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PWL#047 - FSW, Spatter Reduction,Testing Filler Metals,Abrasive WaterJet Cutting, CaseHardening, MMC June 27, 2007 |
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| We hope you will find this Letter interesting and useful. Let us know what you think of it. PWL#047 - FSW (Friction Stir Welding), Spatter Reduction in FCAW, Testing Filler Metals, Abrasive Water Jet Cutting, Case Hardening, Metal Matrix Composites 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: Friction Stir Welding 3 - How to do it well: Reducing Spatter in FCAW 4 - Testing Filler Metals 5 - Online Press: recent Welding related Articles 6 - Terms and Definitions Reminder 7 - Article - Abrasive Water Jet Cutting 8 - Site Updating: Case Hardening 9 - Short Items 10 - Explorations: beyond the Welder 11 - Metal Matrix Composites 12 - Testimonials 13 - Correspondence: a few Comments 14 - Bulletin Board
1 - Introduction This new 47th Issue of Practical Welding Letter opens with an exposition of a relatively new welding process, namely FSW, that in less than two decades has reached a remarkable development in quite distant fields of industrial applications. This level of acceptance is a clear indication not only of the potential of the basic idea but of the passionate dedication and efforts of all involved and of their determination to demonstrate by ingenuity and hard work that there is no barrier to prevent the will to solve technical problems. Spatter can be a costly problem, especially for Flux Cored Arc Welding if one is at loss in trying to reduce its effects. Nevertheless on should strive to optimize the parameters, especially voltage, the factor having a paramount influence on this drawback. Not all Filler Metals, even if meeting the requirements of a certain specification, can be presumed to be completely equivalent. This is the lesson one can learn from a new document. It is my understanding that, at least for important projects, it would be necessary to test and to approve specific makers of Filler Metals. Then we go on with a description of Abrasive Water Jet Cutting, the only cold process that can be used for heat sensitive and explosive materials like obsolete ammunitions. For our Website Update we recall a few basics on Case Hardening, a suite of different processes aimed at providing surface hardness in specific locations of steel items. And we call a warning against inappropriate welding applications on case hardened materials. Finally we present an article on Metal Matrix Composites, a class of new materials engineered to show special properties and to perform unique functions in harsh environments. The other departments appear at their usual place with more information and additional links. We hope you enjoy what you read. We would like to know if this publication is useful and interesting. To write us your e-mail click on the Contact Us button in any page of our website. 2 - Article: Friction Stir Welding The process known as Friction Stir Welding was invented and patented at The Welding Institute (TWI) of Cambridge, UK, in 1991. Investigations started with modest laboratory testing. Over the years many research projects explored different aspects essential to understanding theory and practice of the process and involving many variations. Tools, materials, working conditions and properties obtained were investigated over the years, to the point that significant industrial applications became possible and successful in diversified industries, from aerospace to automotive, from shipbuilding to production of large aluminum panels made from extrusions. Although applied primarily for welding aluminum alloys, FSW use had been demonstrated for copper, lead, magnesium, zinc and titanium alloys, as well as for steel, stainless steel and dissimilar material combinations. A special rotating nonconsumable heat resistant steel tool of conical form often with some thread like protrusions, and roughly similar to an end mill with a contact shoulder, is plunged and then traversed along the joint while rotating. The tool movement tears bits of the base material reducing it to a mass of solid plasticised phase under the friction generated heat. This material of plastic consistence is moved from the front side of the tool's progress to its rear side, where it cools down making metallurgical contact, i.e. coalescence (welding) to the undisturbed base metal, while the tool advances along the joint completing the weld. The process is performed in the solid phase, inasmuch as the melting point is not reached, permitting application to materials that cannot be fusion welded without unacceptable damage. FSW presents many advantages over fusion processes, especially due to the low heat input and because of the efficient use of energy. The main parameters to be controlled in an automated FSW machine are, the downward force exerted on the tool, the weld speed, the rotational tool speed and its tilt angle, besides the tool shape and material. Distortions are limited, internal defects are virtually absent, welding is performed by a fully controllable machine providing repetitive and reliable results with predictable properties, without the use of skilled labor or of shielding gases and filler metal. Limitations consist in the fact that the machines must be sturdy to clamp rigidly the parts and therefore expensive, that the weld ends with a hole that cannot be filled because filler material is not used (so that circular welds are not done except if a conical plug can be friction welded to fill the keyhole by an additional and different process). Equipment, originally evolving from heavy duty vertical milling machines is fabricated by an increasing number of manufacturers who reached their experience through interaction with dedicated users. It is generally built around the requirements of the items to be welded. A large body of experience and information is readily available. A Review of three Articles on Progress in FSW, published in the March 2006 Issue of the Welding Journal was presented in issue 32 of PWL. Click on PWL#032 to read it. 3 - How to do it well: Reducing Spatter in FCAW Q: How can spatter be reduced in Flux Cored Arc Welding? A: For a general overview of this process see our website page on Flux Cored Arc Welding Tips and other pages referenced thereby. FCAW is often perceived as a low cost process, even for Hobby and Home work, in that shielding gas is not required (flux cored wire is self shielded), thus reducing equipment cost and simplifying procurement of consumables. For industrial applications shielding gas (for steel mostly Argon with 8-25 %CO2) is almost always employed, with remarkable influence of the gas mix on the arc and on the resulting welds. It is also claimed to be easier to master than Gas Metal Arc Welding (GMAW), in that only basic skills are required to obtain acceptable welds in all positions. Penetration and deposition rate are higher than for Shielded Metal Arc Welding. The often cited additional advantage is that flux cored filler material, by virtue of special ingredients in the flux can be more tolerant to the presence of rust or mill scale on the steel. The production of thicker smoke and fumes is considered an advantage when welding outdoors because an occasional light breeze would not remove the shielding effect around the weld. It can be a nuisance and a health risk if welding indoors, unless fume extraction is in place to protect the welder. Slag has to be removed in any case after welding and before any additional weld is done on top of the deposited weld beads. When using traditional constant voltage power supplies the polarity selected is mostly DCEP (Direct Current Electrode Positive) that gives a stable arc, low spatter (at the correct voltage), a good weld bead profile and optimum penetration It is important to know which metal transfer mode is used. At lower currents the short circuit transfer mode is operating, usually when welding steel less than 3 mm (1/8") thick. Spatter is best controlled by using voltage adjustment to obtain a crisp, consistent crackle sound. One should learn from practice to recognize the correct sound associated with short circuit welding. As an indication, the starting voltage for short circuit applications with flux cored wire of size 0.8 - 1.0 - 1.2 mm(0.030 - 0.035 - 0.045") is 16 to 18 V. The corresponding wire feed speed could be 1.8 to 10.7 m/min (70 to 420 inch per minute), that would provide 50 to 170 Amps, 65 to 200 Amps, and 130 to 220 Amps for the three wire sizes. If the crackle of the weld consists in a soft plop sound with some spatter, reduce voltage one volt at a time until the correct sound is generated and spatter is eliminated. If on the contrary the sound is harsh and explosive with no soft sounds, then increase one volt at a time until spatter is substantially reduced. With higher current levels the metal transfer becomes the spray mode. Here the arc length should be kept minimal and again one should strive to obtain the consistent crackling sound already described. Voltage for spray mode would preferably be between 24 and 34 V, a good starting point would be 30V. For 1.0 mm (0.035") wire size the wire feed speed could be between 10.7 and 14.2 m/min (420 and 560 ipm) that would provide 215 to 300 Amps for a normal stickout (electrode extension) between 13 to 16 mm (1/2 to 3/4"). For 1.2 mm (0.045") wire size, the wire feed speed could be between 8.9 and 16 m/min (350 and 630 ipm) that would provide between 250 to 360 Amps. Voltage adjustment in spray mode goes in opposite direction relative to short circuit mode. Decreasing voltage (one volt at a time) shortens the arc, but too low a value will bring the electrode to plunge in the weld pool with consequent spatter. Then the voltage should be increased again until the optimum is reached and spatter is substantially reduced.
4 - Testing Filler Metals Filler Metals are usually selected according to binding specifications or sound engineering practice. While the applicable specification is certainly a good starting point for initial orientation, one should appreciate that there is more than specification to a successful filler metal selection. In fact, different products from various manufacturers may meet formal specification requirements and still be essentially different in minor contributing factors to the point of producing significantly variable results in practical applications. For important projects the selection of a suitable filler material should be made the object of a testing program aimed at identifying the specific material capable of providing consistent high quality welds at high deposition rates. The common practice to select the products available at the least purchase cost may be deleterious to the whole project and far more expensive in terms of total production costs. The testing is performed on test pieces welded according to detailed Welding Procedure Specifications (WPS) and should include nondestructive and destructive tests that demonstrate the adequacy of the tested procedure to meet the project requirements. While selecting the size of the filler wire or electrode for a given application one has to consider that smaller sizes increase current density, that may influence the penetration capability. One should also remember that selecting such a size that permits the use of high current, say more than 360 Amps, risks to be uncomfortable to the manual welder. We can learn a few hints from recommendations listed in FEMA 353, a document dealing with welding involved in seismic shock resistant applications. See FEMA 353 Welding Manual at In particular the recommendations include the following Essential Variables identified as significant factors in assuring consistency of results:
It is believed that by specifying some or all of the additional parameters listed above in WPS documents even when strictly not required, after having thoroughly tested the performance of different filler wires, one can reach an increased level of confidence in production quality and economy. 5 - Online Press: recent Welding related Articles Recent Developments in Abrasive Jet Software Carbide Brazing Video An Article of ours on Planning a Career in Welding From TWI:
6 - Terms and Definitions Reminder Abrasive Blasting is a method of surface cleaning or of roughening of a metallic surface by the use of a powerful jet of water entraining abrasive particles. Arc Cutting is a group of cutting and severing thermal processes whereby metal is melted under heat from an electric arc between electrode and work and then removed. Brazing Sheet is brazing filler metal in form of sheet or strip to be cut and preplaced in a brazing joint. Cutting Torch is a manual device used to direct the thermal source (flame, arc, plasma) to the metal to be severed to perform the cutting. Double Arcing is a faulty arc occurring within the torch between the electrode and the inside surface of the nozzle, diverting energy and disrupting the torch itself. Electrode Setback is the distance that the electrode is set behind the nozzle edge inside an arc torch. Plasma Arc Cutting is an arc cutting process that uses the energy from a constricted arc to melt metal or vaporize other materials and to remove the debris with a gaseous jet. Reducing Atmosphere is an active gaseous mixture capable of reducing metallic oxides to metallic form by reversing oxidation processes at high temperatures.
7 - Article - Abrasive Water Jet Cutting The subject of this Article was first introduced in these pages as early as September 2004 in PWL#013 where, in section 6, the term Abrasive Waterjet Machining was briefly explained. In two later issues, PWL#024 and PWL#030, references to articles from the co-founder of a commercial company illustrating other aspects of this technology were given in section 5. In this very issue reference is made, again in section 5 above, to a new article from the same Author dealing with progress in software for waterjet control. The unique outstanding advantage of this cold cutting method, employing a high pressure water jet with or without abrasive particles entrained in the water flow, is its absolute absence of heat. Therefore it is the only technology that can be employed to cut through obsolete ammunition or containers with traces of explosive gas mixtures. When it is used to cut metals, the low temperature assures absence of thermally induced stresses and deformation, of recast or remelt layers and of hard and brittle transformation structures. Moreover it is used to cut most diverse stuff like granite, stone, glass and composite materials. Cutting itself results from a combination of effects including micro-machining, abrasive erosion and rapidly changing stresses. The combined parameters, including material properties, water pressure, diameter of the nozzle orifice, stand-off distance, abrasive type/quantity and cutting speed influence the surface aspect of the cut, the number and roughness of striations and their drag angle. Excessive speed for a given power will cause the cut to stop short of the through thickness. Relatively low pressure systems work at about 700 bar (10 ksi), high pressure ones are those working anywhere from 2000 to 4000 bar (30 to 60 ksi). Cost of reciprocating pumps increases with pressure, and for the highest pressures high pressure intensifier pumps and high pressure attenuators are used at very high equipment costs. The speed of the water jet can reach 2.5 times the speed of sound. Additional advantages besides the cold cutting capability already remarked are:
Limitations to consider are:
The Abrasive Waterjet Cutting Process is highly versatile and can be used successfully in industries requiring cutting capabilities for vastly different materials. 8 - Site Updating: Case Hardening The Page of this Month deals with surface hardening processes done by heat treatment of items with locally modified chemical composition by diffusion processes, or by local heat treatment of suitable materials. Click on Case Hardening to reach the page. Although the connection with welding may not be readily apparent, it is important to know that welding of case hardened layers should never be attempted. Follow on of new pages of our Website is easily done by reviewing 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 9.1 - Counterboring is the enlargement of a drilled hole for part of its length, to permit the head of a bolt or screw to enter for its whole height in the thickness of the material without protruding from the surface. 9.2 - Checks are minute cracks on the surface of a metal part, that may be associated with thermal treatment or thermal cycling. In plastic parts can appear after long eposure to sunlight and are called crazing. Also fine cracks on the surface of a casting caused by heat gradients during cooling. In a die, strains or pressure have the tendency to produce checks or develop cracks in impression corners. In hot forging dies thermal fatigue can generate areas of small cracks. 9.3 - Chills are block inserts of copper or graphite embedded in a casting sand mold or core, or placed in a mold cavity to change the solidification pattern by increasing the cooling rate of the molten metal at the point of contact. 9.4 - Compressive Strength is the maximum (uniaxial) compressive stress that a material is capable of developing before failing, related to its original area of cross section. The compressive strength has a definite value if the material fails in compression by a shattering or disruptive fracture. Otherwise the value agreed upon as indicative of compressive strength is an arbitrary value depending on the degree of distortion assumed to indicate complete failure of the material. 9.5 - Die Insert is an interchangeable part of a die that contains part or all of the impression of a forging or of a sheet metal part stamping die or of an extrusion die and is fastened to or supported by the master die block. 9.6 - Expendable Pattern of wax or of plastic materials is destroyed, fused or vaporized during preheating or while pouring molten metal for making a casting, after having helped in making the ceramic mold.
10 - Explorations: beyond the Welder Wireless Power Transfer ASM International Strategic Plan The AMMTIAC Quarterly is a technical resource Compressed Air Car What did they do with their Passion
11 - Metal Matrix Composites A short note on this subject was published in Issue 21 of Practical Welding Letter for May 2005 in the Short Items Section (9.1). Here we would like to expand the information by including additional data. Manufacturing Metal Matrix Composites (MMCs) consists in blending a reinforcement, mostly but not only nonmetallic, into a metallic matrix. The useful properties like light weight, corrosion resistance, relatively low melting point and mechanical strength so appreciated in aluminum alloys are a good starting point for exploring aluminum MMCs. Increased stiffness, strength, and wear resistance, along with modified thermal conductivity and a lower coefficient of thermal expansion can be tailored to the sought requirements by controlling the amount, material, size, shape and distribution of the dispersed reinforcement, and by considering the mechanical properties of the selected condition of the matrix material, and the nature of the interface. Aluminum can accept various reinforcing agents, for cast products, the reinforcement constitutes about 10 to 20 volume percent. For powder metallurgy products, the reinforcement percentage averages 20 to 40 volume percent of the composite. Continuous fibers of boron, graphite or refractory metals provide continuous reinforcement. They are produced by diffusion bonding, hot-press molding, and metal infiltration of fiber tows. Generally these products are more expensive than the discontinuously reinforced materials, and they have been restricted to aerospace applications because of economic considerations. Discontinuous particles, short fibers, and whiskers of aluminum oxide (alumina) (Al2O3) or silicon carbide (SiC) are quite common reinforcements. Foundry methods, powder metallurgy processing, extrusion, rheocasting, and other methods are used for incorporating discontinuous reinforcement into aluminum matrix. Discontinuous SiC reinforced aluminum base MMCs are being developed by the aerospace industry for use as structural components like airplane skins, stiffening ribs, and electrical equipment racks. These composites can be optimized to achieve dimensional stability, important for certain applications. The composite Al/SiC has lower density, better thermal conductivity and lower coefficient of thermal expansion than current heavier metals used as heat sinks in the electronics industry. It it therefore considered an advantageous substitute. Cast discontinuous Al/SiC are also being developed for a number of automotive components. Brake rotors made of Al-9.0Si-0.55Mg castings reinforced with 20 volume percent of SiC show the stiffness and wear of ceramics with the low density of aluminum, making parts at half the weight of cast iron drums, with better heat removal and reduced noise and vibration. Metal Matrix materials are mostly aluminum, but also titanium, magnesium, copper and ordered intermetallic compounds (NiAl and Ti3Al) have been investigated and approved for specific uses. Titanium Matrix Composites with continuous silicon carbide (SiC) fiber reinforcement have been developed to extend the elevated-temperature performance of titanium and its alloys for aerospace applications. SiC fibers generally are added in the proportion of 35 to 40 volume percent of the processed material. Titanium alloys selected as matrix material include Ti-6Al-4V, Ti-15V-3Sn-3Cr-3Al, and Ti-15Mo-3Nb-3Al-0.2Si. This last alloy, reinforced with SiC, withstands temperatures up to 800 0C (1500 0F) and is used on Boeing 777 engine nacelles. A number of processing techniques have been evaluated for titanium MMCs, but only a few have been used commercially. Magnesium Matrix Composites are reinforced with continuous graphite for space structures, with randomly cut alumina fiber (Al2O3) for automotive engine parts, and with discontinuous silicon carbide (SiC) or boron carbide (B4C) for other applications. Continuous tungsten fibers are used to reinforce Copper Matrix Composites for advanced rocket engines. Continuous graphite fibers are selected to reinforce Composites used as thermal management elements for electronic components and for satellite radiator panels. Superalloy Matrix Composites reinforced with refractory metal wires (primarily tungsten) have been used for gas turbine engine and other high-temperature applications. Additional applications are still restricted by the high manufacturing costs but will be increasingly be expanded with the development of more economic mass production processes. Joining MMC is problematic at least, and probably it is the main hindrance to a larger use of them in aerospace applications. Conventional arc welding processes destroy those very properties that make MMCs so desirable. It is predicted that considerable efforts will be devoted in the near future to finding better joining solutions. Nonetheless brazing, diffusion bonding, high temperature solid state welding and high energy welding were successfully applied in specific cases.
12 - Testimonials On Fri Jun 15 11:01:13 2007, the following results were submitted from the
"Form 5" on www.welding-advisers.com: First Name: Tim Elia, [Note: Timothy E. Volin, Ph.D., contributed recently to PWL an important article (11) on
On Tue Jun 19 10:06:38 2007, the following results were submitted from the
"Form 5" on www.welding-advisers.com: First Name: alaina I am 17, and I plan on going to college to become a welder and this site has
helped me understand more facts about it. I really enjoy welding and I Think
that it's a good trade to get into, and your site has made me realize all the
opportunities I will have. :) Thank you alaina
13 - Correspondence: a few Comments One of our readers complains that two of the Codes that he is requested to conform with are not of his liking, and that terminology instead of being consistent is slightly different. Unfortunately, once the Codes are selected in a purchase order they are the Law. Either if you are learning for a Certification or if you are interpreting the requirements for your project you have to comply as best as you know whether you like the Codes or not. It is not a matter of preference. Other readers hint to their problems without describing materials, processes, dimensions and welding difficulties. It is quite impossible to guess what are they talking about. They should remember that we know nothing of their concerns unless they care to give us full information.
14 - Bulletin Board 14.1 - 24th Annual ASM Heat Treating Society Conference and Exposition 14.2 - Cold Spray Technology Conference 14.3 - Charting the Course in Welding: U.S. Shipyards
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