Forging process of titanium alloy

Blanking process of titanium alloy forging

For titanium alloy forging, because of its high cost, it is more suitable for forging, which not only improves the internal quality of components, but also saves metal materials. Every link of forging affects the internal quality or appearance quality of forging more or less. Therefore, we must strictly follow the forging process to complete each process.

There is a layer of hard and brittle α layer on the surface of forging (rolling) rod of titanium alloy. Before die forging, the layer should be removed to avoid cracking of blank surface during forging. For bars with a diameter less than 50 mm, the surface layer with a thickness of 3 mm shall be removed, and when the diameter is greater than 50 mm, 5 mm shall be removed. For extruded bars, 2mm can be removed if the diameter is less than 50mm, and 3mm can be removed for those with diameter greater than 50mm. After turning, if some parts still have defects, local grinding can be carried out to eliminate them, and the grinding depth should not be greater than 0.5mm.
Fixed size titanium alloy blanks for forging can be cut by sawing machine, lathe, anode cutting machine, punching shear machine, grinding wheel cutting machine or forging hammer or hydraulic press. The hot cutting efficiency is the highest on the punching and shearing machine.

• (1) The thickness of the disc saw blade is between 2 and 8 mm, which is suitable for cutting large diameter bar. The linear speed of the circular saw is about 30000 ~ 35000 mm / min, and the clean end face can be obtained when the feed rate is small. In order to prevent the metal from sticking to the tool and burning the metal, suspension can be used to reduce friction and cool the tool.
• (2) The width of incision is not more than 3 mm. Sodium silicate with concentration of 1.28-1.32 g / cm3 is used as working fluid. Although the cutting consumption of anode cutting is less, the production rate is low.
• (3) Before cutting on the hammer or hydraulic press, the bar should be preheated to the deformation starting temperature for punching (or cutting). Industrial pure titanium can be cut in cold state on punching and shearing machine.
• (4) When cutting titanium alloy on lathe, the cutting speed should be within 25000 ~ 30000mm / min, and the feed rate should be 0.2 ~ 0.3mm/cycle.
• When the α layer is peeled off, the turning specification is: cutting rate 15000 ~ 20000mm / min. When RA = 0.63 ~ 2.5, the feed is 0.08 ~ 0.1mm/cycle; when RA = 1.25 ~ 5, the feed is 0.1 ~ 0.2mm/cycle; when RA = 2.5 ~ 10, the feed is 0.3 ~ 0.4mm/cycle. When turning, lubricating coolant must be used to supply 1-1.5mpa pressure.
• (5) When the diameter of titanium alloy bar cut by grinding wheel is less than 60mm, grinding wheel should be used. When the diameter of grinding wheel cutting exceeds 20 mm, coolant should be used. The cutting efficiency of grinding wheel is high, but the life of grinding wheel is short.

After the blank is cut, the acute angle of the end should be rounded. Otherwise, it may cause folding when die forging or upsetting on horizontal forging machine. For the blank with diameter less than 50 mm, the radius R is 1.5-2.0 mm; for the blank with diameter more than 50 mm, the radius R is 3-4 mm.

Forging process of titanium alloy

Titanium and titanium alloys are widely used in various fields because of their low density, high specific strength and good corrosion resistance. In order to better use these materials, researchers have done a lot of research on the forging process.

Main forging process of titanium alloy

Forging is a plastic forming process that uses the plasticity of metal to make the blank material obtain certain shape and structure properties under the impact or pressure of tools. The advantage of forging production is that it can not only obtain the shape of mechanical parts, but also improve the internal structure of materials and improve the mechanical properties of mechanical parts.

Free forging

Free forging is usually carried out between two flat dies or dies without die cavity. The tool used in free forging has the advantages of simple shape, high flexibility, short manufacturing cycle and low cost. However, the labor intensity is high, the operation is difficult, the productivity is low, the quality of forgings is not high, and the machining allowance is large. Therefore, it can only be used when there are no special requirements for the performance of parts and the number of pieces is small.

Open die forging

The blank is deformed between two modules with cavity, and the forging is confined in the cavity. The surplus metal flows out from the narrow gap between the two dies, forming burr around the forging. The metal is forced to press into the shape of the die cavity under the action of the resistance of the die and the surrounding burr.

Closed die forging

In the process of closed die forging, there is no transverse burr which is perpendicular to the moving direction of the die. The cavity of closed forging die has two functions: one is to form the blank, the other is to guide.

Extrusion die forging

There are two kinds of die forging by extrusion, namely forward extrusion die forging and reverse extrusion die forging. Extrusion die forging can produce all kinds of hollow and solid parts, and can obtain forgings with high geometric dimension accuracy and more dense internal structure.

Multi direction die forging

It is carried out on a multi direction die forging machine. In addition to the vertical punching plug, the multi-directional die forging machine also has two horizontal pistons. Its ejector can also be used for punching. The pressure of the ejector is higher than that of the ordinary hydraulic press. In multi direction die forging, the slide block acts on the workpiece alternately from the vertical and horizontal directions, and one or more piercing punches are used to make the metal flow outward from the center of the die cavity to achieve the purpose of filling the die cavity. There is no burr of special forging on parting line of barrel part.

Partial die forging

In order to forge large integral forgings on existing hydraulic pressure, sectional die forging and backing plate die forging can be used. The feature of the method is to process the forging piece by section, one part at a time, so the required equipment tonnage can be very small. Generally speaking, this method can be used to process large forgings on medium-sized hydraulic press.

Isothermal die forging

Before forging, the die is heated to the forging temperature of the blank, and the temperature of the die and the blank remains the same during the whole forging process, so that large deformation can be obtained under the action of small deformation force. Isothermal die forging is similar to isothermal superplastic die forging, but the difference is that before die forging, the blank of the latter needs superplastic treatment to make it have equiaxed grains.

Defects of titanium alloy forgings

Titanium alloy is widely used in aircraft and aircraft engine as an important metal structure material because of its high specific strength, good medium temperature performance, corrosion resistance and good welding performance. According to statistics, the weight ratio of titanium alloy used in foreign aircraft has reached about 30%, which shows that the application of titanium alloy in aviation industry has a broad future.
However, titanium alloy also has some shortcomings, such as high deformation resistance, poor thermal conductivity, high notch sensitivity (about 1.5), and the change of microstructure has a significant impact on mechanical properties, which leads to the complexity in smelting, forging and heat treatment, and easy to appear defects in titanium alloy forgings.

Defects of titanium alloy forgings

Segregation defect

In addition to β segregation, β spot, Ti rich segregation and strip α segregation, the most dangerous is interstitial α stable segregation (type I α segregation), which is often accompanied by small holes and cracks, containing oxygen, nitrogen and other gases. There are also al rich type α stable segregation (type II α segregation), which also forms dangerous defects due to cracks and brittleness, which also reduces the thermal stability of the alloy.

Inclusion

There are inclusions on the surface of the blank, and cracks are often formed along the inclusions during forging, or obvious foreign bodies appear after forging corrosion, most of which are high melting point and high density metal inclusions. The high melting point and high density elements in titanium alloy composition are not fully melted and left in the matrix (such as molybdenum inclusion), carbide tool chips mixed with smelting raw materials (especially recycled materials) or improper electrode welding process (titanium alloy smelting generally adopts vacuum consumable electrode remelting method), such as high-density inclusions left by tungsten arc welding [III], Such as tungsten inclusions, in addition to titanium inclusions, such as titanium alloy forgings with inclusions are not allowed to be put into use.

Holes

The results show that the low cycle fatigue crack growth rate will be accelerated and the fatigue failure will be advanced.

Crackle

It mainly refers to forging cracks. Titanium alloy has high viscosity, poor fluidity, and poor thermal conductivity. Therefore, in the process of forging deformation, due to large surface friction, obvious internal deformation heterogeneity and large temperature difference between inside and outside, it is easy to produce shear band (strain line) in the forging, and crack along the direction of maximum deformation stress in severe cases.

Overheated

The thermal conductivity of titanium alloy is poor. In addition to improper heating which causes overheating of forgings or raw materials, it is also easy to cause overheating due to thermal effect during deformation in the forging process, resulting in microstructure changes and overheating widmanstatten structure.
In order to ensure the quality of titanium alloy forgings, in addition to strictly controlling the quality of raw materials, we should also pay attention to the ultrasonic flaw detection of the blank and semi-finished products of the forgings, so as to prevent some defects that may change deformation and physical properties in the subsequent heating process.
Source:  China Titanium Alloy Flange Manufacurer: www.titaniuminfogroup.com