7-21. TITANIUM WELDING
(1) Titanium is a soft, silvery white, medium strength metal with very good corrosion resistance. It has a high strength to weight ratio, and its tensile strength increases as the temperature decreases. Titanium has low impact and creep strengths. It has seizing tendencies at temperatures above 800°F (427°C).
(2) Titanium has a high affinity for oxygen and other gases at elevated temperatures, and for this reason, cannot be welded with any process that utilizes fluxes, or where heated metal is exposed to the atmosphere. Minor amounts of impurities cause titanium to become brittle.
(3) 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 and impurities can raise the transition temperature period and impurities 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).
(4) 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.
(5) The procedures for welding titanium and titanium alloys are similar to other metals. Some processes, such as oxyacetylene or arc welding processes using active gases, cannot be used due to the high chemical activity of titanium and its sensitivity to embrittlement by contamination. Processes that are satisfactory for welding titanium and titanium alloys include gas shielded metal-arc welding, gas tungsten arc welding, and spot, seam, flash, and pressure welding. Special procedures must be employed when using the gas shielded welding processes. These special procedures include the use of large gas nozzles and trailing shields to shield the face of the weld from air. Backing bars that provide inert gas to shield the back of the welds from air are also used. Not only the molten weld metal, but the material heated above 1000°F (538°C) by the weld must be adequately shielded in order to prevent embrittlement. All of these processes provide for shielding of the molten weld metal and heat affected zones. Prior to welding, titanium and its alloys must be free of all scale and other material that might cause weld contamination.
b. Surface Preparation.
The nitric acid used to preclean 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.
(1) 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.
(2) 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).
(3) Metals that are scale free can be cleaned by simple decreasing.
(4) 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.
(5) Metals with a heavy scale should be cleaned with sand, grit, or vapor blasting, molten sodium hydride salt baths, or molten caustic baths. Sand, grit, or vapor blasting 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.
(6) 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.
c. MIG or TIG Welding of titanium.
(1) General. Both the MIG and TIG welding processes are used to weld titanium and titanium alloys. They are satisfactory for manual and automatic installations. With these processes, contamination of the molten weld metals and adjacent heated zones is minimized by shielding the arc and the root of the weld with inert gases (see (2)(b)) or special backing bars (see (2)(c)). In some cases, inert gas filler welding chambers (see (3)) are used to provide the required shielding. When using the TIG welding process, a thoriated tungsten electrode should be used. The electrode size should be the smallest diameter that will carry the welding current. The electrode should be ground to a point. The electrode may extend 1-1/2 times its diameter beyond the end of the nozzle. Welding is done with direct current, electrode negative (straight polarity). Welding procedure for TIG welding titanium are shown in table 7-28. Selection of the filler metal will depend upon the titanium alloys being joined. When welding pure titanium, a pure titanium wire should be used. When welding a titanium alloy, the next lowest strength alloy should be used as a filler wire. Due to the dilution which will take place dining welding, the weld deposit will pick up the required strength. The same considerations are true when MIG welding titanium.
(a) General. Very good shielding conditions are necessary to produce arc welded joints with maximum ductility and toughness. To obtain these conditions, the amount of air or other active gases which contact the molten weld metals and. adjacent heated zones must be very low. Argon is normally used with the gas-shielded process. For thicker metal, use helium or a mixture of argon and helium. Welding grade shielding gases are generally free from contamination; however, tests can be made before welding. A simple test is to make a bead on a piece of clean scrap titanium, and notice its color. The bead should be shiny. Any discoloration of the surface indicates a contamination. Extra gas shielding provides protection for the heated solid metal next to the weld metal. This shielding is provided by special trailing gas nozzles, or by chill bars laid immediately next to the weld. Backup gas shielding should be provided to protect the underside of the weld joint. Protection of the back side of the joint can also be provided by placing chill bars in intimate contact with the backing strips. If the contact is close enough, backup shielding gas is not required. For critical applications, use an inert gas welding chamber. These can be flexible, rigid, or vacuum-purge chambers.
(b) Inert gases. Both helium and argon are used as the shielding gases. With helium as the shielding gas high welding speeds and better penetration are obtained than with argon, but the arc is more stable in argon. For open air welding operations, most welders prefer argon as the shielding gas because its density is greater than that of air. Mixtures of argon and helium are also used. With mixtures, the arc characteristics of both helium and argon are obtained. The mixtures usually vary in composition from about 20 to 80 percent argon. They are often used with the consumable electrode process. To provide adequate shielding for the face and root sides of welds, special precautions often are taken. The precautions include the use of screens and baffles (see (c) 3), trailing shields (see (c) 7), and special backing fixtures (see (c) 10) in open air welding, and the use of inert gas filler welding chambers.
(c) Open air welding.
1. In open air welding operations, the methods used to shield the face of the weld vary with joint design, welding conditions, and the thickness of the materials being joined. The most critical area in regard to the shielding is the molten weld puddle. Impurities diffuse into the molten metal very rapidly and remain in solution. The gas flowing through a standard welding torch is sufficient to shield the molten zone. Because of the low thermal conductivity of titanium, however, the molten puddle tends to be larger than most metals. For this reason and because of shielding conditions required in welding titanium, larger nozzles are used on the welding torch, with proportionally higher gas flows that are required for other metals. Chill bars often are used to limit the size of the puddle.
2. The primary sources of contamination in the molten weld puddle are turbulence in the gas flow, oxidation of hot filler reds, insufficient gas flow, small nozzles on the welding torch, and impure shielding gases. The latter three sources are easily controlled.
3. If turbulence occurs in the gas flowing from the torch, air will be inspired and contamination will result. Turbulence is generally caused by excessive amounts of gas flowing through the torch, long arc lengths, air currents blowing across the weld, and joint design. Contamination from this source can be minimized by adjusting gas flows and arc lengths, 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. Chill bars or the clamping toes of the welding jig can serve as baffles (fig. 7-16). Baffles are especially important for making corner type welds. Additional precautions can be taken to protect the operation from drafts and turbulence. This can be achieved by erecting a canvas (or other suitable material) screen around the work area.
4. In manual welding operations with the tungsten-arc process, oxidation of the hot filler metal is a very important source of contamination. To control it, the hot end of the filler wire must be kept within the gas shield of the welding torch. Welding operators must be trained to keep the filler wire shielded when welding titanium and its alloys. Even with proper manipulation, however, contamination from this source probably cannot be eliminated completely.
5. 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.
6. Most of the auxiliary equipment used on torches to weld titanium is designed to improve shielding conditions for the welds as they solidify and cool. However, if the welding heat input is low and the weld cools to temperatures below about 1200 to 1300°F (649 to 704°C) while shielded, auxiliary shielding equipment is not required. If the weld is at an excessively high temperature after it is no longer shielded by the welding torch, auxiliary shielding must be supplied.
7. Trailing shields often are used to supply auxiliary shielding. These shields extend behind the welding torch and vary considerably in size, shaper and design. They are incorporated into special cups which are used on the welding torch, or may consist only of tubes or hoses attached to the torch or manipulated by hand to direct a stream of inert gas on the welds. Figure 7-17 shows a drawing of one type of trailing shield currently in use. Important features of this shield are that the porous diffusion plate allows an even flow of gas over the shielded area. This will prevent turbulence in the gas stream. The shield fits on the torch so that a continuous gas stream between the torch and shield is obtained.
8. Baffles are also beneficial in improving shielding conditions for welds by retarding the flow of shielding gas from the joint area. Baffles may be placed alongside the weld, over the top, or at the ends of the weld. In some instances, they may actually form a chamber around the arc and molten weld puddle. Also, chill bars may be used to increase weld cooling rates and may make auxiliary shielding unnecessary.
9. Very little difficulty has been encountered in shielding the face of welds in automatic welding operations. However, considerable difficulty has been encountered in manual operations.
10. In open air welding operations, means must be provided for shielding the root or back of the welds. Backing fixtures are often used for this purpose. In one type, an auxiliary supply of inert gas is provided to shield the back of the weld. In the other, a solid or grooved backing bar fits tightly against the back of the weld and provides the required shielding. Fixtures which provide an inert gas shield are preferred, especially in manual welding operations with low welding speeds. Figure 7-18 shows backing fixtures used in butt welding heavy plate and thin sheet, respectively. Similar types of fixtures are used for other joint designs. However, the design of the fixtures varies with the design of the joints. For fillet welds on tee joints, shielding should be supplied for two sides of the weld in addition to shielding the face of the weld.
11. For some applications, it may be easier to enclose the back of the weld, as in a tank, and supply inert gas for shielding purposes. This method is necessary in welding tanks, tubes, or other enclosed structures where access to the back of the weld is not possible. In some weldments, it may be necessary to machine holes or grooves in the structures in order to provide shielding gas for the back or root of the welds.
When using weld backup tape, the weld must be allowed to cool for several minutes before attempting to remove the tape from the workpiece.
12. Use of backing fixtures such as shown in figure 7-18 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 (fig. 7-19) 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.
13. Bend or notch toughness tests are the best methods for evaluating shielding conditions, but visual inspection of the weld surface, which is not an infallible method, is the only nondestructive means for evaluating weld quality at the present time. With this method, the presence of a heavy gray scale with a nonmetallic luster on the weld bead indicates that the weld has been contaminate badly and has low ductility. Also, the weld surface may be shiny but have different colors, ranging from grayish blue to violet to brown. This type of discoloration may be found on severely contaminated welds or may be due only to surface contamination, while the weld itself may be satisfactory. However, the quality of the weld cannot be determined without a destructive test. With good shielding procedures, weld surfaces are shiny and show no discoloration.
(3) Welding chambers.
(a) For some applications, inert gas filled welding chambers are used. The advantage of using such chambers is that good shielding may be obtained for the root and face of the weld without the use of special fixtures. Also, the surface appearance of such welds is a fairly reliable measure of shielding conditions. The use of chambers is especially advantageous when complex joints are being welded. However, chambers are not required for many applications, and their use may be limited.
(b) Welding chambers vary in size and shape, depending on their use and the size of assemblies to be welded. The inert atmospheres maybe obtained by evacuating the chamber and filling it with helium or argon, purging the chamber with inert gas, or collapsing the chamber to expel air and refilling it with an inert gas. Plastic bags have been used in this latter manner. When the atmospheres are obtained by purging or collapsing the chambers, inert gas usually is supplied through the welding torch to insure complete protection of the welds.
(4) Joint designs. Joint designs for titanium are similar to those used for other metals. For welding a thin sheet, the tungsten-arc process generally is used. With this process, butt welds may be made with or without filler rod, depending on the thickness of the joint and fitup. Special shearing procedures sometimes are used so that the root opening does not exceed 8 percent of the sheet thickness. If fitup is this good, filler rod is not required. If fitup is not this good, filler metal is added to obtain full thickness joints. In welding thicker sheets (greater than 0.09 in. (2.3 mm)), both the tungsten-arc and consumable electrode processes are used with a root opening. For welding titanium plates, bars, or forgings, both the tungsten-arc and consumable electrode processes also are used with single and double V joints. In all cases, good weld penetration may be obtained with excessive drop through. However, penetration and dropthrough are controlled more easily by the use of proper backing fixtures.
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.
(5) Welding variables.
(a) Welding speed and current for titanium alloys depend on the process used, shielding gas, thickness of the material being welded, design of the backing fixtures, along with the spacing of chill bars or clamping bars in the welding jig. Welding speeds vary from about 3.0 to 40.0 in. (76.2 to 1016.0 mm) per minute. The highest welding speeds are obtained with the consumable electrode process. In most cases, direct current is used with straight polarity for the tungsten-arc process. Reverse polarity is used for the consumable electrode process.
(b) Arc wander has proven troublesome in some automatic welding operations. With arc wander, the arc from the tungsten or consumable electrode moves from one side of the weld joint to the other side. A straight, uniform weld bead will not be produced. Arc wander is believed to be caused by magnetic disturbances, bends in the filler wire, coatings on the filler wire, or a combination of these. Special metal shields and wire straighteners have been used to overcome arc wander, but have not been completely satisfactory. Also, constant voltage welding machines have been used in an attempt to overcome this problem. These machines also have not been completely satisfactory.
(c) In setting up arc welding operations for titanium, the welding conditions should be evaluated on the basis of weld joint properties and appearance. Radiographs will show if porosity or cracking is present in the weld joint. A simple bend test or notch toughness test will show whether or not the shielding conditions are adequate. A visual examination of the weld will show if the weld penetration and contour are satisfactory. After adequate procedures are established, careful controls are desirable to ensure that the shielding conditions are not changed.
(6) Weld defects.
(a) General. Defects in arc welded joints in titanium alloys consist mainly of porosity (see (b)) and cold cracks (see (c)). Weld penetration can be controlled by adjusting welding conditions.
(b) 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 agitating the molten weld puddle and adjusting welding speeds. 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.
1. 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.
2. 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.
3. Cracks due to hydrogen may be prevented by vacuum annealing treatments prior to welding.
(7) Availability of welding filler wire. Most of the titanium alloys which are being used in arc welding applications are available as wire for use as welding filler metal. These alloys are listed below:
(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.
d. Pressure Welding. Solid phase or pressure welding has been used to join titanium and titanium alloys. In these processes, the surfaces to be jointed are not melted. They are held together under pressure and heated to elevated temperatures (900 to 2000°F (482 to 1093°C)). One method of heating used in pressure welding is the oxyacetylene flame. With suitable pressure and upset, good welds are obtainable in the high strength alpha-beta titanium alloys. The contaminated area on the surface of the weld is displaced from the joint area by the upset, which occurs during welding. This contaminated surface is machined off after welding. Another method of heating is by heated dies. Strong lap joints are obtained with this method in commercially pure titanium and a high strength alpha-beta alloy. By heating in this manner, welds may be made in very short periods of time, and inert gas shielding may be supplied to the joint. With all of the heating methods, less than 2 minutes is required to complete the welding operation. With solid phase or pressure welding processes, it is possible to produce ductile welds in the high strength alpha-beta alloys by using temperatures which do not cause embrittlement in these alloys.No tags for this post.