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PWL#046 - MPW, Tin to Stainless, Brazing Ti Alloys, Lamellar Tearing, Consultation, GTAW Tips May 31, 2007 |
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| We hope you will find this Letter interesting and useful. Let us know what you think of it. PWL#046 - Magnetic Pulse Welding for Sheets, Welding Tin to Stainless, Filler Metals for Brazing Titanium Alloys, Lamellar Tearing, Welding Consultation, GTAW Tips for Pipes of Stainless Steels, Nickel Alloys and more...
This publication brings to the readers practical answers to welding problems in an informal setting designed to be helpful and informative. We actively seek feedback to make it ever more useful and up to date. We encourage you to comment and to contribute your experience, if you think it may be useful to your fellow readers.
You are urged to pass-along this publication to your friends, if you like it, and if you want to help them. If you received this from a friend and if you like what you read, please subscribe free of charge and you will also receive a bonus book on Practical HARDNESS
TESTING Made Simple.
1 - Introduction 2 - Article: Progress in Magnetic Pulse Welding 3 - How to do it well: Welding Tin to Stainless Steel 4 - Filler Metal for brazing Titanium Alloys 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article - Lamellar tearing 8 - Site Updating: Welding Consultation 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contribution: Alloy Pipe GTA Welding Tips 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction Here we are again with a new issue, in fact the 46th, of Practical Welding Letter The first article we propose deals with a new twist of Magnetic Pulse Welding, adapted to weld together overlapping sheets of aluminum and steel. It appears that this combination, quite impossible to perform with regular means, has interest for auto makers that strive to reduce weight while maintaining strength of the car body and resistance to impact. Once demonstrated the feasibility, it remains to be seen if economics will dictate adoption of this application in practice. A reader asked how to weld tin to stainless. A quite strange request that deserves nonetheless an answer. Titanium alloys are enjoying increasing diffusion for new applications that were not cost-efficient until some time ago. Of the applicable joining processes, brazing may present economic and fabrication advantages in certain situations. Filler metals for brazing Titanium alloys can be found within traditional types or as a special material class developed for unique uses. Lamellar tearing may be an insidious risk in certain welding constructions. We dedicate an article to this subject in response to renewed interest from the part of interested correspondents. For our Site Update we prepared this time a new page on Welding Consultation. Many industries were hard hit by the need to restructure and to reduce the number of professional staff members. Therefore the expertise that was available in house years ago, may be lacking in more recent times. If Management wants to maintain performance, productivity and quality at acceptable levels, it may need to be alert and ready to look for help where it is available, that is outside the organization. Despite the urge to find solutions inside, one has to understand the influence of production problems on the bottom line, and to overcome old habits and prejudice. We signal in the Contributions section a recent article from the Welding Journal offering tips for Gas Tungsten Arc Welding of pipes in stainless, nickel and special alloys. Those working on this type of welds may profit from the tips if they are ready to learn from this article. Other departments are in their usual places. We hope you will find interesting and useful subjects, and knowledge you can use in practice. We would like as usual to get your comments and feedback. Click on Contact Us.
2 - Article: Progress in Magnetic Pulse Welding Our readers may recall that we published an
article on Magnetic Pulse Welding Recently, in issue 43 of PWL for March 2007, we published an article (in section 7) on welding together incompatible materials like aluminum to steel and also a short note (section 4) where two commercial proposals are offered by industrial companies who developed other applicable processes. The requirements of automakers that look constantly for lighter structures, drive an intensive research in this hot subject, joining aluminum to steel. As MPW is a non fusion, cold welding process perfectly suitable for joining together these dissimilar materials, it is not surprising that new applications are sought trying to perform successfully those problematic joints. A new research conducted in Japan is reported in the Welding Journal for May 2007 (see Online Press, further down in section 5). The reasons for incompatibility of fusion welding of the two materials, already exposed in our article above, are summarized neatly in this report. Dissimilar Lap Joints made between overlapping sheets were investigated, with the magnetic pulse weld running parallel to the edges, somewhere in the middle of the overlapping area, similar in appearance to a resistance seam weld (that would not be successful with this material combination). Essential to the favorable results of these trials are the development of suitable The research demonstrated that effective Magnetic Pulse Welding was achieved in the experiments performed (as observed in metallographic sections through the interface) and that the mechanical properties of the joint exceeded those of the weakest of the two materials. Interested readers are invited to seek the original research report.
3 - How to do it well: Welding Tin to Stainless Steel Note: This is a real question from a correspondent. Q: How do you weld tin to stainless steel? A: You don't, due to the huge difference in melting point. If you need soldering on stainless you might pre-electroplate the stainless with tin, possibly by duplex coating (first nickel, then tin). Finally you may solder your connection to the plated tin layer.
4 - Filler Metal for Brazing Titanium Alloys Titanium Alloys have unique properties that recommend their selection for specific applications. In particular their excellent corrosion resistance is due to the adherent and stable titanium oxide readily formed on the surface. Having lower density than steel, and favorable specific strength (that is strength per unit weight) in certain temperature intervals, they are the preferred solution for applications like compressor parts of gas turbine engines. Titanium alloys can be described by the main phase in their microstructure. The alpha alloys have hcp (hexagonal close packed) structure, good strength, toughness and creep resistance. As they do not undergo ductile-to-brittle transition at any temperature (DBTT), alpha alloys are suitable for cryogenic (very low temperature) applications. The beta alloys have bcc (body centered cubic) structure, they undergo ductile-to-brittle transition and therefore are not suitable for low temperature applications. They have good formability and elevated strength to weight ratio. Mixed structures also exist depending on additional elements in the metal composition that tend to influence the proportions of the two basic structures. Alpha and near alpha compositions do not harden by heat treatment. Therefore heating and cooling for brazing do not affect their strength. The filler metal selected should not cause galvanic corrosion. Also it should not produce brittle titanium intermetallic compounds with copper, nickel and aluminum. Intermetallics with silver are less brittle, therefore silver base brazing alloys for joining titanium are sometimes preferred (like Ag, Ag-5%Al, Ag-7.5%Cu, Ag-5%Al-0.5%Mn, but also 72%Ag-28%Cu [eutectic]). Aluminum and alloys 4040 (Al-Si) and 3003 (Al-Mn) can be used. More complex alloys are Titanium based and include a large proportion of Zirconium. Titanium alloys are usually brazed in vacuum, sometimes with argon backpressure to avoid vaporization of silver and other elements. Torch brazing is not practical. Rapid brazing can be performed by induction or by infrared heating under inert atmosphere. Fast heating cycles may be preferred because of reduced risks of intermetallic formation. Beta and alpha-beta alloys are responsive to heat treatment, therefore the brazing cycle should be selected with care not to undermine the titanium alloy properties. Beta alloys can be brazed near the solubility temperature of the alloy (followed by quenching and aging at lower temperature, usually between 480 and 650 0C (900 and 1200 0F). Alpha-beta alloys can be brazed below the transition temperature from alpha to beta but only if the volume fraction of the beta phase is less than 20%. Fixtures should not be made of stainless or nickel alloys, because the mere contact at high temperature may result in unwanted joining. In the following research articles, evaluations were performed using different filler metals for brazing of Titanium Alloys. Brazing of Ti-6Al-4V and Niobium using Three Silver-Base Braze Alloys Brazing of Titanium Using Low Melting Point Ti-Base Filler Metals Electron Beam Brazing of Titanium for Construction in Space
5 - Online Press: recent Welding related Articles Application of Magnetic Pulse Welding for Aluminum Alloy and Steel Sheets Modular System of Powder Nozzles for Laser Beam Build-up Welding Using Laser Powder Cladding to Build Up
Worn Compressor Blade Tips Weldbonding From TWI
6 - Terms and Definitions Reminder Brazeability represents the relative ease (or difficulty) of brazing given materials with certain processes using fluxes or atmospheres as recommended. It is a concept similar to that of wettability that indicates if the filler metal tends to spread at the brazing temperature and to wet the base material. Caulking is the plastic deformation of weld metal and nearby surfaces by hammering or pressing to seal discontinuities. Gas Lens is a collar of porous material that is attached to the torch of Gas Tungsten Arc Welding (Tig), to provide laminar flow of the shielding gas (usually argon) around the electrode. Advantages of its use are longer electrode extension possible, unimpeded view and better gas flow. Positioning of fusion welding is the mutual relationship of weld pool, joint, members and heat source during welding. Preferred whenever possible is the flat position that guarantees the deepest penetration and the highest deposition rate. Other positions requiring more skill are horizontal, vertical (both -up and -down), and overhead. If pipes can be rotated, welding can be performed in flat position, if not all the positions come into play in different sections of the joint. Press Fit or Interference Fit is an assembly method, temporary or definitive, that is based on expansion or contraction of one or both of the components. It is usually performed by generating a thermal differential by heating or cooling, and possibly using a press to insert one of the elements around or inside the other. A tight fit, having sufficient interference to oppose adequate resistance to displacing forces, can be used instead of welding if practical. Self fixturing is the preferred method of assembling elements and holding them in place for furnace brazing. Different joint details are suitable, depending on the shape and requirements of components. Solubility of gases in liquid metal can be much higher than in solid, causing problems, especially if the gases are expelled from the solidifying mass and cause porosity and cracks. Thermal conductivity is the property indicating the ability of a material to conduct heat or the rate at which heat is transmitted by conduction. Large differences exist between different metals and must be taken into account when welding, because of the influence on heat dispersion (or on the contrary on local heat accumulation) and on the tendency of causing deformations in weldments. The good heat conductors aluminum and copper are difficult to weld by oxyacetylene (low heat source), because heat is readily dispersed by conduction.
7 - Article - Lamellar tearing Following renewed interest from our correspondents in this subject, that was dealt with briefly in section 6 of issue 11 of PWL for July 2004, we are going to expand on it hereafter. Lamellar tearing is a kind of decohesion occurring inside rolled plates as a consequence of stresses, running perpendicular to the rolling plane, that usually originate in nearby welds, either immediately or some time later. Sure enough decohesion originates at nonmetallic inclusions, usually sulfides or silicates, deformed by the rolling process into thin patches. Thick plates of strong and hard steels appear to be more susceptible to tearing. In metallographic cross sections lamellar tearing looks like cracks, often similar to parallel steps, possibly joined by short branches. Tearing appears in the base material, not in the fused or heat affected zone, and normally it does not reach the surface. Of the non destructive methods, only ultrasonic testing is suitable for finding this defect. Tearing can also appear as a consequence of flame cutting or cold shearing, caused by stresses in the thickness direction. Three factors are said to cause lamellar tearing when they appear simultaneously:
The tendency to the formation of lamellar tearing is primarily countered by paying special attention to the joint design so that stresses are reduced through the thickness. For plates meeting at 900 (the edge of the vertical plate being flush with the upper surface of the horizontal one) the bevel should be made in the vertical susceptible plate, while the other one should be welded across its edge. Sometimes partial penetration is preferred, when acceptable, because of lower stresses being generated. Low yield filler metal, if selected, lowers stress level in the weld and reduces tearing risk. The plates can be specified with low sulfur content, 0.010% max. or a treatment of molten steel (before rolling) with rare earth elements can be prescribed, that will modify the morphology (the form) of inclusions, to resist deformation under rolling, therefore avoiding their flattening, dangerous to tearing. Special ultrasonic testing can be requested to reject plates with laminations. Tensile test per ASTM A770 of specimens machined in the thickness direction of plates can be specified to show minimum 20% reduction of area: this gives a good chance of freedom from lamellar tearing.
8 - Site Updating: Welding Consultation The Page of this Month in our website deals with the need to look for external help, when in house experts cannot overcome the difficulties. The recent trends of restructuring and globalization hit hard on the delicate balance of expertise among in-house available professional workforce. The problems may be evident if production is reduced and scrap rate increases. But they may be hidden, they only cost time and money and nobody seems to notice. Especially dangerous are ill-advised changes introduced under the deceptive pressure of saving. Once in a while it would pay to take stock of welding production, before being blocked by lack of workforce or of suitable means. There may be resistance to acknowledge the need for help. But is it wise to refuse stubbornly to accept the limitations of reality? It is easy to inquire what could be done to get out of choking hindrances. See the new page by clicking on Welding Consultation. For seeing the titles of all website pages and for reviewing a brief description of their content click on the Site Map. Let us have your feedback by using the form available by clicking on the Contact Us button from the NavBar of any page of the website.
9 - Short Items Charpy Test is an impact test in which a V-notched, keyhole-notched, or U-notched specimen, supported horizontally at both ends, is struck behind the notch by a pendulum striker. The striker is mounted at the lower end of a pivoted bar. Its energy derives from its mass and from the speed it gains while falling from the height of its resting position to the lowest place, where it strikes the specimen. The energy that is absorbed in the test is calculated from the height to which the striker would have risen in the absence of specimen (almost equal to the starting height) and the one it reaches after fracturing the specimen. Fractography is a term describing the science of studying aspect and features of materials fracture, with reference to presence or absence of characteristic signs or marks, generally on photographs of the fracture surface at different magnifications as suitable for the investigation. Both optical and electronic microscopes are used. It is an important tool for failure analysis. Fretting Corrosion is the accelerated deterioration at the interface between contacting surfaces as the result of corrosion and slight oscillatory relative movement between the two surfaces. It is often characterized by the detachment of particles and subsequent formation of oxides, which are often abrasive and contribute to increase the wear. High-Strength Low- Alloy (HSLA) Steels are designed to provide better mechanical properties and/or greater resistance to atmospheric corrosion than conventional carbon steels. They are generally characterized by properties, not by chemical composition. HSLA steels have low carbon contents (0.05 to 0.25% C) in order to produce adequate formability and weldability, and they have manganese contents up to 2.0%. Microalloying is the purposeful addition of limited quantities of chromium, nickel, molybdenum, copper, nitrogen, vanadium, niobium, titanium, and zirconium in various combinations to enhance specific properties or fabrication capabilities. Izod Test is a type of impact test in which a V-notched specimen, mounted vertically, is subjected to a sudden blow delivered by the falling weight at the end of a pivoted arm. The energy absorbed while breaking off the specimen's free end is a relative measure of the impact strength or toughness of the material. Weldbonding is a hybrid joining method combining resistance spot welding and adhesive bonding. A spot weld is made through the adhesive applied to one sheet of a lap joint. The spot weld displaces locally the adhesive without impairing its properties. The adhesive is then cured to complete the process. See on this subject an article in section 5 above. 10 - Explorations: beyond the Welder Boeing Helicopters being made EWI Corporate Video Green Gold in a Shrub When Milky Way and Andromeda Collide... Home Surveillance Guide
11 - Contribution: Alloy Pipe GTA Welding Tips The fine tips given by someone of rich experience are gifts that should not be overlooked by anyone, whatever his or her previous proficiency with the matter. This is the case with the informative article published in the most recent issue of the Welding Journal for June 2007, at page 58. Although we try to present here the highlights, the thorough reader would be well advised to seek the original source. The authors advise that previous knowledge of gas tungsten arc welding practice with carbon steel, although important, has to be adapted when upgrading to weld complex alloys like stainless steels, nickel base and related alloys especially in pipe shape. In particular one should remember that the added elements, while contributing to mechanical and other properties like corrosion and heat resistance, make pipe welding more difficult. First tip is to slow down to the correct speed for each metal, because the weld pool is less fluid and more time has to be allowed to wet the joint and to obtain full penetration. Also it is suggested to resist the desire to input more heat to make up for speed, lest composition is altered with serious consequences. Provisions for purging the pipe with shielding gas must be carried out as needed to protect the inside against oxidation. Poor backing gas coverage is readily found upon inspection. Argon is generally used at 10 to 20 cuft/h (4.7 to 9.4 l/min), but mixtures with helium at higher flow (1.5 to 3 times more) are recommended for nickel alloys thicker than 1/8" (3.2 mm), as helium promotes a hotter arc for increased penetration and speed. Installing a gas lens (see section 6 above) assures a smoother flow, the use of the largest suitable cup is suggested. 2% Thoriated tungsten electrode extension should be kept short, 3/16" (4.8 mm) or less on butt joints, may be longer for thicker materials or fillet welds. Arc length should be short for better control. Power supply should be set for DCEN (electrode negative). High frequency start is preferred. Metal area and filler material should be cleaned with a solvent, (WARNING: flammable!) to be removed from the weld area before striking the arc. Cleaning and proper preparation are an important factor in welding success. Root opening should be wide, filler metal can be used to set the dimension if suitable. Additional detailed tips are offered regarding position of torch and filler, comments on tack welds, on tie-in, on convex beads, on stringer beads, on crater cracks and on extended postflow. A copy of the original article should be obtained, studied and kept for reference until satisfactory results are obtained with gas tungsten arc welding of these difficult alloy pipes. 12 - Testimonials Sun May 06 08:36:41 2007 Thanks for the abundant information and sharing it
with this novice.
From: Mahsa Seyyedian (E-mail address removed for security) Dear Mr. Levi,
yours faithfully,
13 - Correspondence: a few Comments 13.1 - I have nothing against students that submit to me examination questions they should answer to. The may learn also by this process. I would like however that they tell me that much, not presenting the questions as coming from their own pressing urge to know. 13.2 - It happens again. People who receive my newsletter click on Replay just to send me a short note. Please click on Contact Us button instead, do not copy the whole newsletter again, this only fills up the mailbox with no other useful outcome.
14 - Bulletin Board 14.1 - Explosion of New Processes 14.2 - Weld Cracking VI From SiteSell: 14.4 - Make Your Knowledge Sell! 14.5 - Watch the Sitesell.com
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