How To Prevent The 7 Major Types And Causes Of Titanium Alloy Forging Cracking?

Titanium alloys with low density, high strength, high temperature resistance, corrosion resistance, non-magnetic and biocompatibility and other excellent comprehensive performance, in contemporary aerospace, weapons equipment, chemical and petroleum, medical, nuclear power and other fields of application is becoming more and more widely used, most of which are used in the deformation of titanium alloys such as bars, forgings and rolled profiles. Forging deformation is to ensure that the titanium alloy material to obtain the ideal organization and properties of the important means, but in the titanium alloy forging, especially ingot to bar billet production process, there are often various forms of cracking, not only to increase the cost of production and manufacturing, but also to reduce the production efficiency and the rate of material, and when the seriousness of the product scrap. In view of this, this paper analyzes and summarizes the cracking phenomenon that occurs in the process of titanium alloy forging over the years, and puts forward the measures to prevent and solve the cracking, so as to ensure the production efficiency and product quality of titanium alloy forging.
1,Titanium alloy forging cracking classification and causes
1.1 End face cracking
End face cracking is a kind of cracking that occurs on the end face of the billet in the process of titanium alloy forging, and the cracks are generally larger and more common. End face cracking has a greater impact, usually leading to forging can not continue according to the process. The main reasons for end face cracking: ① ingot head shrinkage holes or the end of the cold segregation is not completely clean; ② upsetting process, the billet end face and the anvil contact heat is absorbed, pulling the long process, billet end face bulge cooling fast; ③ end face position by different directions of the extrusion, deformation is not uniform, or pulling the long time deformation speed is too fast; ④ some of the billet deformation resistance is large, poor mobility, the end of the heart of the concave.
1.2,Folding crack
Folding cracking is mainly three kinds of cases: First, the raw material titanium alloy ingot height to diameter ratio ≥ 2.5 or ingot sampling in the middle of the ingot, residual sampling groove, upsetting and folding; Second, the forging process of the billet in the middle part of the crack, grinding into a deep pit, upsetting and folding again; Third, the other auxiliary processes generated by the sharp corners of the bumps and craters are not in accordance with the provisions of the grinding discharge injuries. Figure 1(a) shows the folding of a TB6 titanium alloy billet in the middle part due to deep pit after grinding. Figure 1 (b) for a TC4-LC titanium alloy billet sawing slant when splitting, turn 180 ° to continue sawing, sawing surface in the middle of the residual sharp edges are not polished, and then pull the length of the more serious folding occurred.
1.3 Tear
Tearing is a kind of transverse cracking of titanium alloy billet localized tearing of the billet when drawing length and deformation. Forging tearing will increase the amount of grinding, reduce the material into the material rate, while the deeper tearing in the forming process may lead to the blank can not meet the product machining size requirements and scrap. The main reason for tearing is a single pass under the amount is too large (single side greater than 50mm) or under the rate is too fast, Figure 2 (a) for a TB6 titanium alloy slab tearing. Secondly, the hammer anvil deformation due to prolonged use of its edge curvature is not smooth enough, or the hammer anvil preheating temperature is not enough, the regular deformation under the pressure will also lead to billet tearing. In addition, some step shaft bar or forging with step, because of the large difference in the local size of the wrong platform, pulling long forging is more likely to tear, Figure 2(b) for a TC4-DT titanium alloy forging step at the tear.

1.4, internal crack
Internal cracking generally refers to a single point, continuous or intermittent continuous cracking within the titanium alloy billet in the forging process, which is usually difficult to find, and needs to be detected by low inspection or ultrasonic flaw detection. This type of crack destroys the continuity of the metal and can only be scrapped. The reasons for internal cracking can be summarized into two categories: First, some titanium alloy materials contain tungsten, molybdenum and niobium and other refractory metal elements, the local existence of more serious segregation, inclusions or loose, cold segregation and other metallurgical defects in the forging and deformation process into a source of cracks; Secondly, for some small specifications (quasi 90 mm or Ф90 mm or less) titanium alloy materials, drawing long process of cooling fast, the inverted edge of the temperature is low or need to be Turnaround forging, billet local temperature gradient, easy to produce internal cracks. Figure 3 for a TA15 titanium alloy quasi-Ф85mm finished bar flaw detection found near the heart of the position of the internal fracture morphology, after analyzing the second case.

1.6. Brittle cracking
Some difficult to deform high temperature titanium alloy material itself poor plasticity, improper process control, easy to cause severe brittle cracking, cracks deep and difficult to remove. Below the process requirements of the final forging temperature forging, billet under greater pressure is prone to brittle cracking. There are some high-temperature titanium alloy materials do not take 200 ~ 300 ℃ low-temperature furnace or segmented heating process is unreasonable, the temperature is too fast, due to the poor thermal conductivity of titanium alloys, resulting in the heating process of the billet end and the middle, the surface and the heart of the existence of a large temperature gradient, out of the forging, serious cracking occurs. Figure 5 (a) shows a high-temperature titanium alloy test material cracking, nearly broken into two sections, after analyzing the heating process and heating process is too fast. Figure 5 (b) shows a Ti3Al ingot forging process occurs during brittle cracking, analyzed as a result of improper cooling operation. Due to equipment failure stop forging air-cooled, operating machine clamping ingot parts wrapped in insulation cotton is not cleaned up in time, the ingot cooling is not uniform, and the material is more sensitive to the temperature, in the heating and forging, in the ingot wrapped in insulation cotton border cracking.
1.7,Surface cracking through the body
Ingot metallurgical quality is poor, the surface defects are not removed thoroughly, or the heating process is unreasonable leading to cracking on the surface of the billet in the forging process. General area is relatively large, serious harm. Figure 6 for the incoming material processing TC4-LC titanium alloy ingot first fire drawing long, near the head position of the side of the serious cracking (box in the figure), after analyzing the furnace number of ingot smelting quality is poor, the ingot surface cold segregation and subcutaneous air holes are not cleaned up resulting from. Company a titanium alloy test material ingot casting first fire forging through the body cracking serious, after analysis of the test material phase change point 1020 ℃, while the opening temperature is only 1050 ℃, heating temperature is low, casting state organization plasticity is poor, resulting in serious cracking, and then warmed up to 1150 ℃ reopen the billet, the cracking is significantly reduced.
2,Forging cracking characteristics of titanium alloy
2.1 Serious harm
Titanium alloy cracking is a serious hazard, not only to increase the labor intensity of workers grinding, reduce production efficiency, but also reduce the product yield. Part of the serious cracking due to grinding deeper, need to increase the fire times trimming, increase manufacturing costs, for batch production of products due to the violation of the cured process specifications, easy to cause product quality fluctuations. More serious forging cracks can lead to product obsolescence due to the inability to meet product delivery size or weight requirements.
2.2, congenital
For Ti3Al, Ti2AlNb, IMI 834, as well as some new components of difficult to deform high-temperature titanium alloys, due to their own structural and organizational factors, the deformation ability is poor, itself more prone to cracking. Certain titanium alloy ingot metallurgical quality is poor, there are segregation, inclusions, subcutaneous porosity or cold segregation and other metallurgical defects, in the subsequent forging process is prone to multi-fire cracking and more difficult to eliminate.
2.3 Difficult to repair and grind
Titanium alloy cracking with difficult grinding characteristics: ① cracks, must be cleaned up, need to control the grinding width to depth ratio of ≥ 10, grinding a larger amount; ② part of the poor plasticity of titanium alloy, due to the grinding of the billet local temperature rise, the existence of a temperature gradient, cooling due to the role of the stress, cracks will continue to extend, i.e., while grinding cracking; ③ some of the small sizes of titanium alloy rods or forgings, due to the weight of the size of the small, grinding is not easy to fix; ④ Cracking at the end of some longer bars and forgings, which need to be polished vertically, is difficult to operate and poses a safety hazard; ⑤ Certain shaped parts have irregular shapes and sizes, and there is no place to start with the grinding of internal cracks.
2.4 Difficult inspection
Titanium alloy cracking has difficult inspection characteristics: ① cracks need 100% inspection, inspection workload; ② cracks become smaller, easy to be covered by grinding dust, the inspector can not determine with the naked eye whether to remove thoroughly; ③ some irregular shape, cracks are more difficult to inspect; ④ cracks occur inside the billet or under the skin, not visible to the naked eye, usually finished product inspection or low inspection to find out when it is too late; ⑤ easy to crack the internal cracks due to irregular shape and size, the cracks can not be found; ⑤ easy to crack the internal cracks due to irregular shape and size, the cracks can not be found; ⑤ easy to crack the internal cracks due to irregular shape and size, the cracks can not be found. ⑤ easy to crack or forming before the billet coloring penetration test, increasing the difficulty of the inspector's work.
3,Titanium alloy forging crack prevention and solution
According to the above seven types of titanium alloy cracking forms, causes and characteristics, and combined with cracking examples, the development of targeted preventive measures. At the same time, it is necessary to focus on improving the level of staff operation and tacit understanding, to ensure that the key equipment in perfect operating condition, the development and strict implementation of appropriate grinding, inspection specifications, control the key points and important details.

3.2 Folding and cracking prevention
First, control the raw material titanium alloy ingot or intermediate process billet height to diameter ratio ≤ 2.5, the more appropriate height to diameter ratio range of 1.8 ~ 2.3; Second, the middle part of the ingot residual sampling groove and billet folding cracking parts of the machine or sanding treatment, be sure to round and smooth over the width-to-depth ratio of ≥ 10 is appropriate; Third, be sure to ensure that sawing, machining and other auxiliary processes generated by the sharp corners of the bumps and craters (such as machining positioning holes, etc.) in accordance with the internal control Requirements for grinding off, similar to the billet steps shown in Figure 7, positioning holes, etc. must be polished clean before continuing forging.
3.3 Tear prevention
First, fast forging machine forging single pass unilateral under pressure control 20 ~ 30mm range, under pressure rate should not be too fast (to 5 ~ 10mm / s appropriate); Second, to ensure that the edge of the hammer anvil curvature ≥ R40mm, deformation occurs in a timely manner to maintain the processing and to ensure that the anvil preheated in advance to 300 ~ 400 ° C; Third, for the steps of the shaft bar or step forging, step forging at the forging be sure to use the knocking stick light pressure, then The hammer should be pressed down slowly, and the deformation of single hammer should be strictly controlled within 30 mm, and when tearing occurs, the hammer should be repaired or the forging should be stopped in time for grinding.
3.4,Prevention of internal crack
First, for titanium alloy materials containing refractory metal elements such as tungsten, molybdenum and niobium, we should start from the source, strengthen the ingot melting process research, improve the level of melting technology, to reduce or eliminate segregation, inclusions, loose or cold segregation and other metallurgical defects; second, for small-size titanium alloy billet, the drawing process must be timely chamfering, and try to avoid billet clamping end of the head of the forging, the conditions permit should be promptly returned to the furnace to be forged.
3.5 Prevention of longitudinal cracking
First, the titanium alloy billet drawing process in a timely chamfering, prohibit the emergence of sharp edges, black edges; Second, some of the temperature-sensitive high-temperature titanium alloy forging after air cooling or cover insulation cotton cooling, try not to use the air-cooled, water-cooled, oil-cooled, and other rapid cooling methods.
3.6 Brittle cracking prevention
For difficult deformation of high-temperature titanium alloy materials, should be used "package cotton forging": that is, the billet heated to the specified holding time, the billet is removed, wrapped in a layer of thermal insulation cotton (thickness of about 10mm, uniformly sprinkled with a layer of glass protective agent), return to the furnace to continue to heat (time according to the size of the billet and the package cotton fast and slow), out of the forging, the process of timely increase cover thermal insulation cotton, minimize the temperature of the billet, to minimize the temperature of the billet. Cotton, minimize the loss of billet temperature, to ensure that always in the process of forging temperature above the final forging temperature, practice shows that the operation can significantly reduce cracking. In addition, this type of titanium alloy heating should be taken 200 ~ 300 ℃ low-temperature furnace, multi-stage heating, slow heating (50 ~ 100 ℃ / h), to ensure that the billet is heated evenly, reduce the temperature gradient in different parts. Finally, the forging process is prohibited from dropping material, avoiding violent collision with the workpiece; the cooling process is slow and uniform; crack grinding is completed for color penetration test, etc..
3.7, surface cracking prevention
First, the surface cracking caused by poor metallurgical quality of ingots, should strengthen the melting process research, improve the melting level, forging before the need to completely remove the ingot cold segregation, subcutaneous porosity and other metallurgical defects. Secondly, when formulating the heating and forging process, we should deeply summarize the lesson that the low heating temperature of a test material leads to serious cracking, and comprehensively consider various factors to ensure that the process is optimal.
4,Conclusion
(1) Titanium alloy forging cracking includes end cracking, folding cracking, tearing, internal cracking, longitudinal cracking, brittle cracking, etc., with various forms and controllable causes; and it is characterized by serious hazards, congenital, hereditary, difficult to repair and grind, and difficult to inspect.
(2) To reduce and avoid titanium alloy forging cracking, prevention should be the main focus, and reasonable and feasible preventive measures should be developed. At the same time, it should ensure that the important processes such as forging, grinding and inspection are under control, and the key control points of the measures are strictly implemented.