BEST MIG AND TIG WELD PRACTICES FOR TITANIUM.
[] Titanium low weight, high strength and good corrosion resistance.
[] Titanium has low thermal conductivity, low density and low thermal expansion.
[] Titanium alloys match or exceed the tensile strength of structural steels.
[] Titanium alloys stronger than steel yet approx. 40% lighter.
[] Titanium will work at temperatures up to 600C yet also work with liquid notrogen.
[] Titanium alloys usually have a lower modulus of elasticity (stiffness) than steel, but much
higher than aluminum or magnesium.
[] In comparison to aluminum and steel alloys, titanium alloys have a 30% or greater strength to weight ratio.
[] Most titanium alloys are softer than steel.
[] Titanium has poor wear properties.
[] Titanium tensile strength increases as the temperature decreases.
[] Titanium has low impact and creep strengths.
[] Titanium is reactive metal that forms a thin layer of titanium dioxide on it's surface. The oxide provides protection from many corrosives materials.
[] Titanium is bio-compatible in that it does not react with the human body.
[] Titanium will burn in pure nitrogen.
[] Titanium has the characteristic known as the ductile-brittle transition. This refers to a temperature at which the metal breaks in a brittle manner, rather than in a ductile fashion. The recrystallization of the metal during welding can raise the transition temperature. Contamination during the high temperate period can raise the transition temperature so that the material is brittle at room temperatures. If contamination occurs so that transition temperature is raised sufficiently, it will make the welding worthless. Gas contamination can occur at temperatures below the melting point of the metal. These temperatures range from 700°F (371°C) up to 1000°F (538°C).
At room temperature, titanium has an impervious oxide coating that resists further reaction with air. The oxide coating melts at temperatures considerably higher than the melting point of the base metal and creates problems. The oxidized coating may enter molten weld metal and create discontinuities which greatly reduce the strength and ductility of the weld.[] Titanium Property Values
[] Melting point 1933 K
[] Boiling Point 3560 K
[] Tensile strength 234 MPa
[] Yield strength 138 MPa
Common Titanium Alloys:
The following are the common titanium alloys. Selecting base titanium metals or titanium weld filler metals to meet specific corrosion or mechanical properties requires many considerations, if you need this material data I recommend you check in with the International Titanium Association www.titanium.org -- http://www.titaniuminfogroup.co.uk. www.timet.com
For Titanium electrode data check with any titanium electrode manufacturer. Filler materials used for Welding-titanium alloys are described in AWS 5.16 - Specification for Titanium and Titanium Alloy Weld Electrodes and Rods.The maximum useful temperature range for titanium structural applications is 800 to 1100 0F, depending on the selected alloy and it's condition. Titanium is used in cryogenic applications as it has no dangerous ductile-brittle transition temperature. Unalloyed titanium is usually selected for its excellent corrosion resistance, especially in applications where high strength is not required. Besides the unalloyed or commercial titanium, different classes of titanium alloys are described by reference to metallurgical types called alpha, alpha-beta and beta, which indicate the main phases present in the microstructure.
TITANIUM WELD WIRE SUPPLIER: Looking for a Titanium weld wire supplier and good practical Titanium data.
The use of extremely high quality filler metal is vital for producing good titanium welds, especially for aerospace and aircraft repair industries, as well as for other critical applications. The filler metal must be free of metallic and nonmetallic impurities, with an extremely clean,smooth surface free of moisture, dirt, lubricants or other contaminants. It takes some additional care during storage and handling of the filler metal to prevent contamination that would cause poor welds. Lancaster Alloys Company guarantees the highest quality titanium filler metal supplied to its customers including uniformity, freedom from any surface defects and contaminant. © Lancaster Alloys Co. http://www.lancasteralloys.co.uk. This company provides great data and supplies it's products in a wide variety of re-sealable packing bags covered by requirements of AMS, AWS and other commercial practices, including vapor barrier envelopes with desiccant, to assure the filler metal containment free as long as it kept in the original packaging.The two most common titanium grades are commercially pure titanium ASTM grade 2 (corrosion resistant) with strength 350 - 450 MPa, and higher strength titanium alloys Ti-6AI-4V that provide strength in the 900 - 1100 MPa range.
Commercially pure titanium alloys: Examine the corrosion resistance and mechanical properties of the many grades available.
The commercially pure (C.P) alloys are widely used for industrial applications as a results of the combinations of good weldability, strength, corrosion resistance and formability.
ASTM Grades 1 -2 - 3 - 4. (alpha) are commercially pure and typically used for their corrosion resistance.. Each of these grades has a different amount of impurity content. Grade 1 is the the most pure. The mechanical properties increase with the grade number.
ASTM Grades 7 - 11 - 16 - 17 are also alpha titanium that contain palladium, these grades have excellent corrosion resistance, while grades 26 and 27 provide even greater corrosion resistance. There are many other grades available.When welding CP alloys you would typically utilize a weld wire one or two PSI strength grades lower than the parent metal. The weld dilution with the base metal along with micro contaminants from the base will l increase the strength of the weld metal. One of the important benefits of welding the commercially pure grades of titanium is that they are over 99% pure titanium and there is no concern for segregation. The same is true for commercially pure weld wire or rod.
Unalloyed grades include about 30% of titanium production. Unalloyed titanium are generally weldable and welded joints generally have acceptable strength and ductility. Postweld stress-relief annealing of weldments is recommended.
Alpha titanium alloys:
Alpha alloys are predominantly alpha, usually with small amounts of beta present. Alpha alloys typically contain aluminum and tin, and they can also contain molybdenum, zirconium, nitrogen, vanadium, tantalum, columbium, and silicon. These alloys are weldable in annealed condition and retain their strength at high temperature. They do not generally respond to heat treatment however they can be strengthened by cold work ( strain hardening) and are commonly used for chemical processing, cryogenic applications and airplane parts.
Beta titanium Alloys:
The Beta alloys are the smallest group of titanium alloys. These alloys have good hardenability. They have good cold formability when they are solution-treated, and high strength when aged. Beta alloys are slightly denser than other titanium alloys. These weldable alloys are the least creep resistant alloys, they can provide yield strengths up to 1345 MPa.Alpha-Beta titanium Alloys:
Alpha-beta alloys contain both phases, with more beta than the alpha alloys.Titanium alpha-beta alloys can be welded in certain conditions but with limited weld ductility or heat affected zone ductility, however these alloys can be strengthened by heat treatment. In contrast to the Alpha alloys, the Alpha-Beta alloys can be strengthened by heat treatment and aging. The Alpha-Beta alloys can undergo machining / manufacturing while the material is still ductile, and then be heat treated to strengthen the material, which is a major advantage. The alloys are commonly used in aircraft and aircraft turbine parts, chemical processing equipment, prosthetic devices and marine parts.
Ti-6Al-4V is a common titanium alloy. These alloys are weldable in the annealed condition as well as in the solution treated and partially aged condition (aging can be completed during the post-weld heat treatment). Strongly stabilized alpha-beta alloys can be embrittled by welding, the result of phase transformations occurring in the weld metal or the heat affected zone.
ASTM Titanium Specs
[] B265 Strip Sheet and Plate.
[] B337 Seamless and Welded Pipe.
[] B338 Seamless and Welded Tubes.
[] B348 Bars and Billets.
[] B363 Seamless and Welded Fittings.
[] B367 Castings.
[] B381 Forgings.
[] B861 Seamless Pipe replaces B 337
[] B862 weldded Pipe.
[] B863 Wire.
[] F 67 Unalloyed Titanium for Surgical Applications.
[] Ti-6AI-4V Surgical Applications.
TITANIUM SAFETY:
Storage of titanium chips from machining operations is relatively safe. The storage of fine titanium powder can be fire or explosion hazard. Avoid weld sparks or flames around titanium dust. If a fire does start with the titanium after product and its safe to do so, try to isolate the burning material from the bulk of the scrap. A titanium fire can be extinguished using either a Class D extinguisher or dry powder. A sodium chloride base powder or salt / sand can be used to reduce the oxygen.
Give consideration to the collection or storage of titanium chips and the dust from machining operations. The chips are relatively safe, the concern should be for the titanium dust / powders which could create either a fire or explosion. Fires or explosions may be initiated by exposing a flammable concentration of titanium dust to grinding / weld sparks spark or a naked flame. If a fire does start, try to isolate the burning materials from the rest of the materials. A titanium fire will be extinguished using either dry powder (sodium chloride, salt sand) or a class D fire extinguisher.From Lancaster Alloys: The possibility of spontaneous ignition of titanium or its alloys during welding is extremely remote. Like magnesium or aluminum the occurrence of fires is usually encountered where accumulation of grinding dust or machining chips exists. Even in extremely high surface-to- volume ratios, accumulation of clean titanium particles do not ignite at any temperature below incipient fusion temperature in an ambient atmosphere. However, spontaneous ignition of fine grinding dust or lathe chips, saturated with oil under hot humid conditions have been reported. Water or water-based coolants should be used for all machining operations. Carbon dioxide is also a satisfactory agent. Large accumulations of chips, turnings, or other metal powders should be removed and stored in enclosed metal containers. Dry grinding should be done in a manner that will allow proper heat dissipation, with the powder similarly stored in enclosed containers.
Dry compounds extinguishing agents or dry sand are effective fire extinguishing agents. Ordinary extinguishing agents such as water, carbon tetrachloride, and carbon dioxide foam are ineffective in extinguishing titanium fires.
The toxicity and related health problems associated with Titanium Tetrachloride are discussed by the Agency for Toxic Substances and Disease Registry. http://www.atsdr.cdc.gov/tfacts101.html.
http://www.corrosion-doctors.org/ MatSelect/corrtitanium.htm describes the corrosive effects of some chemicals with titanium.The nitric acid used to pre-clean titanium for inert gas shielded arc welding is highly toxic and corrosive. Goggles, rubber gloves, and rubber aprons must be worn when handling acid and acid solutions. Do not inhale gases and mists. When spilled on the body or clothing, wash immediately with large quantities of cold water, and seek medical help. Never pour water into acid when preparing the solution; instead, pour acid into water. Always mix acid and water slowly. Perform cleaning operations only in well ventilated places.
The caustic chemicals (including sodium hydride) used to preclean titanium for inert gas shielded arc welding are highly toxic and corrosive. Goggles, rubber gloves, and rubber aprons must be worn when handling these chemicals. Do not inhale gases or mists. When spilled on the body or clothing, wash immediately with large quantities of cold water and seek medical help. Special care should be taken at all times to prevent any water from coming in contact with the molten bath or any other large amount of sodium hydride, as this will cause the formation of highly explosive hydrogen gas.
TITANIUM BEST PRACTICES THINK CLEAN: Titanium weld joints are similar to those for other metals, and the edge preparation is commonly done by machining or grinding. In welding Titanium cleanliness is always vital and of course the parts have to be spotless, however give also consideration to the influence of the surrounding atmosphere as this needs consideration. Aggressive wire brushing of the base metal in the weld area using a stainless steel brush designated solely for the titanium only. Use of high-purity acetone for cleaning both the titanium surfaces to be welded and also when TIG welding for the surface of the weld filler wire. Maintaining cleanliness enables manufacturers to avoid porosity and a loss of toughness typically associated with contaminates and titanium welds..
Treat the welded part as a surgeon should treat a patient.
[] Wear white cotton gloves when handling the parts.
[] Store parts in clean dry area.
[] Don't store parts unless they are wrapped and sealed from the atmosphere.
[] A separate highly clean area in the weld shop should be used for welding the titanium.
[] The clean area should be isolated from dirt-producing operations such as grinding, painting machining torch cutting and painting.
[] The weld area should be free of air drafts and the humidity should be controlled.
[] Remember grinding dust and particle contaminates from the weld smoke can end up on the Titanium surface, this will cause weld issues.
[] Any oxide layer must be removed from the titanium surface by grit blasting or pickling.
[] Contaminants such as grease, oils, marker pens and even fingerprints should be removed from any area subject to >400 C, clean with detergent cleaners or non-chlorinated solvents.
[] To clean the parts. Chlorinated Fluoro Carbon solvents are forbidden for cleaning titanium and titanium alloys because they produce embrittlement. Use instead only Acetone or Methyl Ethyl Ketone (MEK).
[] To clean the welds, a new dedicated stainless steel brush should be used to clean the weld joint and
immediate area surface.
[] Take care of those brushes. After use, the stainless brushes should be rinsed in alcohol and stored in a sealed container. Always remember the weld color indicates the success of your cleanliness.
[] Light surface oxides can be removed by acid pickling while heavier oxides on the surface could require
you grit blast then pickle the part.
[] Keep in mind that clamps and fixtures in the proximity of the parts that are in the heat sensitive zone of > 400C. can also contaminate the parts.
[] The weld wire used should not be left in the open, store in a sealed dry area and use clean cotton gloves or new weld gloves if handling.Lancaster Alloys Provides the following advice on cleaning.
[] Surface cleaning is important in preparing titanium and its alloys for welding. Proper surface cleaning prior to welding reduces contamination of the weld due to surface scale or other foreign materials. Small amounts of contamination can render titanium completely brittle.
[] Several cleaning procedures are used, depending on the surface condition of the base and filler metals. Surface conditions most often encountered are as follows:
(a) Scale free (as received from the mill).
(b) Light scale (after hot forming or annealing at intermediate temperature; ie., less than 1300°F (704°C).
(c) Heavy scale (after hot forming, annealing, or forging at high temperature).[] Metals that are scale free can be cleaned by simple decreasing.
[] Metals with light oxide scale should be cleaned by acid pickling. In order to minimize hydrogen pickup, pickling solutions for this operation should have a nitric acid concentration greater than 20 percent. Metals to be welded should be pickled for 1 to 20 minutes at a bath temperature from 80 to 160°F (27 to 71°C). After pickling, the parts are rinsed in hot water.A common pickling solutions of 48 %hydrofluoric acid concentration and 70% nitric acid concentration. The acid ratio 1:5 and 1:9 is effective.
[] Metals with a heavy scale should be cleaned with sand, grit, or vaporblasting, molten sodium hydride salt baths, or molten caustic baths. Sand, grit, or vaporblasting is preferred where applicable. Hydrogen pickup may occur with molten bath treatments, but it can be minimized by controlling the bath temperature and pickling time. Bath temperature should be held at about 750 to 850°F (399 to 454°C). Parts should not be pickled any longer than necessary to remove scale. After heavy scale is removed, the metal should be pickled as described in (4) above.[] Surfaces of metals that have undergone oxyacetylene flame cutting operations have a very heavy scale, and may contain microscopic cracks due to excessive contamination of the metallurgical characteristics of the alloys. The best cleaning method for flame cut surfaces is to remove the contaminated layer and any cracks by machining operations. Certain alloys can be stress relieved immediately after cutting to prevent the propagation of these cracks. This stress relief is usually made in conjunction with the cutting operation
TITANIUM BEST PRACTICES AND WELD GAS FACTS: Properly shielded titanium welds will be bright and silvery in appearence. The quality and coverage of the shielding gas is an extremely important factor for welding titanium. When titanium is subject to heat above 400oC it reacts with the atmosphere oxygen, nitrogen and also will react with carbon and contaminate. If the contaminants are absorbed into the weld, the results can be porosity and low-notch toughness and brittleness. It's essential that adequate inert shielding gas covers the molten weld pool and all areas above 400oC to ensure a good quality weld.
To prevent contamination between the atmosphere and the hot metal and welds, the inert argon or helium must be free of contaminates. The use of -300F liquid argon containers is preferable to traditional, high pressure cylinder grade argon which may contain possible contaminates and moisture in the gas. If cylinder gases are used, consider the use of specialty gas argon and order ultra high quality grade and dont use cylinder gas mixes below 300 psi as the moisture content rises with pressure drop.
Its recommended to weld small parts in a purge chamber.
It's important to supply the inert gas at the correct flow rate to any side of the titanium parts that are heated above 400°C. A grooved drilled copper back bar fed with argon is effective in shielding the under bead against contamination. On narrow parts remember the part's sides may also need atmospheric protection also. The thinner the part the wider the affected heat zone, the wider the gas coverage.
Huntingdon trail shields
Always ensure that the inert atmosphere protection is maintained until the weld metal temperature cools well below 426°C (800°F), keep those 300C tempsticks handy.
With TIG welds use 20 cuft/hr, with MIG welds use 40 - 45 cuft/hr.
Remember helium when used can change the weld penetration profile. Helium is also lighter than air and the smaller helium molecules do not have the oxide cleaning action that larger argon molecules do. If you add helium to argon a good mix in which you can attain the benefits of both gases, is 60% Argon 40% helium.
Titanium welds typically require three separate gas streams during TIG or MIG welding.
Because of the low thermal conductivity of titanium the molten puddle tends to be larger than most metals. For this reason and because of shielding conditions required in welding titanium, larger MIG and TIG nozzles are used on the welding torch, with proportionally higher gas flows that are required for other metals.
Water-cooled copper backup bars may also be used as heat sinks to help chill the welds. These bars are usually grooved, with the groove located directly below the weld joint. The backup bars provide around 10 cfh of inert gas flow for adequate shielding.
Note: Chill bars often are used for faster cooling and to limit the size of the weld puddle.
For the back up gas and trailing gas shield, typically start out at 10 cuft/hr. The color of the welds will indicate the effectiveness and desired length and width of the trailing shield apparatus. Ensure the purge gas flow rates are sufficient to exclude air but not so great as to cause turbulent in the flow. Gas control is assisted and turbulence minimized by adjusting gas flows and by placing baffles alongside the welds. Baffles protect the weld from drafts and tend to retard the flow of shielding gas from the joint area.
Weld contamination which occurs in the molten weld puddle is especially hazardous. The impurities go into solution, and do not cause discoloration. Although discolored welds may have been improperly shielded while molten, weld discoloration is usually caused by contamination which occurs after the weld has solidified.
Use of backing fixtures can be eliminated in many cases by the use of weld backup tape. This tape consists of a center strip of heat resistant fiberglass adhered to a wider strip of aluminum foil, along with a strip of adhesive on each side of the center strip that is used to hold tape to the underside of the tack welded joint. During the welding, the fiberglass portion of the tape is in direct contact with the molten metal, preventing excessive penetration. Contamination or oxidation of the underside of the weld is prevented by the airtight seal created by the aluminum foil strip. The tape can be used on butt or corner joints or, because of its flexibility, on curved or irregularly shaped surfaces. The surface to which the tape is applied must be clean and dry. Best results are obtained by using a root gap wide enough to allow full penetration. .
Test both the weld and shielding gases before welding. A simple weld test is to make a weld bead on a piece of clean scrap titanium, and notice its color. The resulting weld bead should be shiny. Any discoloration of the surface indicates a contamination.
For welding small parts and you want gas protection
give Huntingdon a call.
Best WELD Practices and TIG / MIG Titanium Welds.
Most of the titanium alloys which are being used in arc welding applications are available as a wire for use as welding filler metal. These alloys are listed below: For more filler wire data contact a reputable weld wire supplier:
ASTM grades with good weldability. 1-2-3-4-7-9-11-12-13-14-15-16-17-18-21-26-27-28-6.6ELI
ASTM grades medium to fair weldability 5-23-24-29
(a) Commercially pure titanium --commercially available as wire.
(b) Ti-5A1-2-1/2Sn alloy --available as wire in experimental quantities.
(c) Ti-1-1/2A1-3Mn alloy --available as wire in experimental quantities.
(d) Ti-6A1-4V alloy --available as wire in experimental quantities.
(e) There has not been a great deal of need for the other alloys as welding filler wires. However, if such a need occurs, most of these alloys also could be reduced to wire. In fact, the Ti-8Mn alloy has been furnished as welding wire to meet some requests. (Lancaster Alloys).Mention welding titanium and for many weld decision makers their is unnecessary concern. From a welding perspective, TIG or MIG welding Titanium has a lot in common with TIG or MIG welding stainless steels. The primary concern when welding titanium is that both the weld and surrounding metal must be meticulously clean and be totally protected from the reactive gases of the atmosphere. Typically inert argon, helium or a vacuum unit is used for this protection. The atmospheric protection must be provided to all sides of the parts that are subject to temperatures above 800F or 426C.
[] Welding pure titanium, use a pure titanium wire. If welding a titanium alloy, the next lowest strength alloy should be used as a filler wire. The weld deposit will pick up through dilution the required strength. The same considerations are true when MIG welding titanium.
[] Don't weld titanium unless you know the alloy and have checked the last heat treatment that was performed.
[] Heating titanium in air at high temperature not only is an oxidation concern but also be concerned about the solid-solution hardening that can result on the surface thanks to a diffusion of both oxygen and nitrogen. A surface hardened zone of "alpha-case" is formed. This layer must be removed, before placing a part in service, by machining, chemical milling, or other means. If not removed , because its presence reduces fatigue strength and ductility.
[] Welding titanium to most dissimilar metals is not feasible as the titanium forms brittle compounds with most other metals. The exception is titanium can be welded to tantalum, zirconium and niobium. Some of the different grades of titanium can be welded to each other. When using no filler for those titanium welds, the autogenous welds that result typically will have properties that are the average of the two parent metals. To join dissimilar metals to titanium, adhesives, mechanical fastening, explosive or frictional welds may be utilized.
[] Reaction of the titanium with both gases and fluxes is one of the reasons oxy fuel welding, flux cored, SAW` or the SMAW processes are not suited. However new gas shielded flux cored wires are under development in 2006, its hoped these wires and the resulting slag will reduce the need for trailing gas concerns. Titanium can be joined by the conventional processes such as TIG - Plasma - MIG - Pulsed MIG - Resistance - Friction - Lasers - Electron Beam and others.
[] When using TIG to weld titanium, the DCEN process is utilized usually argon and the use of high frequency for the arc initiation. Avoid scratch starts as the tungsten can contaminate the titanium. Two percent ceriated or two percent lanthanated tungsten electrodes are recommended. These electrodes will not spit or split, have low erosion rates, and provide consistent arc starts over a wide amp.[] Before starting an arc in TIG or MIG welding titanium, its good practice for at least 10 seconds to pre-purge the TIG torch or MIG gun. Also pre-purge the trailing shields and backup shields.
[] A common sense process approach to welding titanium is to always use use the lowest weld parameters that will sustain consistent weld fusion. Think fast cooling, always be concerned with putting excess heat to the parts.
[] Interpass weld temperatures should be kept below < 200F.
[] When extinguishing the TIG arc use current downslope and the gas shielding should be continued until the weld metal and surrounding area cools well below 700F. Secondary and backup shielding should also be continued. A straw or blue color on the weld is indicative of early removal of shielding gas.
[] If possible on the fixtures use heat sink copper materials to speed the heat quench times.[] Give consideration to the TIG arc length for welding titanium. When no filler is used the arc length should be approx. equal to the tungsten electrode diameter. If TIG electrode wire is added, the max TIG arc length should be about 1.5 times the electrode diameter. When adding TIG filler the weld wire should be fed into the weld zone at the leading edge of the weld pool. The TIG wire should be fed continuously into the puddle. Do not use intermittent dipping technique as possible weld pool turbulence can result.
[] A water-cooled welding torch is typically used and ensure extra large gas nozzle / diffusers are used. A 3/4-inch or one-inch ceramic cup and a gas lens, is recommended for TIG welding and a minimum one-inch cup may be required for MIG welding.
[] Thoriated tungsten electrodes (usually 2% thoria) are recommended for GTA welding of titanium.
[] Pointed TIG electrodes with an optimum 60 degree cone, try a slight blunt end as it may prolong the tungsten life.
[] For TIG use the smallest diameter electrode which can carry the required current should be used.[] Welding-titanium should be followed by stress relieving in order to prevent cracks, and also to avoid stress corrosion cracking in service.
[] Remember with TIG there is opportunity for possible contamination of the hot end of the wire is removed from the gas shield. The contaminants will then be transferred to the weld puddle. Whenever the TIG weld wire is removed from the inert gas shielding, the end of the wire should be clipped back about 25 mm to remove contaminated metal.
[] Plasma welding offers DCEN TIG quality with key hole full penetration AUTOGENEOUS single pass weld capability that offers many benefits.
[] Pulsed MIG is an excellent choice for titanium and far more economical than TIG. Consider an argon 30 to 40% helium mix to help stabilize the weld arc.[] The filler wire selection to weld the commercially pure grades will typically depend on the mechanical properties that are required from the welds, (ductility / strength).
[] The most common weld processes most commonly used for the fabrication of thin titanium or small weld is the TIG and Plasma processes. The MIG spray or pulsed MIG modes are the process of choice for thicker titanium welds. Pulsed MIG welds can offer much lower weld joules than spray transfer.
[] Porosity in titanium welds is a common problem. Porosity is usually caused by gas bubbles formed from contaminates during the weld solidification. It goes back to gas coverage, cleanliness and avoid weld turbulence.
Foreign materials on the surface of the weld wire and contaminates in cylinder gases are two primary causes of weld porosity and weld embrittlement.
The weld wire comes to you with both surface and subsurface inclusions that results during the weld filler wire during drawing. The wire lubricants and micro contaminates are a major cause of weld porosity. Check out your weld wire supplier and the quality system they use to provide clean wires.Because of the low thermal conductivity of titanium, weld beads tend to be wider than normal. However, the width of the beads is generally controlled by using short arc lengths, or by placing chill bars or the clamping toes of the jig close to the sides of the joints.
The army brass decided that the soldiers could die in light weight Humvees lacking armor plate, yet the American tax payers should shell out for costly howitzers made from titanium. God help the soldiers in the Humvees. God help the soldiers who have to do the titanium weld repairs on the howitzers.
Best Practices and Weld Quality Evaluation;
The visual inspection of a titanium welds / parts is a great indication of the resulting weld / part integrity. A macro examination will reveal typical weld defects such as porosity, undercut, lack of root or side wall fusion. A positive feature of welding titanium is the color of the weld beads will give a good indication of the effectiveness of the inert gases on protection of the parts from the atmospheric gases. The aim should always be to produce a bright silver weld. Any discoloration outside the silver weld indicates that some reaction with oxygen has occurred either during the actual welding or during the cool down period.
Any weld discoloration should be cause for stopping the welding operation and correcting the problem. Light straw-colored weld discoloration can be removed by wire brushing with a clean stainless steel brush, and the welding can be continued. Dark blue oxide or white powdery oxide on the weld is an indication of a seriously deficient purge. When the discoloration takes place, stop welding immediately and review the causes of the oxide reaction.For the finished weld, nondestructive examination by liquid penetrant, radiography and/or ultrasound are normally employed in accordance with a suitable welding specification. At the outset of welding it is advisable to evaluate weld quality by mechanical testing. This often takes the form of weld bend testing, sometimes accompanied by tensile tests.
Titanium Weld defects.
Defects in arc welded joints in titanium alloys consist mainly of porosity and cold cracks.
Weld penetration can be controlled by adjusting welding conditions.Titanium Weld Porosity. Weld porosity is a major problem in arc welding titanium alloys. Although acceptable limits for porosity in arc welded joints have not been establish, porosity has been observed in tungsten-arc welds in practically all of the alloys which appear suitable for welding operations. It does not extend to the surface of the weld, but has been detected in radiographs. It usually occurs close to the fusion line of the welds. Weld porosity may be reduced by a slight agitation of the molten weld puddle and adjusting welding speeds, (slow down). Also, remelting the weld will eliminate some of the porosity present after the first pass. However, the latter method of reducing weld porosity tends to increase weld contamination.
Titanium Weld Cracks.
With adequate shielding procedures and suitable alloys, cracks should not be a problem. However, cracks have been troublesome in welding some alloys. Weld cracks are attributed to a number of causes. In commercially pure titanium, weld metal cracks are believed to be caused by excessive oxygen or nitrogen contamination. These cracks are usually observed in weld craters. In some of the alpha-beta alloys, transverse cracks in the weld metal and heat affected zones are believed to be due to the low ductility of the weld zones. However, cracks in these alloys also may be due to contamination. Cracks also have been observed in alpha-beta welds made under restraint and with high external stresses. These cracks are sometimes attributed to the hydrogen content of the alloys. If weld cracking is due to contamination, it may be controlled by improving shielding conditions. However, repair welding on excessively contaminated welds is not practical in many cases.Cracks which are caused by the low ductility of welds in alpha-beta alloys can be prevented by heat treating or stress relieving the weldment in a furnace immediately after welding. Oxyacetylene torches also have been used for this purpose. However, care must be taken so that the weldment is not overheated or excessively contaminated by the torch heating operation.
Cracks due to hydrogen may be prevented by vacuum annealing treatments prior to welding.:
NDT: Traditional radiography is used to detect lack of root or sidewall fusion, weld porosity or tungsten inclusions. Standard NDT methods of weld inspection are utilized such as dye pen, ultrasonic, fluorescent crack detection and acoustic emission.
Contact your weld wire supplier for as welded tensile and charpy impact properties of the common alloys. A common method of comparing the ductility of welds in sheet materials of various alloys is the bend test. The minimum bend radius for titanium welds is defined as the minimum radius around which a sample of titanium containing a weld can be bent without developing a crack, divided by the thickness of the material. Normally, the sample is bent until the permanent deformation equals 105° with the weld transverse to the bend axis. The actual bend is often much greater then 105° because of the "springback" encountered in titanium alloys. Typical minimum bend radiuses are given by companies like Lancaster Alloys.
Bend or notch toughness tests are the best methods for evaluating the gas shielding conditions. Hardness measurements on weld vs. base metal are also sometimes used as an indicator of weld quality. Normally, uncontaminated weld hardness is no more than 30 points greater on the Knoop, Vickers or Brinell hardness scales (5 points Rockwell B) than the hardness of base metal of matching composition.
It should be recognized that heat-to-heat variation in chemistry, within specifications, can result in hardness differentials somewhat higher than 30 Knoop or Brinell without any contamination. In any event, high weld hardness should be cause for concern because of the possibility of contamination.
HARDNESS: The ASME Code suggests that, if the weld metal hardness is more than 40 BHN greater than base metal hardness, excessive contamination is possible. Substantially greater hardness differential necessitates removal of the affected weld-metal area. The Code further specifies that all titanium welds be examined by liquid penetrant. In addition, full radiography of many titanium joints is required by the Code
Heat treatment of titanium welds is not normally necessary. Annealing may be necessary following severe cold work if restoration of ductility or improved machinability are desired. A stress relief treatment is sometimes employed following severe forming or welding to avoid cracking or distortion due to high residual stresses, or to improve fatigue resistance.
Note: Cleanliness of titanium parts to be heat treated is important because of the sensitivity of titanium to contamination at the heat treated elevated temperatures. Titanium fabrications should be cleaned carefully prior to heating, using nonchlorinated solvents or a detergent wash, followed by a thorough water rinse. Handling following cleaning should be minimized to avoid potential surface contamination.Most titanium grades are typically stress-relieved at about 1000°F (538°C) for 45 minutes and annealed at 1300°F (704°C) for two hours. A slightly higher stress relief temperature [1100°F (593°C), 2 hrs.] and annealing temperature [1450°F (788°C), 4 hrs.] are appropriate for the Grade 5 alloy. Air cooling is generally acceptable.
The color of contamination tells the story:
Properly shielded titanium welds will be bright and silvery in appearance.Thanks to the International Titanium Association for
allowing reproduction of this important color chart.Titanium Weld Color?
[] Shiny silver is an indication of an absence of contaminants
[] Yellow is caused by nitrogen from the atmosphere. The Nitrogen needs to be removed with a stainless steel-brush or by using other mechanical tools before a new weld is added to the existing one.
[] Gold or straw color is an indication of minor contamination. Is this acceptable? it will depend on the weld, application and code utilized.
[] Light blue, these welds are contaminated but the contamination may be removed by aggressive cleaning.
[] If the welds are gray, brown, dark blue, or a white powdery color, these welds should be completely removed, usually by grinding and cleaned before welding again.
Note: If a discoloration takes place after welding the color must be removed before visual inspection has been carried out.
