A very interesting thread. Thanks especially to js and also to all who have contributed to the info.
A very interesting thread. Thanks especially to js and also to all who have contributed to the info.
I'm going to add the machining info as a separate post, since most won't be interested, but I'll reference this post in the OP. OK. Here goes:
Traditional Machining of Titanium
The technology supporting the machining of titanium alloys is basically very similar to that for other alloy systems. Efficient metal machining requires access to data relating the machining parameters of a cutting tool to the work material for the given operation. The important parameters include:
Cutting tools and fluids
Guidelines. The following guidelines, based in large part on the inherent factors affecting the machinability of titanium described above, contribute to the efficient machining of titanium:
Use low cutting speeds: A low cutting speed helps to minimize tool edge temperature and maximize tool life. Tool tip temperature is strongly affected by cutting speed. Lower speeds are required for alloys such as Ti-6Al-4V than are necessary for unalloyed titanium.
Maintain high feed rates: The highest rate of feed consistent with good practice should be used. Tool temperature is affected less by feed rate than by speed. The depth of each succeeding cut should be greater than the work-hardened layer resulting from the previous cut.
Use a generous quantity of cutting fluid: A coolant provides more effective heat transfer. It also washes away chips and reduces cutting forces, thus improving tool life.
Maintain sharp tools: Tool wear results in a buildup of metal on the cutting edges and causes poor surface finish, tearing, and deflection of the workpiece.
Never stop feeding while tool and titanium are in moving contact: Allowing a tool to dwell when it is in moving contact with titanium causes work hardening and promotes smearing, galling, and seizing. This can lead to a total tool breakdown.
Use rigid setups: Rigidity of the machine tool and workpiece ensures a controlled depth of cut.
Although the basic machining properties of titanium metal cannot be altered significantly, their effects can be greatly minimized by decreasing temperatures generated at the tool face and cutting edge. Economical production techniques have been developed through application of the previously mentioned basic rules in machining titanium.
Tool Life. Tool life data have been developed experimentally for a wide variety of titanium alloys. A common way of representing such data is shown in Fig. 10.2, where tool life (as time) is plotted against cutting speed for a given cutting tool material at a constant feed and depth in relation to Ti-6Al-4V. It can be seen that tools for machining titanium alloys are very sensitive to changes in feed. At a high cutting speed, tool life is extremely short; as the cutting speed decreases, tool life dramatically increases. Industry generally operates at cutting speeds promoting long tool life.
Figure 10.2: Effect of cutting speed and feed on tool life during the turning of Ti-6Al-4V alpha-beta alloy:
Forces and Power Requirements. Cutting force is important because, when multiplied by the cutting velocity, it determines the power requirements in machining. For general approximations, the power requirements in turning and milling can be obtained by measuring the power input to the drive motor of the machine tool during a cutting operation and by subtracting from it the tare, or idle power. A good approximation of the horsepower required in most machining operations can be predicted from unit power requirements. Table 10.2 shows the power requirements for titanium in comparison with other alloys.
Table 10.2: Average unit power requirements for turning, drilling, or milling of titanium alloys compared with other alloys systems:
Tool Materials. Cutting tools used to machine titanium require abrasion resistance and adequate hot hardness. Despite the use of new tool materials—such as special ceramics, coated carbides, polycrystalline diamonds, and cubic boron nitride—in metal removal of steels, cast irons, and heat-resistant alloys, none of these newer developments have found application in increasing the productivity of titanium machined parts.
Generally, only straight carbide and general-purpose high-speed or highly alloyed tool steels can be used. Carbide tools (such as grades C-2 and C-3), if feasible, optimize production rates. General-purpose high-speed tool steels (such as grades Ml, M2, M7, and M10) also are used. However, better results are generally obtained with more highly alloyed tool steel grades, such as T5, T15, M33, and the M40 series. Cutting tool performance is influenced by many factors. Setup, processing methods, grinding techniques, material quality, and the condition of the machine tool and fixturing all influence cutter performance.
In early studies, the straight tungsten carbide cutting tools, typically C-2 grades, performed best in operations such as turning and face milling, while the high-cobalt, high-speed steels were most applicable in drilling, tapping, and end milling. The situation remains much the same today. C-2 carbides are used extensively in engine and airframe manufacturing for turning and face milling operations. Solid C-2 end mills and end mills with replaceable C-2 carbides find application, particularly in aerospace plants. M7 and the M42 and M33 high-speed steels are recommended for end milling, drilling, and tapping of titanium alloys.
Cutting Fluids. The correct use of coolants during machining operations greatly extends cutting tool life, and this is particularly true for titanium alloys. Chemically active cutting fluids transfer heat efficiently and reduce cutting forces between tool and workpiece. Of course, cutting fluids should not cause any degradation of the properties of the workpiece. Chlorine at one time was considered a suspect element in cutting fluids, regardless of the concentration and specific conditions used in titanium alloy manufacturing operations. The aversion to cutting fluids containing chlorine was based on the early discovery of hot-salt stress-corrosion damage in titanium alloys through mechanical property studies (see Chapter 13) and on the unexpected cracking of titanium alloys in cleaning and heat-treatment operations.
Although the presence of chlorine ions (e.g., those found in fingerprints on a part) can cause stress corrosion in some alloys during processing, it is not thought to always damage titanium alloys during machining. Nevertheless, cutting fluids used in machining titanium alloys require special consideration. If chlorinated cutting fluids are used on alloys that may be subject to stress-corrosion cracking, carefully controlled postmachining cleaning operations must be followed. The general prohibition against the use of cutting fluids containing chlorine is not universally observed.
When specifying cutting fluids for machining titanium, some companies have practically no restrictions other than the use of con-trolled-washing procedures on parts after machining. Other manufacturers do likewise, except that they do not use cutting fluids containing chlorine on parts that are subjected to higher temperatures in welding processes or in service. Also, when assemblies are machined, the same restrictions apply due to the difficulty of doing a good cleaning job after machining. Still other organizations in aerospace manufacturing permit no active chlorine in any cutting fluid used for machining titanium alloys.
Mechanical property evaluations to define the effect of experimental chlorinated and sulfurized cutting fluids on Ti-6Al-4V alloy indicated that no degradation of mechanical properties relative to those obtained from neutral cutting fluids occurred. Similar results were obtained by using chlorinated and sulfurized fluids in machining, or by having those cutting fluids present as an environment during testing. These results and others suggest that under certain conditions, chlorine-containing cutting fluids are not detrimental to titanium alloys.
Usually the heavy chlorine-bearing fluids excel in operations such as drilling, tapping, and broaching. The use of chlorine-containing (or halogen-containing) cutting fluids generally is not a recommended practice, however. There are excellent cutting fluids available that do not contain any halogen compounds. Actually, for certain alloys and operations, dry machining is preferred. Figure 10.3 shows the effect of various cutting fluids on tool life in drilling Ti-6Al-4V.
Machining Speeds and Feeds. Cutting speed and feed are two of the most important parameters for all types of machining operations. Table 10.3 gives some speed and feed data on turning of selected titanium alloys. Because speed and feed rates have a direct influence on tool life, it is desirable to have charts or graphs for all possible tool and titanium alloy combinations, as well as machining techniques. Considering the range of alloys, tool compositions, and machining techniques possible, such charts are not likely to be available for all situations. However, charts such as Table 10.3 have been compiled for some other machining techniques.
Machining recommendations, such as noted above in Table 10.3 and similar sources, can require modification to fit particular circumstances in a given shop. For example, cost, storage, or other requirements can make it impractical to accommodate a very large number of different cutting fluids. Savings achieved by making a change in cutting fluid can be offset by the cost of changing fluids. Likewise, it might not be economical to inventory cutting tools that have only infrequent use. Furthermore, the design of parts can limit the rate of metal removal in order to minimize distortion (e.g., of thin flanges) and to corner without excessive inertia effects.
An illustration of typical machining parameters used to machine Ti-6Al-4V bulkheads containing deep pockets, thin flanges, and floors at an airframe manufacturer is given in Table 10.4. A bulkhead frequently contains numerous pockets and some flanges as thin as 0.76 mm (0.030 in.). Typical bulkhead rough forgings can weigh in excess of 450 kg (1000 lb), but the finished part is less than 67.5 kg (150 lb) after machining. Extensive machining is done on gas turbine engine components, just as is done on the larger airframe components. Table 10.5 lists typical parameters for machining Ti-6Al-4V jet engine components, such as fan disks, spacers, shafts, and rotating seals.
Table 10.4: Some typical machining parameters used to machine airframe bulkheads from an alpha-beta (Ti-6Al-4V) alloy:
Table 10.5: Example of typical parameters for machining gas turbine components from an alpha-beta (Ti-6Al-4V) alloy:
Increased Productivity with Special Techniques. The inability to improve cutting tool performance for titanium alloys by developing new cutting tool materials—coatings in particular—has been very frustrating. Likewise, very little improvement in productivity has been experienced by exploring new combinations of speeds, feeds, and depths. Some developments of interest include specially designed turning tools and milling cutters, along with the use of a special end-mill pocketing technique.
One of the practical techniques for increasing productivity is to determine the optimum cost in machining a given titanium part for a specific machining operation. If specific data are available relating tool life to speed, feed, and depth for a given operation and cutter, it is possible to calculate the overall cost and time of machining as a function of the cutting parameters. Some companies are using computers to perform such cost analyses and to arrive at minimum costs and optimum production rates for specific machining operations.
Fire Prevention. Fine particles of titanium can ignite and burn. Use of water-base coolants or large volumes of oil-base coolants generally eliminates the danger of ignition during machining operations. However, an accumulation of titanium fines can pose a fire hazard. Chips, turnings, and other titanium fines should be collected regularly to prevent undue accumulation and should always be removed from machines at the end of the day.
Salvageable material should be placed in covered, labeled, clean, dry, steel containers and stored, preferably in an outside yard area. Unsalvageable fines should be properly disposed. Titanium sludge should not be permitted to dry out before being removed to an isolated, outside location.
Dry powders developed for extinguishing combustible metal fines are recommended for the control of titanium fires. For maximum safety, such extinguishers should be readily available to each machinist working with titanium. Dry sand retards, but does not extinguish, titanium fires. Carbon dioxide and chlorinated hydrocarbons are not recommended.
Water should never be applied directly to a titanium fire.
And here's what Donachie has to say about welding titanium:
Joining a Reactive Metal
Titanium has a strong chemical affinity for oxygen, and a stable, tenacious oxide layer forms rapidly on a clean surface, even at room temperature. This behavior leads to a natural passivity that provides a high degree of corrosion resistance. The strong affinity of titanium for oxygen increases with temperature and the surface oxide layer increases in thickness at elevated temperatures. At temperatures exceeding 500 °C (930 °F), the oxidation resistance of titanium decreases rapidly and, as noted previously, the metal becomes highly susceptible to embrittlement by oxygen, nitrogen, and hydrogen, which dissolve interstitially in titanium. Therefore, the melting, solidification, and solid-state cooling associated with fusion welding must be conducted in completely inert or vacuum environments. Similarly, the temperatures and times used for solid-state bonding or for brazing require that processing be conducted in an inert or vacuum environment.
Open-air techniques can be used with fusion welding when the area to be joined is well shielded by an inert gas. By and large, however, atmospheric control by means of a "glove box," temporary bag, or chamber is preferred.
Temperatures for all of the customary metallic joining processes can range from low in the alpha-beta range, above approximately 538 °C (1000 °F), to above the melting temperature of the respective alloys. Solidified cast structures can arise in the cast weld metal area. Coarse structures can form in the weld fusion zone or in the heat-affected zone (HAZ) of a fusion-welded joint, due to holding at high temperatures or due to slow cooling rates from the joining temperatures. Coarse structures can arise in solid-state bonding processes, especially because the joining temperature can be high in the alpha-beta range, and cooling rates from joining can be low.
Special Considerations. Because titanium is a very reactive material and interacts with many atmospheres, special considerations are required both before and during joining to ensure successful joints and acceptable strength of titanium and its alloys. Titanium and titanium alloys can be successfully joined for applications ranging from subzero levels to elevated temperatures when proper precautions are taken and correct preparations are made. Most welding techniques are available for titanium. Titanium alloys can be fusion and solid-state welded, as well as brazed. No fluxes are used when fusion or solid-state welding titanium and its alloys, but fluxes can be used in some situations for brazing.
Because titanium alloy welds are commonly used in fatigue-critical applications, a stress-relief operation is generally required following welding. Specific stress-relief temperatures and times depend on the base-metal.
Three principal conditions need to be met in titanium joining:
Detrimental interstitial elements must be excluded from the joint region
Contaminants (e.g., scale and oil) must be excluded from the joint region
Detrimental phase changes must be avoided to maintain joint ductility
The essence of welding titanium and its alloys is adherence to these principles.
The repeated emphasis on lowering the cutting-surface temperature makes me wonder if it would be beneficial to run the (filtered) cutting fluid through a refrigeration unit to further reduce the temperature of the cutting surface.
Nicely done JS
If it's black, nasty & smelly (which describes heavy chlorinated cutting oils) it works like magic in the most difficult Ti operations. Hate to use it as it stains everything but there's nothing better.
For general purpose lathe or mill work the latest development is MQL (minimum quantity lubrication) using vegetable based oils. Many automated machine tools have through the spindle MQL which applies the oil exactly where needed.
One point to remember whenever lathe turning is that the unbroken chips may form a bird's nest which requires stopping the machine to remove the turnings. Ti ribbons are razor sharp & cut like a knife. I forgot this recently & have a deep cut on the tip of my middle finger. Bled like a stuck hog which is inconvenient as blood causes rusting on the highly finished guide ways on a lathe - took probably half an hour to finally stop
Interesting. I've been lubricating my titanium screw threads with Group 5 gear oil, which, while not chlorinated, does have exposed sulfur atoms on the ends of the molecular strands. (Group 5 oil molecules were once described to me as looking like Koosh balls, albeit with fewer strands, as opposed to older oils which are just single strands.) Stuff works great, but it smells a bit like rotten eggs when put under stress. No idea how it would do as a cutting fluid though.
I keep hearing about the work hardening issue while machining titanium, but I don't recall anyone mentionning heat treatment being offered on a titanium flashlight.
Pointless since it's tough enough un-treated ? Probably, but then does a SPY007 actually need to be so perfectly machined the way it is ? Do we actually need sapphire front lens ?
Since some high-end custom offering are seeking the best among the best, why is heat treatment never offered ?
Work-hardening isn't caused by heat, it's caused by flexing. Using a machining bit to "hammer" the titanium just to make it a little harder would be a huge waste of tools. Probably better to get a bead-blasted finish instead.
On a similar note, I have noticed that with my anodizing rig, if I bump the voltage above say 80 V I get essentially a silver finish in the titanium but it seems to be a thick oxide film and rather hard and more scratch resistant. I have also tumbled some titanium parts with some granite rock and they too seem to have a dull and somewhat harder surface.
I suspect that heat treating of Ti is not very common and I imagine it needs to be done in an inert atmosphere which would add to the cost and may not be that readily available. You have the alpha and beta molecular structuring of Ti and as I recall heat treating can alter the surface structure. Some of my failed welds I believe were due to really hard and embrittled (sp) surface metal; probably nitrogen contamination. Mission Knives is one company who doesn't stop with machined titanium but takes it further with heat treating and I think they even have some proprietary processes involved.
I worked in the R&D lab of that aerospace fastener company and it was located in down town Newport Beach, CA right on PCH. There was even a small boatyard as part of the property and the owner of the company kept his private yacht docked at the facility (before my time). I heard of a time that the owner had the R&D shop fabricate a fishing gaff for him out of titanium and he insisted that it be sent out for heat treatment. I gathered that the guys had made a beautiful gaff and one that would never be duplicated due to the effort involved. It had a great curve to it and perfectly sharpened point. They sent it out for heat treatment against their better judgment and certainly a case of major overkill in their opinions. It came back from the heat treatment straight as an arrow. Apparently the treatment process induced some form of memory from the Ti and it completely lost its bent form. Strange metal!
A sapphire window may indeed be overkill but IMHO the alternatives are inadequate in some realistic applications and uses. Heat treating Ti is also an overkill in these lights, in my opinion, but with non heat treated titanium as a viable alternative.
So, I've just read some of the sections in the Donachie book on Heat Treatment. The short of it is this: heat treatment is very difficult for the reasons Don already mentioned: it must be done in an inert atmosphere to avoid oxygen and nitrogen infiltration and consequent embrittlement and degradation.
(The beta transus of Ti-6-4 is 1000C +/- 20 C) (!)Any heat treatment at temperatures above about 427 °C (800 °F) must provide the titanium or titanium alloy with an atmospheric protection that prevents pickup of oxygen or nitrogen and formation of alpha case. The protection also obviates the possibility of undesirable scale formation. (Contamination during heat treatment is discussed later in this Chapter.)
Moreover, it only increase the hardness and strength by a pretty small margin in the case of Ti-6-4. As received from the supplier, grade 5 Ti-6-4 has a yield strength of 141ksi. With further heat treatment, you can increase this to 153ksi. However this will be at the expense of ductility. And, as I just implied, grade 5 Ti-6-4 is already annealed. In fact, all the stock titanium has to be straightened by what is essentially annealing:
Straightening, Sizing, and Flattening. Straightening, sizing, and flattening of titanium alloys are often necessary to meet dimensional requirements because it can be difficult to prevent distortion of close-tolerance thin sections during annealing. Because titanium alloys have excessive springback, the straightening of bar to close tolerances and the flattening of sheet present major problems for titanium producers and fabricators. Straightening, sizing, and flattening can be conducted independently of other related processes or can be combined with annealing (or stress relief) by use of appropriate fixtures.
Unlike aluminum alloys, titanium alloys are not easily straightened when cold, as explained previously. (See the section "Forming" in Chapter 5.) Because of springback and resistance to straightening at room temperature, it is necessary to employ elevated-temperature forming. Therefore, titanium alloys are straightened primarily by creep straightening processes.
Creep straightening uses the concept that at annealing temperatures, many titanium alloys have low creep resistance. The creep resistance can be sufficiently low enough to permit the alloys to be straightened during annealing. With proper fixturing and, in some instances, with judicious weighting, sheet metal fabrications and thin complex forgings have been straightened with satisfactory results. Again, uniform cooling to below 315 °C (600 °F) after straightening can improve results.
Creep flattening consists of heating titanium sheet between two clean, flat sheets of steel in a furnace containing an oxidizing or inert atmosphere. Various jigs and processing techniques have been proposed for annealing titanium in a manner that yields a flat product. Creep flattening and vacuum creep flattening are two such techniques. Vacuum creep flattening is used to produce stress-free flat plate for subsequent machining. The plate is placed on a large, flat, ceramic bed that has integral electric heating elements. Insulation is placed on top of the plate, and a plastic sheet is sealed to the frame. The bed is slowly heated to the annealing temperature while a vacuum is pulled under the plastic. Atmospheric pressure is used to creep flatten the plate.
So I'm guessing that there is no way that heat treatment wouldn't be stupidly expensive, and ultimately more or less just cosmetic for our purposes. Although I am definitely curious to hear what the response to Don's question is!
Regarding the gaff that came back from heat-treatment un-bent: http://en.wikipedia.org/wiki/Nickel_titanium
Your post reminded me of NiTiNOL memory wire I used to play with as a kid.
I have a small electric furnace that can go up to 2400F as I recall. I have used it to memory form nitinol which really is an amazing alloy of Ti. I have also thought about sticking some Ti flashlight parts in there and elevating them in temp perhaps with some cloisonne powder or even organic materials (I remember doing raku ceramics in high school) just to see what might result but then again, it might be akin to a kid playing with an adult chemistry set and unwanted accidents resulting.
The Haiku that survived the fire got me thinking about playing with the furnace again but who know what risks might be involved. At the minimum probably a respirator should be worn.
I remember some weld beads I made on titanium joints that I was hardly able to grind off due to contamination probably but if one could get that kind of surface hardening on a titanium light you would really have to go our of your way to scuff it up! But you might drop the light and have it break.
Not only have you worked in the coolest places, lived (and live, present tense) in the coolest places, but you also have the coolest TOYS! You have an IS as well as a small electric furnace--and presumably a welder and lathe as well? Not to mention flashlights, dive gear, photography stuff, and who knows what else! . . .
That's too cool for words! LOL!
I have spent a lot of time working titanium mainly in my knives
I was not sure but I love the name for Tungsten...
its called Wolfram (from germany originally)
Last edited by gollum; 04-11-2012 at 03:11 AM.
Very interresting stuff on titanium hardening and heat treatment !
Great post on titanium. Hopefully it will help prevent some of the aluminum versus titanium threads that pop up periodically.
It's great to have all the information collected in one thread to reference. By the way, there is a typo in the oxide section. The oxide has a much lower conductivity then the metal.
One thing though is that I would not call titanium inert or corrosion resistant. As you point out in your welding post, it is very reactive. Titanium dioxide, TiO2, however is extremely stable and inert and forms a highly impervious barrier analogous to Al2O3 on aluminum. Also, people do not have allergies to pure gold and pure platinum. It is their alloys that they are sensitive to, most typically alloys with nickel.
After the patient's jaw size is measured, a correct wire size selected and installed. It would follow the patterns of the patient's current teeth that needs to be corrected to a proper curve.
These NiTiNOL wires are pre-set to typical human mouth temperature, so the slow process of 'correcting' and 'aligning' your teeth started
well at least thats how she explained to me heee
They also use special pliers to crimp the wire in various places before installing it, in case they need to modify the curve of the wire to make the patient's teeth fit properly. It aches terribly for the first few days, but after that the nerves give up and go numb.
Thanks! I will clarify the inertness-through-passivation issue in my OP.
Do you have a reference for the conductivity of the oxide layer of titanium? I looked high and low on the internet and only found a technical paper about the use of TiO2 as a semi-conductor and it's conductance vs. the partial pressure of oxygen. I also didn't find any info in the Donachie text, which is more oriented towards mechanical properties than electrical ones. So, I would love it if you could post a link to a reference, or the appropriate section of a hard-copy reference!
And one on the rise of the 58 percent pure platinum alloy:
The Platinum Standard: Purity Issue Divides Jewelers
This is why makers of titanium jewelry tout its hypo-allergenic nature.
Nickel is added to white gold, hence it's incredible durability.
Marduke - Solitaire...I've seen matches which are brighter AND have a longer runtime. 光陰矢の如し
I thought white gold was gold and silver? If a hardener were to be added, I think I'd want the hardener to be platinum or rhodium, not nickel. Nickel is good for hardening industrial alloys, not so much for hardening jewelry.
Another option that one of my friends liked using was palladium white gold; while not as tough as nickel white gold, is better for people allergic to nickel. (Palladium, one of the six platinum group metals is also at times referred to as poor mans platinum.) Rhodium is used for plating white gold, platinum or palladium to give a purely white appearance, but I prefer not to plate because I like the natural appearance as they are.
Marduke - Solitaire...I've seen matches which are brighter AND have a longer runtime. 光陰矢の如し
Huh. Learn something new every day. I'll stick to yellow gold or rose-gold; if I'm going to have something with gold in it, I want it to look like it has gold in it.
This has nothing to do with flashlights but this new David Guetta and Sia song is called Titanium and it's kinda cool.
This is one thread that you will be smarter when you finish reading it. Great info and it busts many of peoples misconceptions of the metal.