Research on the Technology of Eliminating Ultrasonic Flaw Detection Defects in 7050 Alloy Large Forged Pipe by Forging Deformation

The 7050 aluminum alloy large forged pipe formed by free forging (mandrel elongation) passed ultrasonic testing before heat treatment. After heat treatment, it was found to have excessive ultrasonic testing point defects, and multiple batches of forged pipes have such problems. Immediately, production was stopped for investigation and analysis of the cause. The raw materials, heat treatment, and forging processes were investigated one after another, and it was ultimately confirmed that the poor mandrel elongation method resulted in point defects. The defects were amplified by heat treatment to form ultrasonic point defects that exceeded the standard. By adjusting the mandrel elongation operation, it is possible to eliminate excessive ultrasonic point defects in large aluminum alloy forged pipes.

0. Introduction

7050 aluminum alloy is a high-strength deformed aluminum alloy that can be heat treated and strengthened and can produce various products such as thick plates, profiles, forgings, and wires. 7050 aluminum alloy is often overaged, which enables the alloy to maintain a high level of strength and possess excellent comprehensive properties such as good toughness, high fatigue strength, and good stress corrosion resistance.
Our company has undertaken the trial production task of 7050-T74 large aluminum alloy forged pipes for a certain model. This is the first trial production of a large specification 7050 forged pipe. During the trial production, there was a problem of excessive ultrasonic point defects leading to scrapping. An analysis was conducted in response to this problem, and a solution was proposed. After actual production verification, the solution was practical and effective.
After consulting with multiple engineers in the same industry, the opinion on the issue of point defects in forged pipe flaw detection is consistent: metallurgical defects. Understanding and solving measures for such defects: controlling the melting process, improving the purity of the melt, adopting methods such as filtration, degassing, and strengthening stirring to reduce metallurgical defects such as inclusions and gases. The treatment plan is also basically the same: it is removed by machining, and if it cannot be removed, it will be scrapped. However, relevant literature records or reports have yet to be on controlling point defects in ultrasonic testing by adjusting forging operations. This is the first time this article has described it, and it can provide a reference for related research.

1. Problem Description

1.1 Forged Pipe Information

Alloy status: 7050-T74; Execution standard for forged pipes: AMS A-22771; Typical dimensions of forged pipes: length 3070mm, outer diameter 612mm, wall thickness 50mm; Delivery status: heat treatment + rough machining + ultrasonic testing; Ultrasonic testing standard: Grade A in GJB 1580A-2019.

1.2 Process Route

There are various specifications of forged pipes, which can be divided into two types based on the size of the inner hole. Both types of forged pipes have excessive point defects. The difference between the two types of forging pipe forming processes is that small inner hole forging pipes do not require horse frame expansion, while large inner hole forging pipes require the same other processes. The process route for forging pipes with large inner holes is as follows:

Blanking → 60MN press ingot forging → 60MN press reverse extrusion → sawing back extrusion cylinder bottom → 60MN press horse frame expanding hole → 60MN press spindle elongation → sawing length end → 60MN press horse frame expanding hole → rough machining before heat treatment → ultrasonic testing → solid solution → aging → sampling → physical and chemical testing → final rough machining → ultrasonic testing → acceptance and warehousing.

1.2 Discovery and situation introduction of point defects in ultrasonic testing exceeding the standard

After the forging of the forged pipe is completed, it is roughly machined to a wall thickness of 80mm before heat treatment, and the overall surface is smooth. After rough machining, the ultrasonic inspection of the outer circular surface meets the A-level inspection standard in GJB 1580A-2019. However, some forged pipes showed single and multiple ultrasonic defects exceeding the standard after heat treatment (solid solution + aging) and rough machining to the delivery size (wall thickness of 50mm). The first batch produced 15 forged pipes, of which 5 were scrapped due to excessive point defects, with a rejection rate of 33.3%. Production was immediately stopped to investigate and analyze the reasons.
The distribution of point-shaped defects is described in the ultrasonic inspection diagram. From the wall thickness perspective, defects are distributed between 1/2 of the wall thickness and the inner wall, closer to the inner wall. From the perspective of length, there are multiple positions in length, but there are few defects at both ends, mainly distributed within the range of 1500mm in the middle. Figure 1 is a schematic diagram of forged pipes, and Table 1 shows the ultrasonic testing results of four typical oversized forged pipes.

Figure.1 Schematic diagram of forged pipe
Table.1 Forging Ultrasonic Testing Results

Serial number	Defect burial depth/mm	Defect equivalent/mm	Distance from imprint end face/mm	Type
1#	48	φ2.0+2dB	1478	Single point
2#	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 flaw detection and positioning, multiple point defects were sampled and dissected for analysis. By using scanning electron microscopy and metallographic microscopy, it was found that the microstructure of point shaped defects is consistent, all of which are long cracks. The typical metallographic diagram of the typical parts is shown in Figure 2. Energy spectrum analysis was conducted on the composition of the components inside the crack, and it was found that the main component inside the crack is Al₂O₃, which is the oxide film. The oxide film is a common metallurgical defect in aluminum alloys, usually in the form of dots, which is significantly different from the elongated oxide film found in this forging process. Mechanism of oxide film formation: During the melting and casting process of aluminum alloy, the surface of the melt comes into contact with air, and an oxide film is formed due to a high-temperature oxidation reaction, which covers the surface of the melt. When the oxide film on the surface of the melt is broken and drawn into the melt, it ultimately remains in the ingot to form oxide film defects.

Figure.2 Defect Metallography

2. Cause analysis

2.1 Raw Material/Ingot Inspection and Analysis

The dot-like defect was analyzed as an oxide film. Only a small part of the forged pipes produced by the same melting ingot showed excessive point-like defects. It is suspected that the cutting amount of a certain ingot’s head and tail is insufficient, and the defects still need to be completely removed.
Find the position of the billet in the original ingot based on the melting furnace number and ingot section number. 5 forged pipes failed the flaw detection, 2 billets located near the head of the original ingot, and 3 billets located near the middle of the ingot; The 10 forged pipes that pass the flaw detection are distributed at the positions of the raw ingot head, middle, and tail.
Immediately inspect the raw materials. The raw material is flat ingot, and the raw material has passed the factory re-inspection with complete reports. Inspecting the oxide film on the low magnification and fracture surfaces meets the requirements of Class I forgings in GJB 2351-1995. After the occurrence of excessive point defects, the ingots from the same melting furnace were retested for low magnification and fracture, and the oxide film was found to be qualified after inspection.
Other forgings produced by the same melting ingot all meet the A-level ultrasonic inspection, and the oxide film inspection is qualified. Their morphology is point shaped, not elongated.
After inspecting the production status of raw materials and ingots with the same melting time, it was analyzed that the oxide film defect of the raw materials may only be a secondary cause of exceeding the standard point-like defects. The main reason is that a certain process changes the oxide film from point-like to long strip-like.
Before the heat treatment of the forged pipe, the flaw detection meets Class A, and the exceeding point defects are all caused after the heat treatment. It is suspected that there was a problem during the heat treatment process, so the heat treatment process was immediately inspected.

2.2 Inspection and Analysis of Heat Treatment Process

The heat treatment process parameters are developed according to the standard AMS 2772, a mature process parameter used for multiple models of 7050-T74 forgings. After checking the heat treatment records, it was found that the furnace installation requirements, spacing, and furnace temperature curve all meet the requirements of the heat treatment process.
The solid solution treatment furnace meets the requirements of Class II in AMS 2750, and the aging furnace meets the requirements of Class I in AMS 2750. During the heat treatment process of forged pipes, the equipment operates normally without abnormal conditions such as overtemperature or overtime.
The heat treatment process is normal, but the cause analysis is stagnant. We will hold another meeting to discuss and propose a new viewpoint: 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 in the structure of the forging, and the heat treatment inspection is normal, followed by an inspection of the forging process.

2.3 Inspection and analysis of the forging process

The forging process parameters are formulated according to the forging and heating regulations. After inspection of the forging process records, the measured process parameters all meet the process requirements.
A comparison was made on the forging operations, and the differences in the operations of the three forging processes of modification forging, reverse extrusion, and hole expansion were very small and consistent. In contrast, the differences in the core shaft elongation operations were significant.
There are two operations for mandrel elongation: rotary feeding + linear feeding mixed elongation and single linear feeding elongation. The elongation instructions are as follows:

• Rotating feeding: elongate one pass, divide the forging tube into multiple parts according to the width of the V-anvil, each part is about 2/3 of the anvil width, rotate from the beginning to elongate at 90 ° to a certain size, and then feed the next part of the elongation, continuously cycling the elongation to the final size.
• Straight feeding: elongate one pass without rotating from beginning to end. After the end of a pass, rotate 90 ° and then elongate in a straight line from beginning to end, continuously cycling until the final size is reached.
• There are important findings in the comparison of the mandrel elongation process: for forged pipes that fail the flaw detection, the mandrel elongation method is linear feeding + rotary feeding, and the main method is rotary feeding; The forged pipes that were sent in and pulled out in a straight line passed the flaw detection, and no exceeding point defects were found.

2.4 Summary

After inspecting the raw materials, heat treatment, and forging processes, a preliminary conclusion has been formed: poor core shaft elongation operation can tear the point-like oxide film into long strips, and the defects are amplified during the heat treatment process, forming excessive point-like defects.
To verify the accuracy of the conclusion, the core shaft elongation operation should be improved before feeding for trial production.

3. Improve the mandrel elongation process

After discussion and analysis, the process of improving the core shaft elongation is as follows:

• (1) Elongation method. The mandrel is elongated using the straight feed. The specific operation is to maintain octagonal elongation, with a straight feed from beginning to end for each pass, even during the final chamfering and shaping stages, maintaining a straight feed and listing rotating feed as a prohibited operation.
• (2) Improve operational consistency. Clearly define the forging heat, deformation amount, and tools used in the process. The mandrel elongation tool has been changed from an upper flat 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 deformation zone more uniform and symmetrical.
• (3) Strengthen lubrication. Before the core shaft is elongated, the V-shaped anvil and horse screw (core shaft) are evenly coated with oil-based graphite to reduce friction and facilitate the material feeding of the forging.
• (4) Increase the temperature of the billet and tooling, increase plasticity, and reduce deformation resistance. The heating temperature of the billet during mandrel elongation is controlled according to the upper limit of its alloy forging temperature, and the heating temperature is increased from 440 ℃ to 450 ℃. The preheating temperature of the tooling is increased, and the preheating temperature of the forged V-shaped anvil and horse bar (mandrel) is increased to 300 ℃ -400 ℃.

4. Product validation

After improving the mandrel elongation operation, 2 forged pipes were the first trial produced. After heat treatment, the forged pipes passed the flaw detection, and no exceeding standard point defects were found. The trial production was successful. Subsequently, 5 forged pipes were produced and passed the flaw detection after heat treatment. All 7 consecutive forged pipes are qualified and have achieved the expected goals. Immediately after the batch production, 10 forged pipes were put into operation, and the final flaw detection of 10 forged pipes was also qualified. The qualification rate of flaw detection has been increased from 66.7% to 100%, and the improvement measures have achieved ideal results.

5. Analysis and Discussion

7050 is superhard aluminum, which reduces impurities such as Fe, Si, Mn, and Ti based on 7075 aluminum alloy, increases Cu and Zr, and has poorer plasticity than other aluminum alloys. During the process of mandrel elongation, if the forged tube is fed in a straight line, the inner and outer walls are fed synchronously along the length direction. When the feeding method changes from linear feeding to rotary feeding, the material temperature decreases, the outer wall moves faster, the inner wall moves slower, the outer wall metal deformation is large, and the inner wall metal deformation is small. There is a transition zone with a deformation difference between 1/2 wall thickness and the inner wall. The transition zone will twist and deform, causing the original small defects inside the ingot to be amplified. Before heat treatment, the defects are still within the qualified inspection standards.
Heat treatment solid solution is a rapid cooling process where the internal stress increases and the elongated oxide film is torn, forming longer crack defects. After heat treatment, the defects exceed the standard during the inspection.

6. Conclusion

Poor elongation and deformation of the core shaft can amplify existing metallurgical defects, and the internal stress during the solution process can tear away existing metallurgical defects, resulting in unqualified inspection and product scrapping. The mandrel is elongated using a straight-line feeding method, which improves the anvil tool and material temperature, avoiding the amplification of defects and producing qualified large forged pipes.
Author: Wang Xueqiang