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PWL#103 - Spot weld qualification, Sequential weld repair, Brazing Titanium, Plasma nano coatings March 01, 2012 |
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| We hope you will find this Letter interesting and useful. Let us know what you think of it. PWL#103 - Spot weld qualification, Sequential weld repair, Brazing Titanium, Plasma nano coatings, Plasma cutting retrofit PWL#103 - Destructive Testing of Spot Welded Specimens, How many weld repairs are allowed?, Brazing Titanium with Low Temperature Filler Metals, Plasma Spray Nano structured Coatings, Automated Plasma Cutting Retrofit, Welding Unknown Materials, Welding Copper and much more...
March 2012 - Practical Welding Letter - Issue No. 103
DON'T USE REPLY to send your messages! Use Contact Us instead. 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 think it may 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. The addresses reported hereafter were live and correct at the time of their publication. There is no guarantee that they will always be so, because they are administered by the sources themselves and are under their control. Note: References to articles or other documents are given here in If they are URL's (Uniform Resource Locator), which is the analogue of an address, they begin with "http://..." or "www.". These are not live and must be copied and pasted entirely into the browser (after having selected them with the mouse or otherwise). If they are long they may be displayed in two or more lines. In that case one has to care that the URL be copied completely in a single line without any space, and Enter. If the information is important to you as we hope, you may save the selected pages in a suitable folder on your Computer for easy reference. You are welcome to forward this page to those of your friends who may profit of this information.
1 - Introduction 2 - Article - Destructive Testing of Spot Welded Specimens 3 - How to do it well: How many weld repairs are allowed? 4 - Brazing Titanium with Low Temperature Filler Metals 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article - Plasma Spray Nano structured Coatings 8 - Site Updating: Welding Unknown, Welding Copper (R) 9 - Short Items 10 - Explorations: beyond the Welder 11 - Contributions: Automated Plasma Cutting Retrofit 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board (Sponsored Links)
1 - Introduction This 103rd Issue of Practical Welding Letter begins with an overview (2) of the testing methods used to qualify resistance spot welding schedules. This is an answer to queries requesting indications for destructive testing machine as if a single model, type and size could cover all requirements. Then the review of a research is reported (3), concerning the changes of properties obtained after sequential mock welding repairs performed on a certain test piece. In this report, dealing with a low carbon steel, no dangerous loss of properties was found even after five successive repairs. Follows the abstract (4) of a research conducted to develop low temperature (lower than beta-transus) brazing filler metals suitable for Titanium. While readily applicable successful results are reported, additional research is needed. The Office of Naval Research (ONR) sponsored studies for the development of suitable thermal spray materials reflecting the superior properties of nano structured materials in bulk. A short note reports (7) on successful achievements of these efforts that produced effective corrosion prevention materials now adopted for specific situations where they contribute to substantial maintenance savings. In the website update note (8) a new page is announced on Welding unknown materials, advising readers on what to do if the materials to be welded are unknown. Also a revised page on Welding Copper was updated with a note on copper contamination of steel and possible cracking. Automated Plasma Cutting Retrofit (11) reports on what seems a new process variation, proposed by a commercial company. Without any endorsement or recommendation it may be worth of readers' exploration to check if it may improve their operations. The other sections can be found at their usual place. Your comments and feedback are sought and welcome, as well as your contributions, reflecting your knowledge and experience. Please use the Contact Us Form. 2 - Article - Destructive Testing of Spot Welded Specimens More than once I got a query running like this: "We need to buy spot-weld destructive machine, please help to recommend the Brand and contact Info". Unfortunately such inquirers don't know what they really need. It is certainly my task to try to enlighten them, explaining the complexity of the problem. Therefore it may be useful to dedicate some more space here to show the origin of the requirements. The requirements are usually established in any given Specification that was selected by the designer as properly describing the suitable quality sought for assuring successful service of the item designed. A few Specifications or Recommended Practices are mentioned in my pages Resistance Welding Processes and Resistance Welding Tips. Alternatively similar requirements could be established internally by professionals with sufficient experience, to define the characteristics of any new design. These documents establish how quality is determined and which features must be examined and verified for conformance. There are visual requirements that are checked by visual examination and measurements, to check excessive surface marking, correct location and spacing, workpiece distortion, maximum depth of indentation allowable, absence of visible cracks or evidence of expelled metal. In preparation for a more complete testing, there are a few plain destructive tests that can be performed with simple tools while working on the development of a spot welding schedule. These tests are used for first orientation until promising parameters seem to have been found. A short description of some of them follows hereafter. Lap-shear type test-pieces, consist in two short sections of strip overlapping for a length and joined by the test spot weld in the middle of the overlapping area. The very first tests are done by clamping one end of the test piece in a vise and trying to rip apart the weld with a chisel forced by hammer blows to penetrate the overlap space. Any weak weld promptly falls apart, giving proof of weld inadequacy. Alternatively, for certain materials and thickness ranges, practically hard aluminum alloys around 1 mm, the spot weld may be destructively tested in a vise. For this to occur, the two parts of the specimen should be slightly offset before welding in the direction perpendicular to the long direction. Then the welded specimen is clamped in the width direction between the vise jaws and sheared by the force generated in it by turning by hand the screw bar to close the jaws. With somewhat longer specimens, a peel test is performed. The long leg of one part is clamped in a vise in the thickness direction and the specimen is vertical. The part is bent to one side, the other leg is caught in a pair of pincers or other simple tool and rolled back to pull the spot weld out. All the above tests are performed manually, without any instruments. If the parts separate too easily, the spot weld was inadequate to sustain any load. Only if much effort is needed to separate the elements and the base metal is torn apart, an instrumented test may be performed to measure the actual strength at rupture and to check if it is adequate. Spot weld strength per specification requirement depends on material, condition and thickness. It is known that a simple overlap specimen will undergo a certain rotation while being pulled for tensile test, because the two parts don't lay on the same plane. This means that the stress sustained by the weld is not pure shear, but is complex, combined with bend stress. In practice this consideration does not detract from the usefulness of the results. The reason why destructive testing is applied on spot welded specimens is due to the relative ease by which such testing is performed, and to the fact that measured strength values are significant and can be collected and studied as a baseline for assuring continuing quality production. Normally there is a procedure to follow in order to qualify a given set of relevant parameters, called a schedule, to be established as an official document for any needed job. Part of this qualifying procedure consists in performing a specified number of spot welds in representative panels, typically over 100, and then separating the test pieces by cutting or sawing them out of the panels (without altering material properties). The majority of the test pieces will be pulled in a calibrated tensile testing machine, to provide the value of maximum load at rupture of each test piece in turn. The load is expressed in force units per spot weld, like newtons or kilonewtons (or pounds). These machines, that prompted the inquirers to ask, can be manual, mechanical or hydraulic, and will usually perform in a range of forces. Selection depends on the maximum forces likely to be required for any given job, and on the additional features preferred. Furthermore the precision and the complexity of the machine will determine the price range. The measuring element capable to show the actual force will be probably a load cell, an electronic measuring device, with attached instrumentation. The specification minimum strength permitted is given in a series of tables, arranged per material, condition and thickness. There will probably be also a minimum average permitted, higher than the bare minimum, as a further condition designed to limit the variability of the results. Besides tensile requirements, the welds of the proposed schedule must be examined also for internal structure, minimum nugget dimension, maximum penetration in base metal, absence of cracks, voids or other objectionable discontinuities. This is done by sectioning the test specimens reserved for this test, mounting in a suitable resin for handling, grinding, polishing and etching and then examining them under an optical microscope to measure nugget position and dimensions, metallurgical structure and absence of defects. These operations are routinely done with suitable equipment in metallurgical laboratories. A minimal version describing what is needed for simple tests is proposed in my page on Weld Macro. Nothing forbids to delegate the metallurgical examination to a well equipped and competent external laboratory, provided the logistics of specimens delivery and getting reports does not affect production with undue long delays. Destructive testing as above is done when developing a suitable schedule for a new job. A limited series of similar testing may be part of continuing verification requirements during production runs. In principle also non destructive examinations (ultrasonic examination or radiographic inspection) could be performed on production parts to eliminate the danger of occasional defective spot welds. This however is only seldom required for critical specific applications, the routine testing being generally sufficiently reliable. Modern production instruments, capable of reading and recording actual parameters used by welding machines in real time, provide alarms when predetermined limits are exceeded, and permit continuous monitoring of correct operation. Together with fixed maintenance routines and periodic electrode substitution, these practices assure long periods of successful spot welding performance. Next time you need to procure spot weld testing equipment, you should decide on the needed force ranges to cover specimen testing, before asking for specific machines. 3 - How to do it well: How many weld repairs are allowed? You may recall having seen requirements limiting the number of allowed repairs performed on the same location, for the purpose of protecting the construction from weak spots likely to develop due to repeated weld repairs. If you are bound by such a customer requirement you can either argue on its necessity or abide to it. But now you can at least find a reference to a research program intended to test if there is a reason supporting this request. A group of researchers, knowing the limitations but unable to find on the subject precise indication of accepted Standards, undertake a testing program intended to verify, one way or the other, the influence of actual simulated cut and repair cycles on the properties of a welded joint. You will find the summary in the February 2012 Issue of the Welding Journal at page 25. In the specific case the test was performed on low carbon steel, ASTM A 283 GrB, flat plates 3/8 in. (9.5 mm) thick. Test pieces were welded together using direct current, with the gas metal arc welding (GMAW) process, in the flat (1G) position, with a wire of 1.2 mm diameter, type ER 70S-6 per AWS Specification A5.18, recommended for welding low-carbon steel. The shielding gas was 75% argon and 25% carbon dioxide. Six equal test pieces 200 mm wide × 440 mm long were welded after having been prepared with a 60 deg. bevel, manually cut with an oxyacetylene torch and cleaned with a grinding disk. After the face of the specimens was welded, the root was backgauged with a file and rewelded. The first specimen was put aside for testing. The remaining specimens were cut with the same methods and rewelded. The second specimen having one weld and one repair weld after cutting was put aside for testing. The other specimens followed the same procedure. The third had one weld and two repairs, the fourth one weld and three repairs, the fifth one weld and four repairs and the last one had one weld and five repairs. Test pieces removed from the six specimens were tested for bending, ultimate tensile, impact, elongation, average grain size, and metallographic structure of the HAZ. The results showed a remarkable homogeneity, without dramatic conditions likely to alert the researchers of dangerous worsening of basic properties. The conclusion reached in this program was that welding can be performed safely on the same area at least six times (one weld and five repairs) on low-carbon steel. My comment is that the conclusions may be correct for applications reflecting in all details the procedures of this research. It would be risky to extend them to different situations. Interested readers are urged to seek the original article reported above.
4 - Brazing Titanium with Low Temperature Filler Metals Brazing alloys used to braze titanium based materials must provide the required mechanical properties at the elevated service temperatures sustained by advanced components. Furthermore the successful types should be applicable at less than 800 0C (1500 0F), not to cross the alpha to beta transus temperature, that would result in microstructural changes, detrimental to the mechanical properties of titanium, especially in thin-wall applications. An article published in the Welding Journal of February 2012 on page 45 reports on a series of tests performed in an investigation continuing the work on testing aluminum-based brazing filler metals for titanium, applied at temperatures lower than the beta-transus. Some of the tested filler metals are proprietary alloys briefly described in Tables of the manufacturer's page at Two titanium base metals foils or thin sheets, as applicable for heat exchangers and honeycomb structures, were used for the tests, Grade 2 (CP titanium) and Grade 5 (Ti-6Al-4V). Some aluminum-based filler metals were tested in the foil form on the above Grades. It was found that the proprietary alloy TiBrazeAl-665® (Al-2.5Mg-0.3Cr) produced the highest strengths on both grades of titanium, while staying below the beta-transus. The article describes in some detail the results of the tests and concludes that two of the proprietary alloys, one in composite form, the other in bulk, cold-rolled shape, gave the best mechanical properties of the joints when brazed in vacuum. The researchers admonish that further work must be completed to determine the optimum brazing parameters including joint clearance, heating rate, and peak temperature in vacuum. These promising results may suggest studies for further applications of this technology. Interested readers are invited to obtain the original article reported above. 5 - Online Press: recent Welding related Articles GMAW guns, consumables, and more: 7 common questions answered Robot welding gets 5 times faster Learn to be a welder and jobs will be plentiful Nano-welding with a light touch TWI Connect - Download Jan-Feb 12 Issue - from 6 - Terms and Definitions Reminder Aligned Porosity means a localized distribution of porosity oriented along a line. Backing Weld is that made at the root opening, from the bottom side of the joint, to work as backing to the rest of the weld passes, to be deposited from the top side. Coil without support is a form of packaging of continuous wire filler metal, coiled in a compact large diameter bundle without any support, ready to be mounted on a dispenser for delivery to the machine feeder. Double-J groove is a type of joint having a double-J shape prepared surface on one side, abutting a planar surface on the other element. Edge Loss is the material lost over the edge of a thermal sprayed item. Flanged edge shape is a type of edge shape obtained by bending the edge to form a curved surface. Gas Shielded Flux Cored Arc Welding (FCAW-G) is a variation of the flux cored arc welding process where in addition to the shielding produced by the flux, further shielding is supplied by a shielding gas flowing through the torch. High Energy Beam Cutting is a group of thermal cutting processes where the cutting action is provided by High Energy Beams (Electron- or Laser-Beam) to melt and vaporize the material. 7 - Article - Plasma Spray Nano structured Coatings The February 2012 issue of Advanced Materials and Process Magazine, including the official newsletter of the ASM International Thermal Spray Society, reports on two significant achievements in successful plasma spray applications of advanced materials. The first, reflects original development work that was initiated by the Office of Naval Research (ONR) about fifteen years ago, on the favorable characteristics found in certain bulk Nano structured materials. The project aimed at producing Nano structured composite oxides and carbides, for testing then the properties of plasma sprayed coatings, in view of providing extended service life of definite components. One successful development was the processing method that produced a Nano structured alumina-titania agglomerate feedstock for plasma sprayed coatings of superior characteristics. The favorable test results brought about specific applications of n-AT coatings to several Navy components. For a critical shaft of nickel aluminum bronze (NAB), that suffered damage from the aggressive environment, the standard plasma protective coatings had been originally banned as not effective. Based on preliminary testing, a special authorization was obtained for testing on it the n-AT coating application. The examination of such a shaft after seven years of service, demonstrated zero wear of the coating and no signs of spallation. The success, translated in maintenance savings, was remarkable. All similar Navy shafts were therefore treated with the same coating system. The other achievements refers to progress obtained in plasma sprayed coatings for protecting metallic materials from heat and wear in automotive applications. Similarly zirconia ceramic based coatings are now demonstrated for protection of composite materials as carbon fibers, sintered nylon and fiberglass. While the tests are performed on race cars operating in very harsh conditions, the aim is to transfer the technology to commercial cars and to other applications. Interested readers are urged to seek the original articles in the magazine reported above. 8 - Site Updating: Welding Unknown, Welding Copper (R) The first of my Pages of this Month deal with welding in condition of uncertainty, when the materials to be welded are not known. The page can be found at Welding Unknown. The practice of welding anyhow is, IMO, risky and irresponsible. Is is not difficult to ascertain the real consistence of materials: a tutorial can be found in my other page on Material Identification The other page is a revised edition of an old page, refurbished to make it more useful. It is found at Copper Welding (R). Readers' feedback would be helpful if missing interesting subjects could be useful to somebody or if incomplete or even faulty information is occasionally found anywhere. Please let me know, by using the Contact Us button in the NavBar. Pages can be found using the Site Map or the Index Page. Also check the new or revised additions in the Welding Blog. Let your friends and colleagues know of this website. Forward them this page and ask them to Subscribe. 9 - Short Items 9.1 - Full Hard Temper corresponds to a cold-worked state beyond which the material can no longer be formed by bending without cracking. A full hard temper defined in terms of hardness or tensile strength corresponds approximately to a specific percentage of cold reduction (by rolling or drawing) following a full anneal. For aluminum, it could be a reduction of 75% from dead soft. for austenitic stainless steels, a reduction of about 50 to 55%. 9.2 - Galvanneal is a process intended to produce a zinc-iron alloy coating on iron or steel by keeping the coating molten after hot dip galvanizing until the zinc alloys completely with the basis metal. 9.3 - High Energy Rate Forging (HERF) is a process where the high velocity of the ram, rather than its mass, generates the major forging force to effect deformation of the workpiece in one blow. 9.4 - Mesh is the number of screen openings per linear inch of screen, also called mesh size. 9.5 - Nanohardness test is an indentation hardness testing procedure, relying on indentation force versus tip displacement data, to assess the resistance of surfaces to minimal penetrations, of the order of 10 to 1000 nanometer deep. 9.6 - Overheating means heating a metal or alloy to high temperature such that its properties are damaged beyond repair.
10 - Explorations: beyond the Welder How the First Plant Came to Be Microchip Implant Gives Medication On Command Lonely Planet: Social Media Gets on Board Can E-Bikes Displace Cars? NIST Sensor Improvement Brings Analysis Method into Mainstream 11 - Contributions: Automated Plasma Cutting Retrofit Somewhat hidden in an article praising the advantages of retrofitting plasma cutting systems with new components (The Welding Journal, February 2012, page 34), one can find short descriptions of proprietary new plasma cutting processes (page 36). This short note does not intend to endorse or recommend the processes described. However users in need to improve their existing processes might be interested to explore on their own if the advantages heralded in the article could provide them real benefits. One such process called WMS® (water mist secondary) uses nitrogen as a plasma gas and water as the shield for high precision cutting on nonferrous materials. It is said to provide a clean, dross-free, and oxide free cut surface ready to weld. For precision mild steel cut performance, the article suggests to select oxygen plasma with air shield. Precision cutting that eliminates the need of secondary operations is said to save time and effort. The article warns against trials to look independently for suitable retrofit components, explaining that expert support is needed for successful operational integration. Interested readers are urged to seek the original article reported above.
12 - Testimonials Date: 18 Jan 2012, 12:12:36 AM Name: Chris Jurgen
To: welding-advisers Date: 20 Jan 2012, 08:34:37 AM 13 - Correspondence: a few Comments 13.1 - You may recall that our last Mid Month Bulletin No.70 presented online resources on Weld Failure Prevention. A recent article published in the February 2012 issue of the Welding Journal on page 31 is titled Six Ways to Prevent Weld Failures. It deals with production failures, not service failures as our Bulletin. Although not easily found online except by AWS members, it gives serious advice that may help readers interested in the subject. 13.2 - Quite seldom, fortunately, but definitely at certain times, I get rude inquiries devoid of any good manners. It should not surprise anyone that I refuse to answer. 14 - Bulletin Board 14.1 - FABTECH Canada
14.2 - 2nd Int’l Electron Beam Welding Conf.
14.3 - 2012 International Thermal Spray Conference and Exposition
14.4 - How could SBI! help you Retire
14.5 -
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