very good and its surface finish is excellent. The main use of this material is for artistic sculpture. It can be brazed with oxyacetylene and a suitable flux. The filler metals used are low temperature silver base so that the cast metal is not melted in the process. In case the base metal is in highly stressed condition it might develop hot shortness, by cracking upon contacting molten filler metal. To avoid this, the parts should be stress relieved at 246 0C (475 0F) before brazing. Surfaces should be machined or mechanically cleaned just before brazing. Gas Tungsten Arc Welding can be used with direct current straight polarity (electrode negative) and Silicon Bronze filler metal RCuSi-A, if needed for sections thicker than about 3 mm (1/8"). Thoriated Tungsten electrode (EWTh-2) is used for best life and minimum tip maintenance. Shielding gas is usually argon.
4 - Filler Metals for Brazing in Space In our Issue No. 24 of Practical Welding Letter for July 2005 we briefly touched the subject of Welding in Space. Click on PWL#024 to see that note in Section 7 there. The evidence that manufacturing in Space is a hot matter of active development is further emphasized by the fact that a recent issue (October 2005) of the Welding Journal reports on current efforts performed by NASA to investigate and assess vacuum brazing as a joining process for the assembly of large truss structures in space. That interest stems from the need to transport high up the constructive elements in minimum volume, for then being able to assemble the required structures where they are needed. Vacuum brazing is one of the mature terrestrial joining technologies that may be ideally suited, if properly adapted, for use in space. For one thing vacuum is readily available in the environment there, without any effort. The cost effective robotic technology developed for assembling such structures consists in electron beam brazing of ultrathin titanium tubing to special fittings designed to provide the six way nodes or points of convergence of the triangular sections of truss structures. The low temperature (around 600 0C) silver based filler metals and new ones yet to be developed will be preplaced (snap-on) outside of the tube ends. The robot is charged to introduce the tube into the fitting. A brazed joint is to be realized between the external tube surface and the internal surface of one of the short conforming tubular sections of the six way node fitting. The short time electron beam heating for brazing will be achieved by directing the beam on the internal tube surface and moving around the point of impingement to describe a whole circle. A very low power electron beam gun is sufficient to perform the task. Interested readers are urged to see the original article at page 25 in the October 2005 issue of AWS Welding Journal.
5 - Online Press: recent Welding related Articles As announced in the PWL previous issue, No. 27, our Article on Aluminium International Today, 2005 Online Archive can be seen at From AWS From TWI From NIST
6 - Terms and Definitions Reminder Acethylene Feather or feather extension of the flame inner cone is obtained from an excess acetylene or carburizing oxyacetylene flame. Aligned Discontinuities are generally found along a line parallel to the weld seam. If the distance between any two single discontinuities is greater than three times the largest discontinuity dimension, they would generally be considered separate and not aligned. Backing serves the purpose of supporting and shielding the root side of a joint. It may be a metal, in which case it may or may not become partially fused and part of the joint. Or it may be a flux. Carrier Gas in Thermal Spraying is used to transport powder particles, from the metering supply, through the flame or arc and project them upon the part to be coated. Duty Cycle is the percentage of active time in any ten minutes period, when a power source or other device can be operated at a given power level without risk of overheating. Joint Clearance in brazing is the gap controlling capillarity between faying surfaces of a joint. Square Groove is a type of joint having the abutting surfaces parallel to one another and square to the plane surface. Tungsten Inclusion is a flaw consisting in a bit of tungsten electrode (from GTAW) embedded in weld metal. It can be found by x-ray and ultrasonic inspections.
7 - Article: High Performance Ceramics Ceramics are usually associated with pottery and chinaware, materials and technology dating back to more than 10,000 years ago. Commonly used modern ceramics based on clay include tableware, sanitary ware, walls and floor tiles, electrical insulators, as well as ceramics for civil engineering. These inorganic, non-metallic materials, however, are now providing new materials and technology at the forefront of industrial development. They are generally moulded from a mass of raw material at room temperature, and gain their typical properties through a high temperature firing process. Thanks to focused research driven by higher and more exacting requirements, recently developed forming and manufacturing techniques have promoted the production of advanced ceramics. Their properties have the potential to solve technical and engineering challenges that were regarded as out of reach only a few decades ago. Generally the properties are a function of the high quality and purity of the materials used and of the special chemical processing used. High Performance or High-Tech or Engineering Ceramics have only the name in common with their illustrious predecessors. Their unique and extraordinary physical, thermal and electrical properties permit new development opportunities for applications in widely different industries. In general terms, advanced ceramics exhibit exceptional properties that make them highly resistant to melting, bending, stretching, corrosion or wear. Among the most useful properties of high performance ceramics one lists:
Among the drawbacks that hindered in the past more rapid diffusion of ceramics one lists:
On the subject of joining ceramic materials by brazing, see the Section 4 in the July 2005 issue of Practical Welding Letter No. 23, by clicking on PWL#023. Oxide ceramics include:
The most important non-oxide ceramics are:
One important class of High Performance Ceramics is that of Ceramic Matrix Composites. This class, different from that of monolithic homogeneous material, is characterized by reinforcing structures interspersed within the matrix. The purpose of the reinforcing second phase is to improve those very characteristics that, in monolithic ceramics, are unsuitable for certain applications.
8 - Site Updating: Welding Positioners The Page of this Month describes Welding Positioners and the reasons for adopting their use in industrial manufacturing. If well selected and implemented they have the potential to boost productivity, vs. manual welding operations. They are suitable for certain processes only, but are quite adaptable to accept different bodies of the same general shape. Obviously they have limitations, that should be respected. They also improve quality, because welding is consistently performed in the best welding position. They help welders perform a lighter and less physical demanding work. And they reduce the occupational risks connected with moving around heavy structures. To see this new Website page click on Welding Positioner. To review the titles of all pages click on the Site Map.
9 - Short Items Beryllium Mirrors are being manufactured for NASA James Webb Space Telescope. This telescope will display 18 exagonal mirrors arranged to form the primary reflecting surface. The selection of Beryllium is due to its low density and to its performance at cryogenic temperatures. The blanks measuring 1.5 m (almost 5 feet) and weighing 250 kg (553 pounds), obtained from compressed and sintered powder, will be precision machined so that each segment weight will be reduced to 21 kg (46 pounds). Then they will be ground and polished to obtain optical surfaces. The total processing time is estimated to about 53 months(!). Damage Tolerance is a design criterion measuring crack growth rate. It is used to assure a defined service life to components or assemblies in presence of unwanted faults. Their dimensions are assured not to grow to critical size during the useful life. Glow Discharge Mass Spectrometry is a powerful analysis technique for trace elements in materials. Atomization is performed in solid form by sputtering in a low pressure DC plasma. Sputtered atoms are ionized in the plasma and then passed through the mass analyzer for separation and detection. Holographic Inspection uses recordings called Holograms to encode the shape of any arbitrary object. The regenerated three dimensional image obtained by illuminating the Hologram with a coherent beam of radiation is then used as a kind of template against which any deviations in shape or dimensions can be observed and measured. Optical and Acoustical techniques exist that permit performing of sophisticated non destructive inspections for detecting weak points in manufactured structures of various materials and processed parts. Thermal Analysis includes the study of numerous phenomena that occur in metals and alloys following changes of temperature. Phase transition temperatures, dilatometry, thermo-mechanical analysis and determination of thermodynamic properties of advanced materials are among the subjects investigated by this branch of physical study having many implications also for welding technology. X-Ray Fluorescence Spectroscopy is a non destructive analysis technique that permits qualitative identification of materials with simple instruments, but also quantitative determination of composition with more advanced equipment. Atoms excited to metastable condition by x-rays of sufficient energy, regain their stable status by emitting x-rays of characteristic energy. As each element has a unique set of energy levels, it produces x-rays at a unique set of energies, that permit analysis and identification.
10 - Explorations: beyond the Welder On Metallurgical Topics see Drive : Home-fuelled Car Or Vice-versa? On Nobel Prize Winners 2005 Spacecraft makes a grab for asteroid sample 2005 World Malaria Report
11 - Contribution: Brazing Aluminum to Cast Iron A question was submitted on how to braze aluminum alloy to nodular cast iron with filler metal AlSi-12 (Aluminum - Silicon eutectic) or alloy 4032. The aluminum alloy was not defined, so that its melting range is unknown, relative to that of Aluminum Silicon Eutectic. We will presume it is one of the brazeable types, and that the mechanical properties obtained after brazing, although near the annealed condition of the specific aluminum alloy used, are still acceptable for the application involved. The process is not classic brazing, that depends on capillarity for the spread of filler metal, but braze welding. Click on Braze Welding to find our page on this subject in the website. Having access to a technical library one can find information in the ASM Metals Handbook, eighth edition (not the last one), Vol. 6, page 682. Essentially the cast iron has to be protected against oxidation during preheating. That can be done by cleaning first, and then either electroplating with Cu, Ni or Zn, or hot dip coating with Al, Ag, Sn or Zn. Thermal spray can also be used if acceptable. Brittle aluminum-iron phases have to be avoided by the above means. Preheating should be rapid and brazing time must be minimized. Braze-welding should be done with suitable fluxes, preferably by dip brazing in molten flux, but it is also possible by torch brazing. Braze-welding range would be 580 to 600 0C (1076 to 1112 0F).
12 - Testimonials From: NormA1A@aol.com Elia, Thank you so much for your input. [...] Thank you again for your help. I will be telling my friends about your site. Norm Schlinger
From: "Yoppong.Chua" 'Yoppong.Chua@airfoil.com.my'" Dear Sir, Thank you for your quick response; most appreciated. Regards, Chua
13 - Correspondence: a few Comments 13.1 - We are receiving from time to time questions from students busy with their academic assignments. It is for us refreshing and pertinent to be consulted on such subjects. However we find out, most of times, that we are the first to be consulted, without any preparatory work. Therefore the questions tend to be unfocused and too general to be answered in brief. We would like to suggest that at least some bibliographic search be performed in the library before asking us. 13.2 - When we answer with a suggestion to some specific question, it is only natural that we would like to know if our input was of any help. It is true that the advice is based only on the information provided by the inquirer, so that it may be only approximate if any essential data were hold back. Anyhow it would be nice to know the outcome. Keep us informed, please. 13.3 - Although the form used to collect Questions and Feedback includes a line requesting to repeat the e-mail address, (to avoid typographical errors) and we copy and paste addresses exactly, we get sometimes bounced back notices, informing us that the message did not go through. One possible reason might be that the input folder of the recipient is full and will not accept any more messages. Readers are invited to check from time to time their available space and to write us again if they fail to receive our answer. Because we always answer, unless our message is bouncing back undelivered.
14 - Bulletin Board 14.1 - 2006 Intl. Conf. on Tungsten, Refractories & Hardmetals VI 14.2 - 5th International Surface Engineering Congress 14.3 - Do you know a Mom that thinks about working at home?
Best wishes for the coming New Year 2006 Copyright (c) 2005, by Elia E. Levi and welding-advisers.com, |
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