A great tonnage of fabricated metal components requires joining in some form or other. These joining methods may include welding, brazing, soldering, riveting, or bolting. Titanium, therefore, in order to be a useful structural metal for such applications as aircraft, bridges, pipes, tanks, vehicles, and ships, must have the ability to be joined to it self and to other metals.
A joint, in order to be useful, must possess mechanical properties which meet the service requirement specifications of the end product. Intensive research and development of the techniques of titanium joining have reduced a seemingly insurmountable problem to one of practical solution with precautionary techniques.
Welding is the major method employed in joining titanium. Whereas initial attempts at welding titanium indicated the problem to be one of great difficulty, more extensive investigation revealed the problem to be entirely surmountable with application of proper techniques.
With careful choice of materials, sound welds embodying strength, ductility, and resistance to impact loading can be achieved. A selection of material of low interstitial content, as indicated by chemical analysis, should permit a sound weld with good properties as-welded, provided additional contamination is not introduced by the surrounding atmosphere, and the beta content is not too high.
With low interstitial content, unalloyed titanium is readily weldable. However, the typical alpha-beta alloys, containing manganese, chromium, iron, vanadium, and molybdenum, when welded usually have a much lower bend ductility and notch toughness. This bend ductility and notch toughness decrease severely as the total alloy content of beta stabilizing elements exceeds 3%. Such alloys may contain amounts of the alpha stabilizers, aluminum and tin, to permit a greater alloy content without further loss of ductility.
Resistance Welding. Spot, seam, and flash welding are all resistance-welding processes, which have been successfully applied to titanium. Resistance welding is a pressure welding process wherein the heat is obtained by the resistance of the metal to the flow of electric current. In spot welding the heat is restricted to a fairly small section of the lapped area of the parts to be joined.
In flash welding, the heat is obtained from an arc established between the pieces to be welded by electrical resistance. When sufficient heat for proper welding is obtained, pressure is applied to consummate the weld. This procedure is applicable to both unalloyed and alloyed titanium. In the case of the alloyed material there is some loss in ductility which may be restored by post-heat treatment.
Pressure Welding. In this method of welding, the metal is joined under high pressure with or without heat. In cold welding titanium, the metal surfaces arc brought into intimate contact, and high pressures are applied to produce a high strength bond across the weld. The deformation, in the form of upset or bulged metal, is removed to restore the original shape.
The pressures required are dependent upon the size and type of material. The process also requires at least 85% deformation for a high strength bond and thus necessitates using ductile material. Fairly strong bonds of titanium with itself, copper, and steel have been produced.
Fusion Welding. The most common method of welding is the fusion technique. Fusion welding involves various processes in which metals are joined together by contact in the molten state at the surfaces to be joined. This is accomplished either with or without the use of a filler metal and without the application of mechanical pressure.
The most common production method employs a welding torch designed to permit the inert gas to flow through it. This technique develops an envelope of protective gas around the electrode and weld metal. The envelope moves with the torch and thus is constantly covering the new weld puddle. This movement of the shield, however, uncovers the solidified hut still hot weld deposit and here some contamination is picked up.
Helium is particularly effective for hacking because it is lighter than either air or argon and consequently lies up against the underside of the weldment. Because of the very great reactivity of titanium at elevated temperatures, the inert-gas shield must be of high purity or it will in itself be a source of contamination.
Fusion welding of titanium, therefore, differs from that of other metals in that its high reactivity requires careful control of the surrounding atmosphere. The problems encountered in joining titanium to dissimilar metals by welding methods still remain to be resolved. However, joining of titanium to dissimilar metals by methods other than welding has shown better promise.
Brazing and Soldering
Where welding is neither practical nor economically feasible, it may be desirable to consider the brazing or soldering of titanium. Brazing is a joining process wherein the filler metal has a melting point greater than 800°F (430°C), but less than that of the materials to be joined.
Soldering, on the other hand, employs filler metal which melts at less than 800°F (430°C) and is commonly applied to thin-gauge material or wire.
Of the conventional metals employed in the brazing of steel, only pure silver and aluminum have been applicable to titanium with satisfactory results. High strength aluminum alloys have produced brittle impractical joints. Other metals such as zinc and tin will not adequately wet the titanium surface.
Gas Brazing. Of the many methods for gas brazing other metals, only the oxyacetylene gas has been found effective to date with titanium in that the gas employed does not embrittle the material. With a similar brazing method employing pure aluminum in place of pure silver, reasonably ductile but low strength joints are obtained.
In gas brazing aluminum to titanium, the titanium part is dipped into molten aluminum; the aluminum-clad titanium is then brazed to the aluminum part by conventional aluminum brazing techniques.
Furnace Brazing. By brazing in a furnace with a protective atmosphere, the complexity of the gas brazing procedure is simplified. In place of the special fluxes a simple mixture of silver chloride and potassium chloride or manganese chloride and potassium chloride is sufficient. Also, multiple brazing operations are capable of being simultaneously performed with heat applied uniformly to all surfaces of the part.
These advantages, however, are applicable only to small parts limited by furnace dimensions. Longer time cycles are required in furnace brazing because of the absence of localized heating. This results in thicker melting layers which somewhat decrease ductility and strength, especially with the aluminum-brazed joint.
Resistance Brazing. Resistance brazing differs from resistance welding in that a low melting metal is placed between the two surfaces to be brazed. Water-cooled copper electrodes have proved preferable to graphite and other high electrically resistant but contaminating materials. Titanium in itself has sufficient electrical resistance necessary to heat the surfaces. With high currents, short times, and low pressures, strengths superior to those obtained in gas brazing have been achieved in a pure silver.
Soldering. The successful soldering of titanium has been demonstrated. To obtain a rapid economical joint requiring little strength, soldering is preferable to other methods of joining. Samples should be prepared by depositing of thin films of silver, copper, or tin on titanium from their chloride salts. This is usually accomplished by heating the chloride salt-coated titanium in a helium atmosphere furnace. The resultant film can he wetted with either a 60% tin-40% lead or a 50% tin-50% lend solder employing commercial soldering fluxes.
Riveting is the joining of two metals by means of metal fasteners which mechanically lock them selves in position. This method of joining is especially applicable to the joining of highly stressed parts, the forming of a discontinuous joint, and in cases where the work is accessible from only one side.
Titanium is being riveted with stainless steel, Monel, high strength aluminum, and titanium rivets by conventional techniques. With titanium rivets the driving time is increased 65% over that employed for high strength aluminum rivets. Rivets are cold-driven, and rivet holes require the maintenance of close tolerance to insure good gripping. When it is necessary to have flush-head rivets, dimpling is carried out at temperatures of 500 to 600°F (260 to 315°C).
When riveting titanium to dissimilar metals or when riveting titanium with aluminum rivets, precautions must be taken to suppress galvanic corrosion.
Bolting. In this method of joining, the mechanical bond can be readily removed without destruction of the part. Titanium can be joined to itself or dissimilar metals with titanium bolts or with cadmium-plated steel bolts.
With titanium bolts, the locknuts are plated with rhodium or silver to prevent galling and seizing of the nut to the bolt. Some increase in galvanic corrosion has been noted with the application of the cadmium-plated fasteners. Teflon coatings on the threaded parts have been reported to reduce greatly seizing and galling of titanium. Galvanic corrosion is also minimized by the corrosion resistance of the teflon.