Titanium material metallurgy defects
In the titanium material ingots, billets and products, due to the formation of metallurgical defects. Generally divided into internal weaknesses and surface defects. Internal defects include inclusion, segregation, delamination, internal cracks, loose, shrinkage, uneven tissue, coarse tissue, overheating and so on. Surface defects are cracks, peeling, folding, scratching, pressure pit, surface contamination and so on. Some of the above drawbacks can be eliminated in subsequent press working or hot isostatic pressing processes, some of which cannot be eliminated. Parts of the working conditions are different, the requirements of the material are also atypical, the same flaw, for important uses of aviation, aerospace timber, subject to more stringent control. According to the characteristics of titanium materials, there is mainly inclusion, segregation and uneven organization, such as coarse tissue defects.
Inclusion
Ingot, billet and products are not fully melted particles. Divided into metal inclusion and non-metallic inclusions.
(1) metal inclusion. Mainly not completely melting high-melting point metal or solid solution, such as tungsten, molybdenum and carbide cutting pieces. This inclusion is not readily corroded by common etchants commonly used in titanium alloys. Metal inclusion is usually exposed during the subsequent machining of the material. Such defects can be identified with ultrasonic flaw detection, X-ray detection and high and low magnification test methods. Metal inclusion arises from the electrode preparation process. It may be due to: ingredients due to the addition of high melting point alloy material inappropriate, it may be mixed with the electrode refractory metal block, or mixed with Carbide cutter chip fragments car chip ; If the use of TIG welding electrode welding, falling on the electrode of tungsten fragments will form a metal inclusions. Therefore, we must strictly control the electrode preparation process, including the use of reasonable intermediate alloy ingredients and control their size; to strengthen the management of raw materials to prevent foreign matter mixed; electrode welding using plasma welding and other welding methods; Quality alloy head pieces, to strengthen the return of the management and inspection materials.
(2) non-metallic inclusion. Mainly oxides or nitrides and other non-frit. In the autobiographic sample showed a bright spot-like, strip. Often accompanied by material discontinuities. Such inclusion brittleness, deformation process prone to loosening of defects, holes, delamination or crack, the shape and α layer similar to the gradual transition to the normal organization of the material. Such weaknesses can be ultrasonic flaw detection, high and low magnification test methods for inspection. The production of non-metallic inclusions is also associated with the electrode preparation process. During the preparation of the electrodes, sponge titanium mixed with oxidized particulates and combustion particles and oxidized particulates, mixed with clinker of the intermediate alloy, and utilized unqualified return materials. In order to prevent non-metallic inclusions, the quality of the sponge titanium, the master alloy and the return material must be strictly controlled, and if necessary, artificial screening can be performed. It is also important to prevent oxidation during electrode welding to reduce the occurrence of non-metallic inclusion. Metal inclusion and non-metallic inclusions affect the mechanical properties of the deformation and the use of easily lead to cracks, so the aviation, aerospace materials, the organization does not allow macroscopic inclusions.
Segregation
Alloy products in the normal content of the region. Therefore, the amount of α phase and β phase in these regions is obviously distinct from that of the matrix. Titanium smelting using vacuum consumable melting method, while melting solidification, so the chemical composition of the non-uniform is inevitable. Segregation that constitutes a defect refers to the depletion or enrichment of an alloying element in a region of a certain size, or the enrichment of oxygen, nitrogen, carbon and the like as the impurity elements. Segregation in low magnification showed obvious coarse grain group, punctate or banded tissue and other abnormal corrosion zone. In high magnification, there are gravely varying grains, different forms of tissue or areas of varying degrees of corrosion. It can be divided into the following two types:
(1) Metallographic samples have weak corrosion zone (bright stripes, bright spots), usually α-phase enrichment zone. It may be due to abundant alpha stable elements (aluminum, etc.) formed, it may be due to oxygen and nitrogen and other gas impurities formed in the region. Aluminum-rich, oxygen-rich, nitrogen-rich areas, the micro-hardness was significantly greater than the matrix, are brittle segregation. There are areas of poor alloying elements is also a dilute corrosive zone, its composition, microhardness and microstructure are similar to pure titanium.
(2) Metallographic samples have a deep corrosion zone (dark spots), dark spots within the α phase less, or even not, it is well known as β spot. Beta-stable element in the region rich in β transition temperature is lower than the normal region, so in the heat processing and heat treatment in the area of Β transition occurs in advance, resulting in β-spot.
Segregation of such defects can be high and low magnification test methods for inspection. Some of the acoustic impedance and the matrix difference between the larger segregation, ultrasonic flaw detection can also be checked out. Brittle segregation is generally not allowed, and other segregation of titanium la allows removal of segregation and inspection of the corresponding and faces after use. To eliminate or reduce segregation, electrode preparation and smelting processes need to be addressed both in terms of strengthening the management of raw materials, controlling the particle size of the raw materials (titanium sponge, alloying elements, intermediate alloy, return material), improving the uniformity of the mixture, controlling the electrode density , To prevent its occurrence in the melting process off the block caused by uneven composition; choose a reasonable smelting process, the appropriate increase in the number of melting, can reduce the size of inclusions and segregation zone.
Inhomogeneous organization, uneven organization Tissue uneven performance of various parts of the semi-finished products ranging in thickness, fine grain in the matrix with coarse grain region, or on the contrary, coarse grain microstructure area, the local area of The block like α-phase and strip-like organization. Tissue course at high magnification showed coarse grains, overheated tissue, original residual β grain boundaries. In the low magnification organization, there are obviously visible macular grains. Select improper segregation and pressure processing specifications, will make the billet to produce uneven organization caused by the segregation of uneven, in the subsequent pressure processing is impossible to eliminate. Due to processing caused by uneven tissue, reflected in low magnification, corrosive lighting effects of different bands or regions. For example, the most common "forging crosses." Due to the lower surface temperature of the part contacted by the billet and the mold during processing, there is friction and other reasons to form the hard-to-deformable zone, that is, along the cross 45 along the cross. Direction, the formation of large deformation of the shear zone, is the product of intense local metal flow, low magnification corrosion showed dark line. If the forging temperature is too high, the deformation is too large, the restricted area may produce local overheating.
Organizational uneven, coarse tissue defects such as defects can be detected by flow detection or microstructure inspection. For titanium materials, caused by the organization of non-uniform and coarse tissue, it is difficult to improve by heat treatment, should choose a reasonable processing technology, so that the process of deformation of various parts of the product as uniform as possible, can also be selected on the right blank forging The coating insulation and lubrication to reduce the deformation of non-uniformity. If the above defects have occurred,
The obvious unprocessed grains in the low magnification tissue can be eliminated by means of homogenization treatment, deformation heat treatment or the like, and the organization can also be improved by exerting a large amount of deformation in the α + β phase region. Serious organizational inequalities and thick tissue will result in a decline in the overall performance of the material, so for aviation materials is not permitted.
