Titanium machining. High tech. Milling austenitic and duplex stainless steel

Titanium is one of the most interesting and difficult metals to work with. Its unique properties are widely used in various industries. Mechanical processing of titanium, in comparison with ordinary steel, is more than five times more difficult, therefore, special techniques and equipment are used to create products from it.

The main problems encountered in titanium processing, and the means of their solution

The main problem when machining titanium is its tendency to gall and stick to the tool. Also one of the complicating factors is its low thermal conductivity. Most metals resist melting to a much lesser extent, therefore, upon contact with titanium, they dissolve in it, forming alloys. This leads to rapid wear of the tool used.

To reduce scuffing and sticking, as well as to remove the generated heat, the following methods are used:

• when cutting, as well as other processing of titanium, coolants are used;
• sharpening of products is performed using tools made of hard metal alloys;
• metal processing with cutters is performed at much lower speeds in order to avoid unnecessary heating.

The sticking and scuffing effects of titanium are due to its high coefficient of friction, which is considered a serious disadvantage of this metal. For the most part, titanium products are quickly susceptible to wear, therefore the pure composition of this metal is rarely used for the manufacture of products that are used under friction and sliding conditions. During friction, titanium adheres to the rubbing surface, causing a bonding effect and reducing the speed of movement of the communicating parts. Methods that eliminate this negative effect are titanium nitriding and oxidation.

Titanium nitriding is a technological process, which consists in heating a titanium alloy product to a temperature of 850 0 С - 950 0 С and holding it for several days in an environment of pure gaseous nitrogen. As a result of the chemical reactions taking place on the surfaces of the product, a titanium nitride film is formed, which has a golden hue and is more hard, as well as more resistant to abrasion. Products that have undergone such processing have increased wear resistance and are not inferior in their characteristics to products made of surface-hardened special steels.

Titanium oxidation is a widespread method, which consists in heating a titanium product to 850 0 С and its sharp cooling in an aqueous medium, which causes the formation of a dense film on the surface of the workpiece, which is well connected to the main layer of the material. At the same time, the resistance to abrasion and the overall strength of the product increases 15-100 times.

Some features of cutting and drilling titanium

Slicing blanks is very difficult technological process accompanied by the use of special tools and equipment. Sheets are cut with guillotine shears, and billets from long products are sawn with a power saw. Small diameter rods are cut using lathes.

Milling titanium remains the most challenging method for machining titanium. It sticks to the teeth of the tool (cutter), which greatly complicates the work with the workpiece. Therefore, for this method, tools made of a hard alloy of metals are used, and the processing process is accompanied by the use of cooling lubricants and liquids that have a high viscosity.

When performing drilling operations, it is important that drilling chips do not accumulate in the bleed channels, otherwise this can lead to premature wear and tool breakage. When drilling, cutters made of high-speed steel are used.

Features of the connection of titanium products and their elements

If a titanium product acts as a structural element, then the following methods can be used to connect parts made of titanium alloys:

• welding;
• soldering
• mechanical connection using rivets
• bolted connection.

The main joining method is welding, which is common industrial technology. To ensure the strength of the weld, the elements are connected in an inert gas atmosphere or special oxygen-free fluxes. Also for this, the seam is protected with the use of various protective elements. Interaction of molten titanium with such chemical elements as hydrogen, oxygen and nitrogen contained in the air mixture, when heated, leads to the growth of metal grains, a change in its microstructure and the brittleness of the weld. Welding work is performed at high speed.

There is also a method of welding in a controlled environment, which is used for jobs that require a lot of responsibility. If it is necessary to connect small-sized elements, they are placed in special chambers filled with an inert gas. In the case of connecting larger elements, welding is performed in special hermetically sealed rooms. Titanium welding is a responsible job that is entrusted exclusively to trained professionals with the necessary practical experience and skills.

Brazing titanium is used in cases where welding is impossible or impractical. It is also complicated by chemical reactions. Titanium in the molten state exhibits high reactivity and is strongly bonded to the oxide film formed on the surfaces of the workpiece. Most of the common metals are unsuitable as a solder for joining titanium elements; only pure aluminum and silver are used for these purposes.

Mechanical connection of titanium elements using rivets and bolts is also performed using special materials. In most cases, the rivets are made of aluminum, and the bolts used are coated with silver or synthetic Teflon powder. This is due to the fact that when screwing, titanium exhibits its adhesion property and bulges, as a result of connecting the elements, they become unreliable, do not provide strong fixation.

It is generally accepted that titanium lends itself to machining like stainless steels. This means that machining titanium is 4-5 times more difficult than conventional steel, but this is still not an insoluble problem.
The main problems in titanium processing are its high tendency to sticking and galling, low thermal conductivity, as well as the fact that almost all metals and refractory materials dissolve in titanium, as a result of which it is an alloy of titanium and a hard material of the cutting tool. Such processing causes rapid wear of the cutter.

Coolants are used to reduce adhesion and scuffing and to dissipate large amounts of heat generated during cutting. The workpiece is turned using carbide cutters, and the processing speed is usually lower than when turning stainless steel.

If it is necessary to cut titanium sheets, then this operation is carried out on guillotine shears. Sections of large diameters are cut with mechanical saws using coarse-toothed cutting blades. Less thick bars are cut on lathes.

When milling, titanium stays true to itself and sticks to the cutter teeth. Cutters are also made of hard alloys, and lubricants with high viscosity are used for cooling.

When drilling titanium, the main attention is paid to ensure that chips do not accumulate in the take-off grooves, as this will quickly damage the drill. High-speed steel is used as a material for drilling titanium.

When titanium is used as a structural material, titanium parts are connected to each other and to parts made of other materials by different methods.

The main method is welding. The earliest attempts to weld titanium were unsuccessful, due to the interaction of the molten metal with oxygen, nitrogen and hydrogen in the air, grain growth during heating, changes in the microstructure and other factors leading to the brittleness of the weld. However, all these problems, which previously seemed insoluble, were solved in the shortest possible time. Nowadays, titanium welding is a common industrial technology.

But while the problems have been resolved, welding titanium has not become simple and easy. Its main difficulty and complexity lies in the need for constant and rigorous protection of the welded seam from contamination by impurities. Therefore, when welding titanium, not only an inert gas of high purity and special oxygen-free fluxes are used, but also a variety of protective visors, gaskets that protect the cooling ones.

To minimize grain growth and reduce changes in microstructure, welding is carried out at a high speed. Almost all types of welding are carried out under normal conditions, using special measures to protect the heated metal from contact with air.

But world practice also knows welding in a controlled atmosphere. Such protection of the welded seam is usually necessary when performing particularly critical work, when one hundred percent guarantee is required that weld will not be contaminated. If the parts to be welded are not large, welding is carried out in a special chamber filled with an inert gas. The welder clearly sees everything he needs through a special window.

When large parts and assemblies are welded, a controlled atmosphere is created in special spacious sealed rooms where welders work using individual life support systems. Of course, these works are carried out by welders of the highest qualifications, but ordinary welding of titanium should be carried out only by people specially trained in this matter.

In those cases when welding is not possible or simply not advisable, they resort to soldering. Soldering titanium is complicated by the fact that it is chemically active at high temperatures and is very strongly bonded to the oxide film covering its surface. The vast majority of metals are unsuitable for use as solders in titanium brazing, as brittle joints are obtained. Only pure silver and aluminum are suitable for this purpose.

It is possible to connect titanium with titanium, as well as with other metals, mechanically - by riveting or using bolts. When using titanium rivets, the riveting time is almost doubled compared to the use of high-strength aluminum parts, and nuts and bolts made of new industrial metal are invariably covered with a layer of silver or synthetic Teflon material, otherwise, when screwing the nut, titanium will, as is invariably inherent in it, stick and bulge and the threaded connection will not be able to withstand high stresses.

The tendency to sticking and scuffing due to the high coefficient of friction is a very serious disadvantage of titanium. This leads to the fact that titanium alloys wear out quickly and cannot be used for the manufacture of parts operating under sliding friction conditions. When sliding on any metal, titanium sticks to its surface, and the part sticks, seized by the sticky layer of titanium.

However, it is wrong to say that titanium alloys cannot be used in the manufacture of rubbing parts. There are many ways to harden the titanium surface and eliminate the sticking tendency. One of them is nitriding.

The process consists in the fact that parts heated to 850-950 degrees are kept in pure gaseous nitrogen for more than a day. A golden-yellow titanium nitride film of high microhardness is formed on the metal surface. The wear resistance of titanium parts increases many times and is not inferior to products made of special surface-hardened steels.

Another common method for eliminating the scuffing tendency of titanium is oxidation. In this case, as a result of heating, an oxide film forms on the surface of the parts. During low-temperature oxidation, free air access to the metal is hampered and the oxide film is dense, well connected with the main titanium layer.

High-temperature oxidation means that parts are kept in air heated to 850 degrees for 5-6 hours, and then they are sharply cooled in water to remove loose scale from the surface. As a result of oxidation, the wear resistance increases 15-100 times.

LLC "Turning" will carry out (from W1-0 onwards) on CNC machines, batches from 1 piece, extensive experience.

One of the main specializations of our company is turning titanium of all possible grades (main VT1-0, VT3-1, OT4-1, PT3V, VT16) and types (bar, plate, sheet, pipe, forgings). We also carry out welding and aging of titanium and parts based on it. The photo below shows a very complex product made by our company from VT1-0 titanium with welding and aging!

To calculate the cost of titanium turning, send a request with drawings to email ... Call 8 3439 38 00 81, 8 3439 38 98 01, delivery throughout Russia.

Turning titanium is accompanied by numerous difficulties that distinguish it from other metals. This is explained by the fact that titanium has:

- high strength and significant weight;

- low thermal conductivity and excellent corrosion resistance.

Due to these properties, titanium is very popular among manufacturers who are engaged in turning parts. At the same time, these characteristics make this metal very inconvenient for cutting and processing. This is how vibration appears. The cutting element wears out quickly.

If these phenomena can be compensated for, then the processing process becomes extremely effective. The use of the most advanced turning and milling machines, compressor units and other necessary equipment made the processing process much easier.

For titanium machining on a lathe, the workpiece is securely fixed on a powerful lathe. The cutting unit is selected correctly. However, the creation of ideal conditions is sometimes impossible to carry out, because parts can have a complex shape and too thin walls.

With such complexities, the units on which titanium turning takes place quickly become unusable. Parts with a complex shape sometimes cannot be fixed properly.

Titan does not lose its specifications and during processing. At the same time, a lot of heat is generated. Therefore, there is a great risk of defects on the surface of the part, which means that choosing the cutting element correctly and correctly is an extremely important stage. Practice shows: an excellent option is to use fine-grained metal alloys as raw materials for creating a cutter. Thus, cutting and drilling becomes efficient.

In addition, when machining titanium on a lathe, the chips will pick up and adhere to the cutting elements. This disadvantage is eliminated by oxidation: the titanium billet is heated to 900 degrees Celsius and exposed in this form in the open air. After that, the blank must be quickly cooled in water and the turning of titanium parts must be continued.

Properties of titanium: it is the toughness and thermal conductivity that causes the cutter to get very hot. As a result, even extremely durable and high-quality turning and milling tools are quickly destroyed. Due to the significant vibration that occurs when working with titanium, powerful machines are required, the frame of which is securely fixed on the bed.

Manufacturing of titanium parts

To make titanium parts on a CNC lathe easily, it is necessary that:

- machines with high power were used, where it is possible to regulate the speed of rotation of the workpieces;

- tools and blanks were fed with a small overhang;

- the moving parts fit securely and perfectly.

In addition, cutting tools and fixing assemblies must have high thermal resistance, because titanium, while remaining cold, heats the cutter metal to the utmost, together with the surrounding cutting site.

Much attention should be paid to the vibration of titanium parts, which occurs when titanium is machined on a CNC lathe. It arises due to:

- small dimensions of parts;

- the use of a long cutting tool for turning titanium parts;

- the toughness of the metal. Strong heating and high revolutions lead to the fact that the standard spindle taper very quickly becomes unusable.

We will make a prompt calculation according to your drawings, send them by e-mail ... You can call 8 3439 38 00 81, 8 3439 38 98 01, delivery throughout Russia.

You can solve this problem:

- by reducing the distance that separates the part and the spindle;

- precisely fitting the moving parts of the machine;

- rigidly fixing both the frame of the unit, on which the titanium is turned, and its fixed units.

Accordingly, vibration compensation (elimination) is possible if:

- accurately and accurately adjust absolutely all blocks of the machine;

- carefully fit a small size heat-resistant cutting tool;

- bring the cutter attachment point and the part itself as close as possible to each other.

Thanks to these measures, the machine will be able to work for a long time, if you do not increase the dimensional tolerances on the workpiece.

There are a number of additional methods to ensure the stability of the titanium turning process. It is advisable to reduce the number of revolutions, to accurately adjust the position of the cutter, securely fixing it, because the runout of the tool destroys the cutting tool assembly completely.

Titanium alloys are widely used in modern technology because their high mechanical properties and corrosion resistance are combined with a low specific gravity. Alloys of various compositions and properties have been developed, for example: commercially pure titanium (VT1, VT2), alloys of titanium-aluminum (VT5) systems, titanium-aluminum-manganese (VT4, OT4), titanium-aluminum-chromium-molybdenum (VTZ), etc. According to the general classification of hard-to-machine materials, titanium alloys are grouped into VII group (Table 11.11).

Just like stainless and heat-resistant steels and alloys, titanium alloys have a number of features that determine their low machinability.

1. Low ductility, characterized by a high coefficient of hardening, approximately two times higher than that of heat-resistant materials. At the same time, the mechanical characteristics of titanium alloys are lower than those of heat-resistant alloys. Reduced plastic properties of titanium alloys in the process of their deformation contribute to the development of advanced micro- and macrocracks.

Chips formed by appearance resembles a drain, has cracks dividing it into very weakly deformed elements, firmly connected by a thin and strongly deformed contact layer. The formation of such chips is explained by the fact that with an increase in speed plastic deformation at high temperature and pressure flows mainly in the contact layer, without affecting the cut layer. Therefore, at high cutting speeds, it is not drainage, but element chips that are formed.

Shear angles when cutting titanium alloys reach 38 ... 44 °, under these conditions, at cutting speeds greater than 40 m / min, chips may form with a shortening factor K l < 1, т. е. стружка имеет большую длину, чем путь резания. Подобное явле­ние объясняется высокой химической активностью титана.

Reduced ductility leads to the fact that when processing titanium alloys, the force P Z is approximately 20% lower than when processing steels, and the forces P y and P x are higher. This difference indicates a characteristic feature of titanium alloys - the cutting forces on the flank surface when machining them are relatively higher than when machining steels. As a consequence, with increasing wear, the cutting forces, especially Ru, increase sharply.

2. High reactivity to oxygen, nitrogen, hydrogen. This causes intense embrittlement of the surface layer of alloys due to the diffusion of gas atoms into it with increasing temperature. Chips saturated with atmospheric gases lose their plasticity and in this state do not undergo normal shrinkage.

The high activity of titanium with respect to oxygen and nitrogen of the air 2 ... 3 times reduces the contact area of ​​the chips with the front surface of the tool, which is not observed when processing structural steels. At the same time, the oxidation of the contact layer of the chips increases its hardness, increases the contact stresses and cutting temperature, and also increases the wear rate of the tool.

3. Titanium alloys have extremely poor thermal conductivity, lower than that of heat-resistant steels and alloys. As a result, when cutting titanium alloys, a temperature arises that is more than 2 times higher than the temperature level when processing steel 45.

The high temperature in the cutting zone causes intense build-up, seizure of the processed material with the material of the tool and the appearance of scoring on the processed surface.

4. Due to the content of nitrides and carbides in titanium alloys, the material of the cutting tool is highly abrasive. However, as the temperature rises, titanium alloys decrease their strength more strongly than stainless and heat-resistant steels and alloys. Cutting along the skin of many forged, pressed or cast billets of titanium alloys is complicated by the additional abrasive action on the cutting edges of the tool of non-metallic inclusions, oxides, sulfides, silicates and numerous pores formed in the surface layer. The heterogeneity of the structure reduces the vibration resistance of the titanium alloy machining process. These circumstances, as well as the concentration of a significant amount of heat within a small contact area on the leading surface, lead to the predominance of brittle wear with periodic chipping along the leading and trailing surfaces and chipping of the cutting edge. At high cutting speeds, thermal wear is intensified, and a hole develops on the front surface of the cutter. In all cases, however, the limiting wear is on its rear surface.

The level of cutting speed V T when processing titanium alloys is 2.5 ... 5 times lower than when processing steel 45 (see table. 11.11).

5. When processing titanium alloys, special attention should be paid to safety issues, since the formation of fine chips and, moreover, dust can lead to its spontaneous ignition and intense combustion. In addition, dusty shavings are harmful to health. Therefore, it is not allowed to work with feeds less than 0.08 mm / rev, the use of a blunt tool with wear of more than 0.8 ... 1.0 mm and with cutting speeds of more than 100 m / min, as well as the accumulation of chips in a large volume (an exception is made for VT1 grade, processing of which is allowed at cutting speeds up to 150 m / min).

When processing titanium alloys, technological media are widely used (Table 11.12).

The correct choice of cutting fluid can increase the tool life by 1.5 ... 3 times, reduce the height of microroughness by 1.5 ... 2 times. A characteristic feature of the use of cutting fluids in the processing of titanium alloys is the low efficiency of additives containing sulfur, nitrogen, phosphorus, since these elements are readily soluble in titanium. Halogens, and primarily iodine, are much more effective as additives.

Milling titanium requires certain conditions

Compared to most other metals, machining titanium is more demanding and more restrictive. Titanium alloys have properties that can significantly affect both the cutting process and the cutting material. If the tool and cutting conditions are selected correctly, as well as with good rigidity of the machine and the reliability of clamping the workpiece, the process titanium processing will be highly effective. Many of the problems that traditionally arise in titanium processing can be avoided. It is only necessary to overcome the influence that the properties of titanium have on the processing process.

Many of the properties that make titanium such an attractive component material affect its machinability, namely:

• high strength-to-weight ratio, and its density is usually only 60 percent of the density of steel,
• has a lower modulus of elasticity and is more pliable than steel,
• has a higher resistance to corrosion than stainless steel,
• low thermal conductivity.

These properties mean that titanium generates relatively high and concentrated cutting forces during machining. This causes vibration during machining, which leads to rapid wear on the cutting edge. In addition, titanium does not conduct heat well. Therefore, titanium processing requires a high redness resistance from the tool material.

Difficulties in titanium processing

It is generally accepted that titanium is difficult to efficiently machine. But this is not typical of the processing methods. Part of the difficulty stems from the fact that machining of titanium is a new field, and there is not enough experience in it. In addition, the challenges are often relative to expectations or different experiences, especially when the experience is about machining materials such as cast iron or low alloy steels that have lower requirements and are more error-forgiving. Titanium can also be difficult to work with compared to some stainless steels.

Although titanium typically needs to be machined at different speeds and feeds and with a number of precautions compared to other materials, it can be quite easy to machine. If a rigid titanium part is clamped securely on a machine of the correct capacity, in good condition and equipped with an ISO 50 taper spindle with short overhang, there should be no problem - provided the correct cutting tool is selected.

But ideal, stable conditions are not always present when milling. In addition, many titanium parts are complex shapes with small, narrow or large and deep pockets, thin walls and chamfers. To successfully machine these shapes, a longer tool is inevitably required, which can lead to deformation of the tool. Potential vibration problems are more common when machining titanium.

Fighting vibration and heat

Other factors present in less than ideal conditions include the fact that most machines are equipped with spindles with an ISO 40 taper. Due to the intensive use of these machines, they do not stay new for long. Besides, design features the workpiece is often hampered by its effective fastening on the machine. Compounding the problem is that machining typically involves grooving, contouring, or edging, which can - although should not - cause vibration. Therefore, it is necessary to constantly take measures to prevent it, if possible increasing the rigidity of fixing the part. One way to solve the problem is multi-stage workpiece clamping, in which the workpieces are located closer to the spindle, which reduces vibration.

Because titanium retains its hardness and strength at high temperatures, powerful forces and loads are applied to the cutting edge of the insert. In this case, a significant amount of heat is generated in the cutting zone, and this means the danger of deformation hardening of the part. Therefore, the key to successful processing becomes right choice grade of grade and geometry of the indexable insert. Historically, fine-grained uncoated cemented carbide grades have performed well in titanium machining, and today PVD-coated inserts can significantly improve efficiency.

Necessary conditions for calculating cutting conditions

The accuracy of the radial and face runout of the tools also has great importance... For example, if inserts are not properly seated in the cutter body, all cutting edges can quickly be damaged. Low tolerances in the manufacture of cutter bodies or holders, the degree of wear, the presence of defects or low quality tool holders or machine spindle wear have a greater impact on tool life when machining titanium. Due to these factors, a decrease in resistance of up to 80% was observed.

While a positive rake geometry is generally preferred, a slightly more negative rake angle tool is capable of machining at significantly higher feed rates, which can be as high as 0.5 mm per tooth. In this case, the rigidity of the machine and the reliability of clamping the workpiece are very important.

When milling deep pockets, it is useful to use different tool lengths with adapters instead of performing the entire operation with one long tool.

The minimum recommended feed for titanium milling is typically 0.1 mm per tooth. The spindle speed can also be reduced to obtain the original feed rate. An incorrectly selected spindle speed can reduce tool life by 95% at a minimum feed per tooth.

Once conditions are stable, the spindle speed and feed can be increased proportionally to achieve optimum efficiency. Another solution is to remove multiple inserts from the cutter, or choose a cutter with fewer inserts.