Test di processo per migliorare la tenacità all'impatto di piccoli forgiati in lega di titanio Ti-4Al-1,5Mn

Ti-4Al-1.5Mn alloy is a medium strength, high plasticity near-alpha lega di titanio, which has been widely used in the aerospace industry. The recent Ti-4Al-1.5Mn small forgings produced by our company have the problem of insufficient or even unqualified impact toughness in wide varieties and batches, seriously affecting the delivery progress. Our company analyzed the problem of the low impact toughness of Ti-4Al-1.5Mn forgings, proposed two methods to improve the impact toughness of Ti-4Al-1.5Mn forgings, and carried out process trial verification to guide the design of Ti-4Al-1.5Mn hot processing program.

Chemical element content of Ti-4Al-1.5Mn (%)

Component	Fe	C	Mn	N	Al	H	O	Other single	Total Other	More
Minimum value	-	-	0.8	-	3.5	-	-	-	-	Ti: margin
Maximum	0.3	0.08	2	0.05	5	0.012	0.15	0.1	0.4

1. Problem Cause Analysis

Forging requirements

Our company undertakes the Ti-4Al-1.5Mn small forgings of a host plant, whose acceptance standard is type-specific, and the room temperature mechanical property requirements are shown in Table 1. according to the standard requirements, the microstructure of the forgings should be uniform after processing in the α+β two-phase area and delivered in the annealed condition. The recommended trattamento termico system is 740-790℃, holding 1-2h, and air cooling.

The direct cause of low-impact toughness

According to domestic research results, the chemical composition, microstructure, and fiber flow line of titanium alloy forgings will have a more noticeable impact on the impact toughness of the forgiati.
(1) The influence of the chemical composition of the forgings.
Our Ti-4Al-1.5Mn titanium alloy bar is mainly from Baotian and Western Superconductor Company, the chemical composition is in line with the type-specific standards, and the chemical composition of each furnace bar is relatively stable. Still, the Ti-4Al-1.5Mn small forgings produced by each furnace bar have low impact toughness, so the influence of chemical composition can be excluded.
(2) The effect of the fiber flow line.
According to the statistical results, both the die forgings and the long shaft class free forgings have the problem of low impact toughness. In the deformation of die forgings, some metal is discharged along the parting surface, which will cause the fiber flow line near the parting surface to deviate from the mainstream direction, which may lead to the low-impact toughness problem. Long shaft-type free forgings are formed by drawing, and the central fiber flow line is conducive to improving the impact toughness, but there is still the problem of low impact toughness. Therefore, the influence of the fiber flow line can be excluded.
(3) The effect of microstructure.
The study shows that the striped α-phase content in the microstructure of Ti-4Al-1.5Mn titanium alloy has a more noticeable effect on the impact toughness of Ti-4Al-1.5Mn titanium alloy forgings, and increasing the striped α-phase content can improve the impact toughness of Ti-4Al-1.5Mn titanium alloy forgings. Usually, the microstructure of annealed Ti-4Al-1.5Mn titanium alloy small-size bars has a shallow content of secondary strip α phase (most of them ≤10%).
Ti-4Al-1.5Mn titanium alloy forging is usually heated at 30-50°C below the phase change point, i.e., T_β-(30-50) °C. Because of the low forging heating temperature, only a tiny amount of the incipient α-phase is transformed into β-phase during heating. After reaching the holding time, the billet is discharged from the furnace and forged. Because of the small size of the billet, the billet cools down rapidly during the contact between the billet and the die, and the small amount of transformed β-phase is too late to precipitate more stripe α-phase, which in turn may lead to less secondary stripe α-phase in the microstructure of the forging. According to the type standard, the expected annealing heat treatment temperature is 740-790℃, which is lower than the forging temperature, and also cannot effectively enhance the content of the secondary strip α phase.
Therefore, the low content of the second alpha phase in the microstructure should be the main reason for the low impact toughness of Ti-4Al-1.5Mn small forgings.

2. Problem-solving ideas

According to the analysis results, the main direction to solve the problem of low impact toughness of Ti-4Al-1.5Mn small forgings should be to improve the content of the second alpha phase in the microstructure of forgings. Research shows that the heating temperature during forging and heat treatment significantly affects the content of the second alpha phase in forgings. When the forging or heat treatment temperature is higher, more primary α-phase transforms into β-phase in the microstructure of forgings, and more secondary bar α-phase precipitates in the subsequent air-cooling process, thus improving the impact tough. Based on this, two main optimization ideas are used in this paper.
Table.1 Room temperature mechanical properties requirements of an exceptional standard for Ti-4Al-1.5Mn forgings

State	Direction	Resistenza alla trazione/MPa	Yield strength/MPa	Elongation/%	Reduction of area/%	Impact toughness/(J/cm2)	Hardness HB (d)/mm
Annealing	Fiber longitudinal direction	685 – 885	≥585	≥10	≥30	≥35	3.5 – 4.2
	Fiber transverse direction	685 – 885	≥585	≥9	≥20	≥35

Figure.1 Typical microstructure of Ti-4Al-1.5Mn small forgings with low impact toughness (500×)

After forging increase near β annealing

Checking the process records and process documents, the forging temperature of our Ti-4Al-1.5Mn small forgings is generally controlled at 30-50℃ below the phase change point, i.e., T_β-(30-50)℃, but the microstructure of the obtained forgings still has less secondary strip α phase. Therefore, the first optimization idea of this paper adds near-β annealing after the forging process, i.e., the thermal processing route is forging → near-β annealing → ordinary annealing, and the near-β annealing temperature after forging for this process test is chosen to be 10°C and 20°C below the phase change point, i.e., T_β-10°C and T_β-20°C.
(1) Test material.
Ti-4Al-1.5Mn titanium alloy ϕ80mm specification round bar was used, and its phase change point temperature was measured as T_β=(976±3)℃.
(2) Test scheme.

• 1) Under ϕ80mm×50mm specification, test material 2 pieces, respectively numbered test piece 1 and test piece 2.
  2) Forging conventionally: test material is first heated in the resistance furnace, with resistance furnace accuracy of ± 10

2) Forging conventionally: test material is first heated in the resistance furnace, with resistance furnace accuracy of ± 10 ℃; the heating temperature is set to 936 ℃, and the furnace temperature reaches the set temperature after the start of the timing, holding time is set to 100 min; after reaching the holding time, the billet out of the furnace forging, in 750kg free forging hammer will be drawn long billet to 50mm × 50mm × 100mm, after forging air cooling to room temperature.
3) Test piece 1 and test piece 2 were treated differently according to the heat treatment scheme in Table 2. After the heat treatment, the longitudinal mechanical properties and microstructure at room temperature at a 1/2 thickness position were sampled and tested at the same position.
(3) Results and analysis.
1) Microstructure comparison.
Figure 2 shows the comparison of the microstructure of test pieces 1 and 2 at the same sampling position at 1/2 thickness; it can be seen that the microstructure of test piece 1 contains ≤10% of secondary strip α phase, while the microstructure of test piece 2 contains about 20%-30% of secondary strip α phase, i.e., compared with the conventional heat treatment state (test piece 1), more secondary strip α phase is obtained after forging and increasing near β annealing (test piece 2) The expected results were achieved.
2) Performance comparison.
Table 3 compares the two test pieces’ room-temperature longitudinal mechanical properties at the same sampling position. It can be seen that the impact toughness of the forgings was significantly improved after forging with near-β annealing (test piece 2) compared with the conventional heat treatment condition (test piece 1). Still, the strength of the forgings was significantly reduced compared with the conventional heat treatment condition, but it met the standard requirements.

3) Test piece 1 and test piece 2 were treated differently according to the heat treatment scheme in Table 2. After the heat treatment, the longitudinal mechanical properties and microstructure at room temperature at a 1/2 thickness position were sampled and tested at the same position.

(3) Results and analysis.
1) Microstructure comparison.
Figure 2 shows the comparison of the microstructure of test pieces 1 and 2 at the same sampling position at 1/2 thickness; it can be seen that the microstructure of test piece 1 contains ≤10% of secondary strip α phase, while the microstructure of test piece 2 contains about 20%-30% of secondary strip α phase, i.e., compared with the conventional heat treatment state (test piece 1), more secondary strip α phase is obtained after forging and increasing near β annealing (test piece 2) The expected results were achieved.
2) Performance comparison.
Table 3 compares the two test pieces’ room-temperature longitudinal mechanical properties at the same sampling position. It can be seen that the impact toughness of the forgings was significantly improved after forging with near-β annealing (test piece 2) compared with the conventional heat treatment condition (test piece 1). Still, the strength of the forgings was significantly reduced compared with the conventional heat treatment condition, but it met the standard requirements.
Table.2 Heat treatment scheme of the test piece

Number	Heat treatment scheme
Test piece 1	780 ° C x 120min, outlet air cooling
Test piece 2	956 ° C x 120min, outlet air cooling
	780 ° C x 120min, outlet air cooling

Figure.2 Comparison of the microstructure of test piece 1 and test piece 2

Increase near β annealing before forging.

To obtain more secondary strip α phase, an isothermal forging process has been developed in China, which is usually heated at 15-25°C below the phase change point; the die used for forging is heated to the same temperature as the billet and kept at a constant temperature during the forging process, and air-cooled to room temperature after forging. However, isothermal forging has special requirements for dies and forging equipment. The dies need to be made of high-temperature alloys, and the forging equipment can only be used with hydrostatic presses and die-heating tooling, which is costly and inefficient.
If forging or die forging is carried out directly after heating at 15-25°C below the phase change point, the risk of overburning of Ti-4Al-1.5Mn small forgings increases dramatically due to the backseat phenomenon during forging, which can lead to the scrapping of the whole batch of forgings if not adequately controlled.
Therefore, the second optimization idea of this paper is to add near β-annealing before the forging process, that is, the thermal processing route is near β-annealing → forging → general annealing, and the near β-annealing temperature before forging for this process test is chosen to be 20℃ below the phase change point, that is, T_β-20℃.
(1) Test material.
Ti-4Al-1.5Mn titanium alloy ϕ80mm specification round bar is used, and its phase change point temperature is measured as T_β=(976±3)℃;
(2) Test scheme.
1) The next ϕ80mm×50mm specification test material 2 pieces, respectively numbered test piece 3 and test piece 4.
2) Test piece 4 is heated in the resistance furnace, with resistance furnace accuracy of ±5 ℃; the heating temperature is set to 956 ℃, the furnace temperature reaches the set temperature to start timing, the holding time is set to 60 min; after reaching the holding time, the billet out of the furnace, air cooling to room temperature; test piece 3 does not do the pre-forging treatment.
3) Test piece 3 and test piece 4 are forged conventionally: the test material is first heated in the resistance furnace; the accuracy of the resistance furnace is ±10℃; the heating temperature is set to 936℃, the furnace starts timing after reaching the set temperature, and the holding time is set to 100min; after reaching the holding time, the billet is discharged from the furnace and forged, and the billet is drawn to 50mm×50mm×100mm on the 750kg free forging hammer and air-cooled to room temperature after forging;
Table.3 Comparison of the longitudinal mechanical properties of the test piece 1 and test piece 2 at room temperature

Number	Flaky phase a content	Sampling direction	Impact toughness/(J/cm2)	Resistenza alla trazione/MPa	Yield strength/MPa	Elongation/%	Reduction of area/%
Test piece 1 (conventional)			36	820	745	13.5	55
			34	817	743	16.5	50
Test piece 2 (optimized)			55	772	734	17	46
			63	768	712	18	46
Standard requirements			≥35	685 – 885	≥585	≥10	≥30

Table.4 Comparison of the longitudinal mechanical properties of the test piece 3 and test piece 4 at room temperature

Number	Flaky phase a content	Sampling direction	Impact toughness/(J/cm2)	Resistenza alla trazione/MPa	Yield strength/MPa	Elongation/%	Reduction of area/%
Test piece 3 (conventional)			36	820	745	13.5	55
			34	817	743	16.5	50
Test piece 4 (optimized)			43	806	741	18	50
			44	798	726	17	52

4) Test part 3 and test part 4 heat treatment in the same furnace, heat treatment system: the heating temperature is set to 780 ℃, the furnace temperature reaches the set temperature after the start of time, holding time is set to 120min; after reaching the holding time, the billet out of the furnace, air-cooled to room temperature.
(3) Results and analysis.
1) Microstructure comparison.
Figure 3 shows the comparison of the microstructure of test piece 3 and test piece 4 at the same sampling position at 1/2 thickness; it can be seen that the microstructure of test piece 3 contains ≤10% secondary strip α phase, while the microstructure of test piece 4 contains about 15%-25% secondary strip α phase, i.e., compared with the conventional hot working state (test piece 3), the increase in near β annealing before forging (test piece 4) has obtained more secondary strip α-phase, achieving the expected results.
2) Performance comparison.
Table 4 shows the comparison of room temperature longitudinal mechanical properties of the test piece 3 and test piece 4 at the same sampling position. It can be seen that the impact toughness of the forgings was significantly improved compared with the conventional hot-worked condition (test piece 3) after near-β annealing before forging (test piece 4), and there was no significant difference in the strength of the forgings compared with the conventional hot-worked condition. Compared with the increase in near-β annealing after forging, the increase in near-β annealing before forging has a smaller increase in impact toughness but hardly causes any loss of room temperature strength, which should be attributed to the fact that the secondary striped α phase generated during near-β annealing before forging is partially broken up during forging, and the forgings are thus dislocation strengthened.

3. Conclusione

• (1) By adding near-β annealing after the forging process, more secondary strip α phases can be obtained in the microstructure of Ti-4Al-1.5Mn forgings, and the impact toughness of the forgings is significantly improved. Still, the room temperature strength will be more significantly decreased.
• Figure 3 Comparison of the microstructure of test piece 3 and test piece 4
• (2) By adding near β-annealing before forging, the microstructure of Ti-4Al-1.5Mn forgings can obtain more times of raw α-phase, and the impact toughness of forgings can be significantly improved. The room temperature strength will not be significantly decreased.
• (3) The addition of near-β annealing before forging can be used to optimize all types of Ti-4Al-1.5Mn small forgings; the addition of near-β annealing after forging can be used to guide the heat treatment reworking process for forgings with unqualified impact toughness problems.

Authors: Che Anda, Zhang Yuandong, Liu Xiuliang, Ma Siqin, Zhang An

Fonte: Cina Titanium Alloy Forgings Manufacurer: www.titaniuminfogroup.com