Molybdenum is number 42 on the periodic table. With a melting point of 2610°C, molybdenum has a density of 10.22 gm/cc. It has many properties that make it an excellent candidate for fabricated parts that must be made of a refractory metal.

Molybdenum has been used for many years in the lamp industry for mandrels and supports, usually in wire form. Today, several unique properties of molybdenum that satisfy more demanding industry requirements have increased the use of molybdenum as a material in applications requiring other mill forms.

Physical Properties of Molybdenum

PROPERTY
Atomic Number	42
Atomic Weight	95.94
Density (20°C)	10.22 g/CC
Melting Point	2896 K, 2610°C, 4753°Fm
Boiling Point	4912 K, 5560°C, 8382°F
Coefficient of Thermal Expansion (20°C)	4.9 x 10-6/°C
Electrical Resistivity (20°C)	5.7 microhms-cm
Electrical Conductivity	30% IACS
Specific Heat	.061 cal/g/°C
Thermal Conductivity	.35 cal/cm2/cm°C/sec
Modulus of Elasticity (20°C)	46 x 106 psi

Molybdenum has several alloys. For the purpose of this brochure, only alloys that are predominantly molybdenum will be considered.

• TZM (titanium, zirconium, molybdenum)

  Molybdenum's prime alloy is TZM. This alloy contains 99% Mo, 0.5% Ti and 0.08% Zr with a trace of C for carbide formations. TZM offers twice the strength of pure moly at temperatures over 1300°C. The recrystallization temperature of TZM is approximately 250°C higher than moly and it offers better weldability.

  The finer grain structure of TZM and the formation of TiC and ZrC in the grain boundaries of the moly inhibit grain growth and the related failure of the base metal as a result of fractures along the grain boundaries. This also gives it better properties for welding. TZM costs approximately 25% more than pure molybdenum and costs only about 5-10% more to machine. For high strength applications such as rocket nozzles, furnace structural components, and forging dies, it can be well worth the cost differential.

  TZM is available in sheet and rod form in basically the same size range as moly with the exception of thin foil. Rembar is experienced in the fabrication of TZM. Refer to the separate TZM sheet.
  

• Moly/30% Tungsten

  This is another moly alloy that offers unique properties. It was developed for the zinc industry. This alloy resists the corrosive effects of molten zinc. Mo/30W has also proved effective in rocket nozzles and has the potential of offering enhanced performance in applications where any erosive effects are a factor.
  

• Moly/50% Rhenium

  This alloy offers the strength of moly with the ductility and weldability of rhenium. It is a costly alloy and it is only available in a very limited size range. It offers significant advantages in thin foil applications for high temperature delicate parts, especially those that must be welded. Note that, although this alloy is nominally 47% rhenium, it is customarily referred to 50/50 moly/rhenium. Other moly/rhenium alloys include moly/rhenium sheet with 47.5% and 41% rhenium. The moly/41%Re alloy does not develop sigma phase. This makes the material even more ductile after exposure to high temperatures.

There is an increasing demand from the electronics and aerospace industries for materials that maintain reliability under ever-increasing temperature conditions. Because its properties meet these requirements, molybdenum also is experiencing an increasing demand.

Characteristics that support the demand for molybdenum in many electronics applications are its:

• exceptional strength and stiffness at high temperatures
• good thermal conductivity
• low thermal expansion
• low emissivity
• low vapor pressure
• electrical resistivity
• corrosion resistance
• purity
• ductility and fabricability
• machinability

Molybdenum can be furnished in many mill forms such as wire, ribbon, foil, plate, sheet, rod, billet, slab, bar, extruded shapes, tubes, and powder.

Molybdenum has a straight-line expansion. The mean coefficient of expansion is 4.9 x 10^-6 measured between 20°C and 500°C. Molybdenum is suitable for sealing to hard glass since it has approximately the same coefficient of expansion and a transition temperature below 700°C.

Molybdenum oxides dissolve readily in glass. The adhesion between glass and this metal is very satisfactory and gives an absolutely tight seal.

It is essential for the surface of the metal to be correctly oxidized before it comes into contact with the glass. This is easily affected, provided that the surface is clean and free from grooves and cracks. The molybdenum supplied by Rembar is produced with extreme care to obtain a uniform oxide film.

The best method of oxidizing the surface is to heat it for a short time in an air-gas or ozygen-gas flame. Excessive oxidation must be avoided since it results in incomplete absorption of the oxide in the glass. This can possibly render the seal to be porous.

Molybdenum should be oxidized by rapid heating, maintained at high temperature for a short period. The gas flame itself is a guard against excessive oxidation. This is indicated by a slight emission of smoke. Conversely, the reducing part of the flame provides insufficient oxidation and, therefore, must be avoided.

The most favorable sealing-in temperature depends upon the viscosity of the hard glass and lies between 1000°C and 1200°C. The pre-oxidized rod, after slight cooling, has a blue color, indicating a low oxide.

Molybdenum used for sealing glass is principally used in the form of wire and rod from about .040 inches in diameter and larger. Seals made with molybdenum are perfectly free from bubbles provided that the glass used is clear and free from bubbles. This is of special importance for high-voltage tubes because bubbles in the glass will reduce the dielectric strength of the seal.

Molybdenum provides corrosion resistance that is similar to tungsten. Molybdenum particularly resists non-oxidizing mineral acids. It is relatively inert to carbon dioxide, ammonia, and nitrogen to 1100°C and also in reducing atmospheres containing hydrogen sulfide.

Molybdenum offers excellent resistance to corrosion by iodine vapor, bromine, and chlorine, up to clearly defined temperature limits. It also provides good resistance to several liquid metals including bismuth, lithium, potassium, and sodium.

Pressed and sintered or recrystallized molybdenum machines very much like medium hard cast iron. Wrought molybdenum machines similar to stainless steel. Once molybdenum's few peculiarities are known and respected, it can be machined with conventional tools and equipment.

The machining characteristics of molybdenum differ basically from those of medium hard cast iron or cold rolled steel in two ways:

• It has a tendency to break out on the edges when cutting tools become dull.
• It is very abrasive and causes tools to wear out much faster than steel. Once the expected tool life has been established for a particular operation, establishing a program of scheduled tool replacement will permit maximum machining efficiency at minimum time and investment.

Data was gathered from work on unalloyed and alloyed molybdenum. Only minor variations were found between the machinabilities of arc-cast molybdenum and the form produced by powder metallurgy.

Most turning operations on molybdenum and molybdenum alloys are consistent with machining practices on steel. However, the cost of machining can be a concern.

The only obvious differences are that:

• Cutting speeds are relatively fast. They are 50% faster than high strength steels and almost 3 times as fast as nickel-base alloys.
• Greater attention must be paid to tool geometry and tool replacement.

In general, any of the straight tungsten carbide tools are suggested for use. General purpose HSS tools may be used for rough turning.

Recommendations for the turning of molybdenum are as follows:

• The lathe should be rigid and well powered.
• The workpiece should be well clamped and rigidly supported.
• Because molybdenum is shock-sensitive, care must be taken when mounting the workpiece.
• Avoid excessive chucking pressure. It could distort the workpiece or cause a fracture even before machining begins. A useful practice is to apply copper shims at all chucking and workpiece contact points.
• Tool overhang should be kept to a minimum.
• Tools must be kept sharp to avoid buildup that can cause failure of the tool or the workpiece. Tool geometry similar to that used for cast iron is generally suitable. Liberal rake and clearance angles can be used.
• Cutting fluids are required to control the tool tip temperature. Water soluble cutting oils work well.

Threading is usually performed by thread grinding, single point turning or chasing operations. Carbide tools should be used and best results are obtained when:

• the back rake is 10°
• the side rake is 5°
• provide clearances of:
  10° on the leading edge
  3° on the trailing edge
• Rough threading is done best at speeds from 70 to 125 feet per minute.

For making fine threads or shallow threads, grinding is usually more practical than turning. The use of dies is not recommended for threading because they have a tendency to tear or pull threads from the workpiece.

If tapping must be performed, care should be taken that the tap is very sharp, perfect alignment is maintained, and a tapping compound is used. Thread rolling is also possible to produce the strongest threads.

For face milling operations, the conventional carbide or carbide-tipped face mills designed for use on cast iron are employed. High-speed steel will also work, but cutter wear is rapid and tool life is very short. Tool angles are the same as used on cast iron. Cutter conditions should be checked frequently and workpiece backup plates should be used, particularly for heavy stock removal.

Rough cutting of molybdenum in depths of .05 inch to .10 inch calls for speeds of 100-160 feet per minute (fpm) at an average feed of .005 inches of feed per tooth. Finish cuts are made mainly at speeds of 350-400 fpm with a range of the depth of cut from .001-.003 inch and afeed in a range of 0.004-0.005 inches of feed per tooth.

Care must be taken to eliminate corner and edge breakout, especially when using multiple cutters. This can be minimized by in-feeding. Breakdown of one cutter can cause vibration from eccentric loading. This will result in a poor surface finish and accelerated breakdown of the other cutters.

A cutting fluid of soluble oil should be used since it has a decided influence on the effective tool life.

Drilling Setup

Drilling molybdenum and molybdenum alloys presents to special problems. Standard carbide drills are used. Because high temperatures will be generated, extra care must be exercised in cooling the drill. Variations of heat expansion between the drill and the workpiece can cause excessive binding that can result in tool failure or damage to the workpiece.

Heavy duty machines with substantial power, absolute rigidity, and a true running spindle with no end play are necessary for successful drilling. The workpiece should be adequately supported at the point of thrust to forestall vibrations. When small parts are to be drilled, this may require fixturing for support.

Drill rigidity is important. In addition to using the shortest drills possible, the use of a drill bushing should also be considered.

For the drilling process itself, the following considerations need to be observed:

• Standard carbide drills are used with a 118° angle to provide maximum drilling efficiency.
• A generous flow of soluble oil coolant should be maintained.
• Maintain a positive, consistent feed rate to avoid work-hardening and loss of tool life. Light feed rates normally will provide significantly longer tool life.

Drilling Tools and Geometry

Standard points ground to 118° angles with clearance angles of approximately 10° are the most widely used.  Carbide insert drills may also be used.

Crankshaft points may also be considered since they reduce the area of contact and minimize heat build-up.

All drills should be carefully checked for sharpness and proper geometry before being put to use.

Drilling Techniques

Positive drill feed should always be maintained. Any riding of the drill inside the hole without cutting causes excessive temperature and reduces tool life. Also, the drills should be examined periodically during production and re-sharpened or replaced at the first sign of wear. Once operating performance has been established, a drill replacement schedule should be established.

A copious flow of soluble drill oil coolant is recommended.

When drilling through holes, the workpiece should be backed up to prevent edge breakout.

Grinding of molybdenum should be considered primarily for finishing, not for major stock removal. Grinding can be handled on conventional machines with standard feeds and speeds. As long as the machines are in good condition and vibration free, standard practices produce good results.

Molybdenum, like some steels, is susceptible to surface heat checking. Therefore, soft grade wheels are used and they should be sharply dressed. Carborundum wheels No. GA-463-J6-V-10 can be used for rough grinding and No. PA-60-H8-V40 wheels can be used for finish and contour grinding.

The chart entitled "Grinding Recommendations for Unalloyed Molybdenum" lists a wide range of recommendations that can serve as starting points for most applications. Individual variations will depend on the equipment used and the particular job operation required.

Copious amounts of standard grinding coolant should always be used. Soluble oil mixtures are recommended over highly chlorinated or highly sulfurized fluids.

Molybdenum saws readily with HSS band or hacksaws. No coolant is required, although it may be used. Approximately 1/8" should be allowed for the kerf and 3/16" for the camber on heavier sections. Abrasive cut-off operations can also be used. Flame cutting, however, is not recommended.

Both of these machining processes work well with molybdenum. Stock removal rates of up to .5 in³/min and +/-.0005" tolerances have been achieved with electrical discharge machining. "EDM wire cutting" is employed for intricate molybdenum shapes.

Electrochemical machining is normally capable of about 1 in³/min stock removal at 10,000 amps. Electrochemical grinding is particularly effective in producing ultra-fine finishes on molybdenum.

Heated to the proper temperature, molybdenum sheet can be accurately formed into complex shapes. Sheets under .020 inches thick will normally take a 180° bend at room temperature.

Red heat may be required for forming heavy plate. If necessary, dies and tools can be warmed with infrared lamps or strip heaters to assist in the bending process.

Conventional equipment is normally satisfactory for this operation and moderate heating is recommended. Sharp tools with close tool clearances of approximately 5% of the sheet thickness are essential to clean cutting action without the sheet cracking or delamination occurring.

Wall reductions of as much as 20% between heat treatments have been achieved with the deep drawing process. Heating of both sheet and dies are suggested for best results on sheets over .020 inches thick. Conventional equipment, tooling and lubricants normally produce acceptable results.

The use of stress-relieved material and the continuous application of heat are the only precautions to observe. Otherwise, molybdenum can be routinely fabricated into a variety of shapes.

Mechanical joining such as bolting, riveting, and lock seams are the simplest methods of joining molybdenum where fluid-tight joints are not required. It is recommended that rivets be heated in place to 400° to 1400°F, depending on the section size. Lacing with molybdenum wires is often employed for parts such as furnace shields.

Joining of molybdenum parts can be accomplished using conventionally accepted welding techniques except for gas. Heli-arc welding is most common and usually provides satisfactory results. Complex welding operations may require more sophisticated or special techniques.

Careful cleaning of the joint surfaces is essential. Controlled weld atmospheres, such as a dry box, are desirable but not necessary. In designing fixtures, all clamping forces should be compressive and should be released immediately after welding to permit unstressed cooling.

The resistance welding of most refractory metals and their alloys is not normally done for several reasons :

• The resulting cast, coarse grained microstructure has essentially no ductility or toughness.
• The coefficients of thermal expansion are much lower than most common metals, that results in cracking upon cooling if dissimilar metals are joined. 
• The Refractory Metals are also chemically reactive and will oxidize unless heated in inert gas or vacuum. Some Electron Beam Welding is perform to join these metals for low stress applications.

Copper-based alloys are normally acceptable in creating relatively low-strength joints. Higher strength joints can be achieved by using gold, platinum or other more exotic base brazing alloys. Refer to the table of Brazing Filler Metals in the Rembar Technical Data Section.

With proper temperature precautions, brazing will normally produce a more ductile joint than welding. Like tungsten, molybdenum has excellent high-temperature properties; however, poor oxidation resistance requires coating protection at higher temperatures. The presence of minute quantities of oxygen, nitrogen, and carbon lower the ductility of molybdenum.

Annealing

Annealing consist of heating the material to a temperature above or within the critical range, then cooling it at a predetermined slow rate (usually in a furnace) to produce a coarse pearlite structure.  Annealing is dependent on many things.  

Some shops prefer to machine dry on almost all operations. High side-rake angles on cutting tools promote efficient chip flow and reduce the incidence of heat oxidation. With such angles, chip oxidation is not encountered until cutting speeds of 400-600 fpm are used with feeds ranging from 0.005 to 0.010 ipr. At this point, cooling can be accomplished by an air blast directed at the chip and cutting edge.

When machining dry, with or without an air blast, care must be taken to prevent the accumulation of chips into a pile. Such piles will cause heat to be retained, hastening oxidation and reducing the scrap value. In this case, the air blast can be used to scatter the chips.

It must also be remembered that molybdenum can also produce fine, abrasive dust under certain machining conditions. This can cause build-up on the tool, contrary to the steel's behavior, as well as cause tool failure and rapid tool mortality unless the dust is removed from the cutting site. This is best accomplished by flooding with fluids.

Another operational problem occurs because of molybdenum's low coefficient of expansion relative to steel tooling. This occurs mainly during drilling, reaming or tapping operations when overheating could cause the tool to expand and bind in the hole. Here, large volumes of coolant must be delivered to the cutting points. Liquid coolants that can be used include cutting oils with additives, kerosene, soluble oil, and trichlorethylene.

A good coolant that does not contaminate molybdenum is a 50/50 mixture of chlorinated cutting oil and trichlorethylene. Note that sulfur-based cutting oils have also been suggested for roughing cuts, but they are not recommended for finish cuts because of their effects on the surface finish.

Good surface finishes are also obtained when trichlorethylene alone is used. It is important to note that, while trichlorethylene is the best cutting fluid, Environmental Protection Agency regulations will all but outlaw its use in 1995.

Because effective tool life is short when these metals are machined - measured usually only in minutes or inches - even a small superiority factor of one fluid over another can mean worthwhile savings in tooling and labor costs.

A cleaning process is designed to deal with one or more of the following:

• surface scale
• general contamination
• removal of a basis metal

Of all the potential contaminants in wrought products, iron is of primary concern. Others, such as aluminum, carbon, calcium, copper, and nickel among others, may also be present as elements, but they are more frequently present in the form of oxides.

Removal of a controlled amount of basis metal may be desired to insure complete removal of contaminants. There are three main methods for cleaning molybdenum.

1. Immerse the material in a glass cleaning etch composed of:
   95% H₂SO₄
   4.5% HNO₂
   0.5% HF and CRO
   (equivalent to 18.8gm/1)
2. First, immerse in an alkaline bath composed of:
   10% NaOH
   5% KMnO₄
   85% H₂O
   The temperature of the bath should be kept between 150 - 180°F (66 - 82°C) with a soak duration of 5 to 10 minutes.
   When immersion in the alkaline bath is complete, immersion in a second bath is required to remove smut that may be formed by the first treatment. The second bath consists of:
   15% H₂SO₄
   15% HCl
   70% H₂O
   6 - 10 wt. % per unit volume of chromic acid
   The second bath should also provide a soak duration of 5 - 10 minutes.
3. The third method is generally applied to the molybdenum alloy TZM. The recommended procedure is:
   1. Degrease the material form 10 minutes in an appropriate solvent.
   2. Immerse in a commercial alkaline cleaner for 2-3 minutes.
   3. Rinse in cold water.
   4. Buff and vapor blast.
   5. Re-immerse in a commercial alkaline cleaner as above.
   6. Rinse in cold water.
   7. Electropolish in 80% H₂SO₄ at 120°F (54°C) with 8 -12 amps.
   8. Repeat the process beginning with step c., above.

Nominal Chemical Composition; %;	Ti: 0.50	C: 0.015
	Zr: 0.08	Mo: Balanced

Operations	Tool Material	Tool Geometry	Tool Used for Tests	Depth of Cut; inches	Width of Cut; inches	Feed	Cutting Speed; ft/min	Tool Life	Wearland; inches	Cutting Fluid
Turning	C-2 Carbide	BR: 0°SCEA: 15° SR:20°;ECEA: 15° Relief:5°NR:1/32"	5/8" square brazed tool bit	0.030	n/a	.009 in/rev	450	25 minutes	0.010	Soluble Oil (1:20)
Turning	C-2 Carbide	BR: 0°SCEA: 15° SR:20°;ECEA: 15° Relief:5°NR:1/32"	5/8" square brazed tool bit	0.060	n/a	.009 in/rev	350	20 minutes	0.010	Soluble Oil (1:20)
Face Milling	T-15 HSS	AR: 0°ECEA: 10° RR:20°;CA: 45° Clearance: 15°	4" diameter single tooth face mill	0.060	2	.010 inch per tooth	100	70 inches per tooth	0.015	Soluble Oil (1:20)
Face Milling	C-2 Carbide	AR: 0°ECEA: 5° RR:0°;CA: 45° Clearance: 10°	4" diameter single tooth face mill	0.060	2	.005 inch per tooth	350	100 inches per tooth	0.015	Soluble Oil (1:20)
End Mill Slotting	T-15 HSS	Helix Angle: 30° RR:10°;CA:45° Clearance:10°	3/4" diameter 4 tooth HSS end mill	0.125	0.750	.004 inch per tooth	190	78 inches	0.012	Soluble Oil (1:20)
End Mill Peripheral Cut	M-3 HSS	Helix Angle: 30° RR:10°;CA:45° Clearance:10°	3/4" diameter 4 tooth HSS end mill	0.125	0.750	.004 inch per tooth	190	142 inches	0.012	Soluble Oil (1:20)

Note: Width of wheel per revolution of work. For sharper radii, finer grit size may be required.

	Surface Grinding			Cylindrical Grinding
	Plain Surfacing		Form	Roughing	Finishing	Snagging Portable	Cutoff	Bench Grinding
Wheel Speed SFPM	5500-6500		5500-6500	5500-6500	5500-6500	9,500	10,000	6,500
Table Speed F2M	50		50	1/3 (see note)	1/6 (see note)	n/a	n/a	n/a
Work Speed SFPM	n/a		n/a	70	120	n/a	n/a	n/a
Crossfeed inches	0.032		n/a	n/a	n/a	n/a	n/a	n/a
Grinding Fluid	Oil/water emulsion		Dry	Oil/water emulsion	Oil/water emulsion	Dry	Oil/water emulsion	Dry
Infeed per Pass inches	0.002		0.001	.001-.002	.001 + sparkout	n/a	n/a	n/a
Wheel Dressing	Open		Open	Open	Fine	None	None	Open
Wheel Grading	Dry	Wet
Grain	Aluminum Oxide		Aluminum Oxide	Aluminum Oxide	Aluminum Oxide	Aluminum Oxide	Aluminum Oxide	Aluminum Oxide
Grain Type	Friable	Friable	Friable	Semi-Friable	Semi-Friable	Semi-Friable	Tough	Tough
Grit Size	60	60	100	465	80	203	60	36
Grade	H	J	I	K	K	R	N	L
Structure	8	8	5	6	6	Normal	6	6
Bond	Vitrified		Vitrified	Vitrified	Vitrified	Resinoid	Rubber	Vitrified
Carborundum Gradings
Dry	AA60-H8-V40		AA100-15-V40	n/a	n/a	TA203-R-B5	n/a	A36-L6-V30
Wet	GA60-J8-V40		n/a	DA465-K6-V11	DA80-K6-V11	n/a	A60-N6-RR	n/a

The REMBAR Company, Inc.
P.O. Box 67
67 Main Street
Dobbs Ferry, NY 10522

Tel.: (914) 693-2620
FAX: (914) 693-2247