The Large forged tube made of 7050 aluminum alloy formed by free forging (mandrel extension), passed ultrasonic test before heat treatment. After heat treatment, it was found that there were excessive defects at the ultrasonic testing point, and several batches of forged pipes had these problems. Production was immediately halted to investigate and analyze the cause. The raw materials, heat treatment and forging processes were examined one by one, and it was finally confirmed that the poor mandrel stretching method caused point defects. The defects were enhanced by heat treatment to form ultrasonic point defects that exceeded the standard. By adjusting the mandrel extension process, it is possible to eliminate excessive errors of ultrasonic points large forged aluminum alloy tubes .
0. Introduction
7050 aluminum alloy is a high-strength deformed aluminum alloy that can be heat treated and strengthened, and can be used to manufacture various products such as thick plates, profiles, forgings and wires. 7050 aluminum alloy is often over-aged, allowing the alloy to retain high strength and have excellent overall properties such as good toughness, high fatigue resistance, and good stress corrosion resistance.
Our company has carried out trial production of large 7050-T74 aluminum alloy forged tubes for a specific model. This is the first trial production of a large forged tube of specification 7050. During trial production, there was a problem with excessive ultrasonic point defects, which resulted in scrapping. In response to this problem, an analysis was carried out and a solution proposed. After actual production verification, the solution proved to be practical and effective.
After consulting several engineers in the same sector, the opinion on the problem of specific defects in detecting flaws in forged tubes is uniform: metallurgical defects. Understand and resolve such defects: Control the melting process, improve the purity of the melt, apply methods such as filtration, degassing and increased agitation to reduce metallurgical defects such as inclusions and gases. The treatment plan is basically the same: it is removed by machining and, if it cannot be removed, it is scrapped. However, there are still no relevant records in the literature or reports on the control of point defects in ultrasonic tests through adjustment of forging operations. This is the first time this article describes this and can serve as a reference for related research.
1. Problem description
1.1 Information about forged pipes
League status: 7050-T74; Forged Tube Execution Standard: AMS A-22771; Typical dimensions of forged pipes: length 3070 mm, outer diameter 612 mm, wall thickness 50 mm; Delivery status: heat treatment + rough machining + ultrasonic testing; Ultrasonic test standard: Class A in GJB 1580A-2019.
1.2 Process flow
There are several specifications of forged tubes, which can be divided into two types according to the size of the internal hole. Both types of forged tubes have excessive point defects. The difference between the two types of tube forging processes is that forging tubes with small internal holes does not require expansion of the horse frame, while forging tubes with large internal holes requires the same other processes. The process flow for forging tubes with large internal holes is as follows:
Punching → 60MN press, ingot forging → 60MN press, reverse extrusion → back sawing of the bottom of the extrusion cylinder → 60MN press, horse frame expansion hole → 60MN press, spindle extension → sawing the length end → Press 60MN, horse frame expansion hole → rough processing before heat treatment → ultrasonic testing → solid solution → aging → sampling → physical and chemical testing → final rough processing → ultrasonic testing → acceptance and storage.
1.2 Discovery and situational introduction of point defects in ultrasonic testing that go beyond the norm
After the forging of the forged tube is completed, it is processed approximately to a wall thickness of 80mm before heat treatment, and the overall surface is smooth. After rough machining, ultrasonic inspection of the outer circular surface meets the level A inspection standard in GJB 1580A-2019. However, some forged tubes had single and multiple ultrasonic defects that exceeded the standard after heat treatment (solid solution + aging) and rough processing to delivery size (wall thickness 50 mm). The first batch produced 15 forged tubes, of which 5 were scrapped due to excessive punctual defects, with a scrap rate of 33.3%. Production was immediately stopped to investigate and analyze the reasons.
The distribution of point defects is described in the ultrasonic test diagram. From the point of view of wall thickness, the defects are distributed between 1/2 of the wall thickness and the inner wall, closest to the inner wall. From the perspective of length, there are various positions in length, but there are few defects at both ends, mainly distributed in the 1500mm area in the middle. Figure 1 is a schematic diagram of forged tubes and Table 1 shows ultrasonic test results of four typical oversized forged tubes. 
Figure.1 Schematic representation of the forged tube
Table 1: Ultrasonic test results of forgings
| serial number | Defect depth/mm | Equivalent error/mm | Distance to print face/mm | Type |
| 1# | 48 | φ2.0+2dB | 1478 | Single point |
| two# | 34 | φ2.0+3dB | 1685 | Single point |
| 3# | 32 | φ1.2+2.5dB | 1875 | Multipoint |
| 23 | φ1,2+4. 5dB | 1880 | ||
| 4# | 30 | φ2.0+1dB | 1955 | Single point |
After detection and positioning of the flaws, several point flaws were sampled and dissected for analysis. Using scanning electron microscopy and metallographic microscopy, it was determined that the microstructure of the point defects is consistent and that they are all long cracks. The typical metallographic diagram of typical parts is shown in Figure 2. An analysis of the energy spectrum of the composition of the crack components was carried out and it was found that the main component of the crack is Al. 2 Ó 3 this is the oxide film . Oxide film is a common metallurgical defect in aluminum alloys, usually in the form of spots, which is significantly different from the elongated oxide film produced by this forging process. Oxide film formation mechanism: During the melting and casting process of aluminum alloy, the casting surface comes into contact with air, and due to a high-temperature oxidation reaction, an oxide film is formed, covering the casting surface. When the oxide film on the surface of the casting breaks and is drawn into the casting, it remains in the ingot and forms defects in the oxide film. .
Figure.2 Defect metallography
2. Root cause analysis
2.1 Inspection and Analysis of Raw Material/Ingots
The sharp defect was analyzed as an oxide film. Only a small portion of forged tubes produced from the same cast billet had excessive point defects. It is suspected that the intersection of the head and tail of a given block is insufficient and the defects still need to be completely removed.
Determine the location of the billet in the original ingot using the furnace number and section number of the ingot. 5 forged tubes failed to detect defects, 2 billets were close to the head of the original billet, and 3 billets were close to the middle of the billet; the 10 forged tubes that passed the defect detection are distributed at the head, middle and end positions of the raw ingot.
Check the raw materials immediately. The raw material is flat ingot and has passed factory re-inspection with full reports. Inspection of the oxide film on low magnification and fracture surfaces meets the requirements for Class I forgings in GJB 2351-1995. After excessive point defects occurred, ingots from the same furnace were tested again for small enlargement and fracture, and the oxide film was found to be qualified after testing.
Other forgings made from the same cast ingot pass A-level ultrasonic test and oxide layer test are qualified. Its morphology is dotted and not elongated.
After checking the production status of raw materials and ingots with the same melting time, it was found that the defect of the oxide film of raw materials may only be a secondary cause of exceeding the standard point defects. The main reason is that a certain process changes the oxide film from sharp to long stripe-like.
Before heat treatment of forged pipe, the detection of defects is class A, and the defects that exceed the point are all caused after heat treatment. It is suspected that a problem occurred during the heat treatment process, so the heat treatment process was checked immediately.
2.2 Inspection and analysis of the heat treatment process
The heat treatment process parameters are developed in accordance with the AMS 2772 standard, a mature process parameter used for various models of 7050-T74 forgings. After reviewing the heat treatment records, it was found that the furnace installation requirements, clearances and furnace temperature curve meet the requirements of the heat treatment process.
The solution annealing furnace meets the requirements of Class II of AMS 2750, and the aging furnace meets the requirements of Class I of AMS 2750. During the heat treatment process of forged tubes, the equipment operates normally without experiencing abnormal conditions, such as overheating or overtime. .
The heat treatment process is normal, but the root cause analysis is stagnant. We will hold another meeting to discuss and propose a new point of view: the defect may not be caused by a single process, but rather the result of multiple processes working together. The forging and heat treatment processes involve changes to the forging structure, and the heat treatment test proceeds normally, followed by the forging process tests.
2.3 Inspection and analysis of the forging process
The forging process parameters are formulated in accordance with the forging and heating regulations. After reviewing the forging process records, all measured process parameters meet the process requirements.
A comparison of forging processes was carried out. The differences between the operations of the three processes of forging, forging modification, reverse extrusion and hole expansion were very small and consistent. In contrast, differences in nuclear axis elongation processes were significant.
There are two chuck extension methods: rotary feed + linear feed, mixed extension and single linear feed extension. The extension instructions are as follows:
- Rotary feeding: Extend a pass, divide the forging tube into several parts according to the width of the V anvil, each part is about 2/3 of the width of the anvil, rotate 90° from the beginning to lengthen it to a certain size, and then feed the next extension piece, with extension occurring continuously in cycles until the final size.
- Direct Feed: Extend a pass from start to finish without rotation. After finishing one pass, rotate 90° and then extend in a straight line from start to finish, continuing the cycle continuously until reaching the final size.
- Comparing mandrel stretching methods, there are important insights: for forged tubes that fail to detect defects, the mandrel stretching method is linear feeding + rotary feeding, and rotary feeding is the main method. The forged pipes sent and pulled straight passed the flaw detection and no flaws exceeding the point were found.
2.4 Summary
After examining the raw materials, heat treatment and forging processes, a preliminary conclusion can be drawn: improper stretching of the central shaft can tear the point-shaped oxide film into long strips, and the defects are increased during the heat treatment process, resulting in excessive point shaped defects.
To verify the accuracy of this conclusion, the central shaft stretching process must be improved before entering trial production.
3. Improving the chuck extension process
After discussion and analysis, the process to improve the elongation of the central axis is as follows:
- (1) Stretching method. The chuck is stretched with straight feed. The specific operation is to maintain octagonal stretching, with straight feed from beginning to end in each pass, even in the final stages of chamfering and forming, maintaining straight feed and listing rotary feed as a prohibited operation.
- (2) Improve operational consistency. Clearly define the forging heat, the degree of deformation and the tools used in the process. The chuck extension tool has been changed from a flat upper anvil and a lower V-shaped anvil to a double V-shaped anvil, with the number of force points changed from three to four, making the stress state in the zone more uniform and symmetrical deformation.
- (3) Increase lubrication. Before the central shaft is extended, the V-shaped anvil and horse screw (central shaft) are evenly coated with oil-based graphite to reduce friction and facilitate feeding of the forged material.
- (4) Increase the temperature of billet and tools, increase plasticity and reduce resistance to deformation. The heating temperature of the billet during mandrel stretching is controlled according to the upper limit of the alloy forging temperature, and the heating temperature is increased from 440°C to 450°C. The preheating temperature of the tools is increased, and the preheating temperature of the V-shaped forged anvil and shank (chuck) is increased to 300°C -400°C.
4. Product validation
After improving the mandrel stretching process, 2 forged tubes were produced as the first sample. After heat treatment, the forged tubes passed defect detection and no point defects other than the standard were found. Trial production was successful. Then, 5 forged tubes were produced that passed defect detection after heat treatment. All 7 consecutive forged pipes are qualified and have reached the expected targets. Immediately after series production, 10 forged tubes were put into operation, and the final defect detection of 10 forged tubes was also qualified. The defect detection qualification rate increased from 66.7% to 100%, and improvement measures achieved optimal results.
5. Analysis and discussion
7050 is a super-hard aluminum, which reduces impurities such as Fe, Si, Mn and Ti based on 7075 aluminum alloy, increases Cu and Zr, and has lower plasticity than other aluminum alloys. During the mandrel extension process, the inner and outer walls are fed synchronously along the longitudinal direction when the forged tube is fed in a straight line. When the feeding method changes from linear feeding to rotary feeding, the material temperature drops, the outer wall moves faster, the inner wall moves slower, the metal deformation of the outer wall is large, and the metal deformation of the inner wall is small. There is a transition zone with a difference in deformation between 1/2 wall thickness and the inner wall. The transition zone twists and deforms, reinforcing the original small defects within the ingot. Before heat treatment, the defects are still within the qualified test standards.
Solid solution heat treatment is a rapid cooling process in which internal stress increases and the extended oxide film cracks, creating longer crack defects. After heat treatment, the defects exceed the standard when tested.
