Scholarly article on topic 'Microstructure Evolution of Ti-6Al-4V during Superplastic-like Forming'

Microstructure Evolution of Ti-6Al-4V during Superplastic-like Forming Academic research paper on "Materials engineering"

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{"Superplastic-like forming" / Ti-6Al-4V / EBSD / Recrystallization.}

Abstract of research paper on Materials engineering, author of scientific article — Mei-Ling Guo, Jun Liu, Ming-Jen Tan, Beng-Wah Chua

Abstract Superplastic-like forming is a recent developed sheet-forming process that combines hot drawing (mechanical pre-forming) with gas forming. It is an efficient way to form sheet metals into complex parts for automotive and aerospace industries. In comparison with conventional superplastic forming process, the forming time for superplastic-like forming can be significantly shortened as the hot-drawing step would have produced a pre-formed component before gas forming. The other advantage of the superplastic-like forming is its capacity for lower temperature forming, for which superplasticity is not possible. Non-superplastic grade Ti-6Al-4V sheets were successfully formed by superplastic-like forming at 800°C in 16min. The maximum percentage thinning of 54% occurred at the outward corners. In this paper, electron backscatter diffraction (EBSD) was used to examine the microstructure evolution of Ti-6Al-4V at different forming stages during superplastic-like forming process. Some small equiaxed grains, regarded as newly recrystallized grains, were observed near the deformed area after hot drawing. Grains became more randomly distributed as the recrystallization continued during gas forming stage. The as-received structures were finally replaced by the equiaxed grains with an almost random misorientation distribution after the forming process. Dynamic recrystallization was considered as the main deformation mechanism for the non-superplastic grade Ti-6Al-4V alloy.

Academic research paper on topic "Microstructure Evolution of Ti-6Al-4V during Superplastic-like Forming"

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Procedía Engineering 81 (2014) 1090 - 1095

Procedía Engineering

www.elsevier.com/locate/procedia

11th International Conference on Technology of Plasticity, ICTP 2014, 19-24 October 2014,

Nagoya Congress Center, Nagoya, Japan

Microstructure evolution of Ti-6Al-4V during superplastic-like

forming

Mei-Ling Guoa, Jun Liua, Ming-Jen Tan a*, Beng-Wah Chuab

aSchool of Mechanical & Aerospace Engineering, Nanyang Technological University, 639798, Singapore bSingapore Institute of Manufacturing Technology, 71 Nanyang Dr, 638075, Singapore

Abstract

Superplastic-like forming is a recent developed sheet-forming process that combines hot drawing (mechanical pre-forming) with gas forming. It is an efficient way to form sheet metals into complex parts for automotive and aerospace industries. In comparison with conventional superplastic forming process, the forming time for superplastic-like forming can be significantly shortened as the hot-drawing step would have produced a pre-formed component before gas forming. The other advantage of the superplastic-like forming is its capacity for lower temperature forming, for which superplasticity is not possible. Non-superplastic grade Ti-6Al-4V sheets were successfully formed by superplastic-like forming at 800 °C in 16 min. The maximum percentage thinning of 54 % occurred at the outward corners. In this paper, electron backscatter diffraction (EBSD) was used to examine the microstructure evolution of Ti-6Al-4V at different forming stages during superplastic-like forming process. Some small equiaxed grains, regarded as newly recrystallized grains, were observed near the deformed area after hot drawing. Grains became more randomly distributed as the recrystallization continued during gas forming stage. The as-received structures were finally replaced by the equiaxed grains with an almost random misorientation distribution after the forming process. Dynamic recrystallization was considered as the main deformation mechanism for the non-superplastic grade Ti-6Al-4V alloy.

© 2014 Publishedby Elsevier Ltd.Thisis an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Selection and peer-review under responsibility of the Department of Materials Science and Engineering, Nagoya University Keywords: Superplastic-like forming; Ti-6Al-4V; EBSD; Recrystallization.

* Corresponding author. Tel.: +65-67905582; fax: +65-67911859. E-mail address: mmjtan@ntu.edu.sg

1877-7058 © 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Selection and peer-review under responsibility of the Department of Materials Science and Engineering, Nagoya University doi:10.1016/j.proeng.2014.10.146

1. Introduction

Ti-6Al-4V has attracted a great deal of attention in the last few decades due to its favorable comprehensive performances, i.e. high specific strength and toughness, superior corrosion resistance, good superplastic behavior and so forth (Singh et al., 2010). Hence, this light weight alloy has found broad applications in aerospace, transportation and biomedical areas (Sergueeva et al., 2002; Zherebtsov et al., 2004; Curran et al., 2006).

Nomenclature

SPF superplastic forming EBSD electron backscatter diffraction

To fabricate these alloy sheets into the desired shape with high geometrical accuracy and good finishes, superplastic forming (SPF) is considered as the potential forming process to make large and complex workpieces in single operation at elevated temperatures by taking advantage of the superplastic flow behavior of the material (Barnes et al., 2007).

The previous studies show that the superplastic behavior of Ti-6Al-4V alloy can be achieved above 900 °C at low strain rates (usually lower than 10 3 s (Nieh et al., 1997). Many efforts have also been devoted to enhance the superplasticity of Ti-6Al-4V alloy by forming the ultrafine-grained structures, which is considered as a necessity in conventional SPF (Salishchev et al., 2001; Sergueeva et al., 2000). Although SPF process has numerous advantages in material formability, complex shape forming and cost saving by avoiding parts joining and secondary machining, it still has intrinsic deficiencies. The slow forming rates, high forming temperatures and poor thickness control severely limit the application of SPF, especially for cases of mass production and the materials without superplasticity at relatively low temperatures (Barnes et al., 2007; Du et al., 1994).

To address the limitations aforementioned, conventional SPF was modified and improved to meet the requirements on forming productivity and part defects that are generated in the high temperature forming process (Liu et al., 2011). This novel superplastic-like forming process incorporates a designed hot drawing process before the final gas blowing process, thus enabling the whole forming to be completed with shortened time and relatively low temperatures (Liu et al., 2013). In this way, the part defects caused by high temperature forming can be reduced to a great extent, and the forming efficiency can be further improved in comparison with the conventional SPF.

In this study, the non-superplastic grade Ti-6Al-4V sheets were successfully formed by superplastic-like forming at 800 °C in 16 min. The maximum percentage thinning of 54 % was found at the outward corners. The micro structure evolution and the underlying deformation mechanism of Ti-6Al-4V at different forming stages during superplastic-like forming process were both investigated.

2. Experimental details

The metal forming technology used in this research is a combination of hot drawing with gas forming, and the schematic of the superplastic-like forming process is shown in Fig. 1. During hot drawing stage, the punch was actuated to mechanically pull a desired amount of the flange material into the die cavity. Subsequently, argon gas pressure was applied onto the pre-formed sheet to complete the gas forming at a targeted strain rate.

Non-superplastic grade Ti-6Al-4V alloy sheets with the size of 210 x 210 mm2 and thickness of 1.6 mm were used in this research for superplastic-like forming at 800 °C and a strain rate of 10-3 s-1. Ti-6Al-4V sheets have been successfully formed by this process within 16 min, showing much higher efficiency than the conventional superplastic forming. The superplastic-like forming was carried out on a Murdock SPF/DB 100 Ton Press. At the hot drawing stage, the sheet metal was drawn into the die cavity by the punch at a constant speed until it reached a total displacement of 43 mm within 9 s. To reduce the friction and give the material the opportunity to deform easily throughout the gas forming stage, graphite spray was used as the lubricant on both metal surfaces. The

forming pressure-time curve was predicted from simulation to enable the sheet to be gas formed at a strain rate of

Lock bead Sheet

.Heaters

(c) Argon

IL L —J Jr

10 s-1

Fig. 1. Schematic of the superplastic-like forming process: (a) heating and clamping; (b) hot drawing and sealing; (c) gas forming.

The metal sheet was almost fully formed by the hot drawing and subsequent gas forming. Thicknesses along the cross-section were measured from the dome center to the edge. Each measurement was named from location #1 to #11, as shown in Fig. 2. The maximum percentage thinning is 54 % at location #4 where the sheet initially came into contact with the punch nose during hot drawing. Material adjacent to this area was then locked against the die surface by friction and gas pressure. The highest level of deformation occurred at this location, and as a result, leads to localized thinning compared to the other areas.

Fig. 2. Cross-section of Ti-6Al-4V part formed at 800 °C for thickness measurements and microstructure observations.

To observe the microstructure and investigate the deformation influence on the microstructural evolution, the thinning area (location #4 in Fig. 2) during hot drawing and gas forming were characterized by electron backscatter diffraction (EBSD), respectively. The EBSD examinations were conducted using a JEOL 7600F field emission scanning electron microscope with an Oxford Instruments HKL EBSD system. All the specimens used for EBSD were machined along the cross-section of sheet thickness. The Ti-6Al-4V samples were initially ground and mechanically polished, and then electropolished in HClO4: CH3COOH (1: 8) solution at room temperature and 30 V for 30 s.

3. Results and discussion

The maximum sheet thinning after deformation occurred in the vicinity of location #4. The micro structure s before and after forming were then observed at this location. The microstructures were examined by EBSD at the different forming stages, i.e. as-received, hot drawing, gas forming at 6-min, gas forming at 11-min and gas forming at 16-min.

3.1 As-received microstructure

The microstructure of the as-received Ti-6Al-4V material is shown in Fig. 3. A large fraction of low angle grain boundaries with misorientations below 15° were found in the whole area. The distribution of high angle grain boundaries is not homogeneous, and some fine equiaxed grains were observed near the big grains.

Fig. 3. EBSD maps of as-received Ti-6Al-4V material.

3.2 Microstructure after hot drawing

In comparison with the microstructure of the as-received material, the overall boundary angle distributions and average grain boundary misorientation angle remained similar after hot drawing process, as shown in Fig. 4. The average grain size reduced from 1.8 ^m to 1.4 ^m, possibly as a result of dynamic recrystallization. These recrystallized grains were identified by measuring the internal grain misorientations below 3°. The fraction of high angle grain boundaries reduced from 25 % to 23 %, implying that the material to some extent was subjected to subgrain boundary migration and rotation during hot drawing.

Fig. 4. EBSD maps of Ti-6Al-4V after hot drawing.

3.3 Microstructure during gas forming

The gas forming can be divided into three stages according to the forming history, as shown in Fig. 5. At the gas forming 6-min stage, obvious difference when comparing with the hot drawing samples can be clearly observed. The dynamic recrystallized grains with the equiaxed shape were distributed evenly throughout the observed zone, as shown in Fig. 5(a). The grain structure seems more randomly distributed after 6-min gas forming. The average misorientation angle and the fraction of high angle grain boundaries were calculated as 36.9° and 62 %, respectively. The increased amount of high angle grain boundaries is attributed to the newly recrystallized grains.

When the gas forming continued, the average grain size decreased from 1.9 ^m to 1.4 ^m and the fraction of the subgrain boundary increased significantly probably caused by subgrain migration, as shown in Fig. 5(b). Some sub-structures with high density of low angle grain boundaries (the white lines) were observed within the big grains. It should be noted that localized thinning occurred at location #4 (see Fig. 2) during this stage by the imposed gas pressure. Stress was assumed to be concentrated at this area, leading to higher strain rate deformation compared to the other areas. The fast deformation would accelerate dislocation movement, which can be evidenced by the increased fraction of sub-structures (or low angle grain boundaries). As a result, some of the grains in Fig. 5(a) may coalesce and be elongated due to the sub-grain boundary migration (see Fig. 5(b)).

At the 16-min gas forming stage, the average misorientation angle and the fraction of high angle grain boundaries were 41.8° and 75 %, matching closer to 60° and 98 % for the random misorientation distribution. In this stage, the forming rate has been slowed down since the sheet material contacted with the die surface. Dynamic recrystallization continued and subsequently produced copious new grains with high angle grain boundaries misorientation, as shown in Fig. 5(c).

imflifd

10 20 30 40 50 60 70 80 90 Misorientation Angle (°)

,22.8°

0 10 20 30 40 50 60 70 80 90 Misorientation Angle (°)

I 41.8

0 10 20 30 40 50 60 70 80 90 Misorientation Angle (°)

Fig. 5. EBSD maps of Ti-6Al-4V at different gas forming stages: (a) gas forming at 6-min; (b) gas forming at 11-min; (c) gas forming at 16-min.

Table 1 summarizes the grain structure characterizations of Ti-6Al-4V at different forming stages. The grain structure during hot drawing does not show many variations as compared to that in the as-received material. As gas forming proceeded, the microstructure was characterized by randomly distributed equiaxed grains. The increased high angle grain boundaries and reduced aspect ratio indicate that the main deformation mechanism of Ti-6Al-4V alloy during gas forming is recrystallization.

Table 1. Summary of grain structures for Ti-6Al-4V obtained from EBSD calculations.

Mean grain size ( |im ) Fraction of high angle grain boundaries Average aspect ratio

As-received 1.8 25 % 1.8

Hot drawing 1.4 23 % 1.5

Gas forming at 6-min 1.9 62 % 1.5

Gas forming at 11-min 1.4 38 % 1.4

Gas forming at 16-min 1.5 75 % 1.4

4. Conclusions

Non-superplastic grade Ti-6Al-4V sheets were successfully formed by the superplastic-like forming which combines hot drawing (mechanical pre-forming) with gas forming at 800 °C in 16 min. The micro structure evolutions in the stages of hot drawing and gas forming were investigated. The conclusions are summarized as follows:

• The maximum sheet thinning after forming is 54 % at the outward corners (the areas adjacent to the die entry radii). The gas forming can be divided into three stages, in which the microstructure changes attribute to the variation of the deformation rate.

• The main deformation mechanism for the non-superplastic grade Ti-6Al-4V alloy during superplastic-like forming is recrystallization. Grains became more randomly distributed as the recrystallization continued during the gas forming stage.

• The as-received structures were transformed into equiaxed grains with an almost random misorientation distribution after the superplastic-like forming process.

References

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