Tantalum Machining

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Most of the procedures in working and fabricating tantalum are conventional, and can be mastered without too much difficulty. At the out set, however, two important characteristics of tantalum must be kept constantly in mind: 

1.    Annealed tantalum, like copper, lead, stainless steel and some other metals, is
       ‘sticky’, having a strong tendency to seize, tear and gall.
 

2.    All forming, bending, stamping or deep drawing operations are normally done cold.
       Heavy sections can be heated for forging to approximately 800 oF.

 

Forming and Stamping:

Most sheet metal work in tantalum is done with metal of thicknesses ranging from 0.004" to 0.060". The instructions given here apply to metal in this thickness range. 

Blanking or punching presents no difficulties. Steel dies are used. The clearance between the punch and die should be close to 6% of the thickness of the metal being worked. Close adherence to this clearance is important, and the use of light oil or perchloroethylene is recommended to prevent scoring of the dies. Trichloroethane is also acceptable. 

Form stamping techniques are similar to those used with mild steel, except that precautions should be taken to prevent seizing or tearing of the metal. Dies may be made of steel except where there is considerable slipping of the metal, in which case aluminum bronze or beryllium copper, should be used. Low melting alloys such as kirksite may be used for experimental work or short runs. Rubber or pneumatic die cushions should be used where required. Annealed tantalum takes a permanent set of forming and does not spring back from the dies. 

Deep drawing is defined here as an operation where the depth of the draw in the finished part is equal to or greater than the diameter of the blank for deep drawing operations, only annealed tantalum sheet should be used. It should be borne in mind that tantalum does not work-harden as rapidly as most metals, and work-hardening begins to appear at the top rather than at the deepest part of the draw. 

If the piece is to be drawn in one operation, a draw in which the depth is equal to the diameter of the blank can be made. If more than one drawing operation is to be performed, the first draw should have a depth of not more than 40 to 50 percent of the diameter. Dies should be made of aluminum bronze, although the punch may be steel if there is not too much slippage encountered, Sulphonated tallow, chlorinated oil, caster oil, or Johnson No. 150 drawing wax can be used. 

Annealing of tantalum is accomplished by heating the metal in high vacuum to temperatures above 2,000 oF. Spinning is done by conventional techniques, using steel roller wheels as tools, though yellow brass may be used for short runs. Yellow soap or Johnson No. 150 drawing wax may be used as a lubricant. 

 

Cleaning:

Cleaning and degreasing presents no special problems and conventional methods and materials may be used, although hot caustics must be avoided. Electronic tube parts, which must be chemically cleaned, require somewhat more careful treatment. Tantalum parts which have been blasted with steel grit should first be immersed in hot hydrochloric acid to remove particles of iron. The hydrochloric acid may be used as hot and strong as desired; it will not attack the tantalum. The parts should then be thoroughly rinsed with distilled water. Tap water often contains calcium salts which may be converted to insoluble sulphates in the subsequent cleaning processes. If the tantalum parts have not been grit blasted, the hydrochloric acid cleaning maybe omitted, and cleaning may begin with the second step which follows. 

Tantalum parts may be made chemically clean by use of the hot chromic acid cleaning solution commonly used for cleaning glass. A saturated solution of potassium dichromate in hot concentrated sulphuric acid may be used for this purpose, but chromium trioxide is preferred to potassium dichromate because its use eliminates the possibility of potassium residues in crevices or elsewhere on the tantalum parts. 

The cleaning solution should be used at about 110 oC, and should be kept red at all times. When the liquid becomes muddy or turns green it should be discarded. After the chromic acid wash, the parts must be thoroughly rinsed, preferably with hot distilled water. If running distilled water is not available, three dip washes will suffice, but it is important that all cleaning solution be removed. The parts should be dried in clean, warm air, free from dust. They should not be wiped with paper or cloth, nor handled with fingers. Tantalum parts must not be cleaned by hydrogen firing.

 

Grit Blasting:

Tantalum parts for electronic tubes are often blasted with steel grit to provide greater radiation surface. The recommended procedure is a blast of a few seconds with No. 90 steel grit at a pressure of 20 to 40 psi followed by thorough cleaning in hot hydrochloric acid as already described. Sand, alumina, silicon carbide or other abrasives should not be used because they become embedded in the tantalum and cannot be removed with any chemical treatment which would not damage the tantalum.

Since the purpose of grit blasting is to increase the amount of surface per unit of area, the blasting should be done in the manner which will produce fine "whiskers" rather than mere indentations on the surface. Sharp particles of grit will do this, while dull ones merely indent the surface. To achieve best results, the blasting nozzle should be held at an angle nearly tangential to the work, rather than perpendicular to the work. 

 

Machining:

In lathe operations, cemented carbide tools such as VR/Wesson Grade 3A5, with high cutting speeds, have been found most satisfactory. Tools should be kept sharp, and should be ground with as much positive rake as the strength of the tool will with stand. The same rakes and angles as are used with soft copper will usually give satisfactory results with tantalum. A minimum speed of 100 surface feet per minute will be found correct for most turning operations. Slower speeds will cause the metal to tear, especially if annealed metal is being cut. Perchloroethylene or trichloroethane is recommended as a cutting medium and the work must be kept well flooded at all times. Even when filing or using emery cloth, the file or cloth must be kept well wetted with one of these compounds. Basic limiting parameters are as follows:

Set-up                          Rigid

Feed/Cut Length            Heavy                        0.015" IPR

Surface Speed              Slow                           40-50 SFPM

Tooling                         Maintained Sharp        5o - 10o side clearance

                                   Heavy Lube Flow         45o side rake

                                                                    5o back rake

                                                                    5o - 10o front clearance

 


 Welding:

Tantalum may be welded to itself and certain other metals by resistance welding and to itself by inert gas arc welding. Acetylene torch welding is destructive to the metal. 

Resistance welding can be done with conventional equipment, and methods are not substantially different from those used in welding other materials. Because its melting point is 2,700 o F higher than that of SAE 1020 steel and its resistivity is only two-thirds that of SAE 1020 steel, tantalum requires a higher power input to accomplish a sound weld. The weld duration should be kept as short as possible in the range of one and ten cycles (60 cps) to prevent excessive external heating. Where possible, the work should be flooded with water for cooling and reduction of oxidation. 

RWMAC lass 2 electrodes are recommended, with internal water cooling. As in all resistance welding, the work must be cleaned free of dirt and oxides. The electrode contours should be kept of constant area and contour to prevent lowering of current and pressure densities. A common mistake in welding tantalum is to apply too much electrode force which causes so little interface resistance that no weld is made. 

Strong, ductile welds can be made by the tungsten inert gas (tig) method. Extreme care must be taken to cover with an inert gas all surfaces which are raised above 600 oF by the welding heat. Helium, argon, or a mixture of the two gases, creates an atmosphere which prevents embrittlement by absorption of oxygen, nitrogen or hydrogen into the heated metal. Where a pure, inert atmosphere is provided, the fusion and adjacent area will be ductile. Extremely high ductility can be obtained in a welding chamber which can be evacuated and purged with inert gas.

Where the use of a welding chamber is not practical, the heated surfaces can be protected by proper gas-backed fixturing. This usually serves three purposes: (1) to hold the work in alignment, (2) to chill the work in order to limit the heat area, and (3) to act as a conduit for the inert gas and to exclude air from the heat area. Weld ductilites in the order of a 180 o bend over on metal thickness can be consistently accomplished where back-up gas fixtures and gas-filled trailing cup are used.