Effect of Heat Treatment Temperature on Microstructure and Properties of Different Formed Surfaces of TC4 Titanium Alloy Sheet by Selective Laser Melting

Ti-6Al-4V titanium alloy, also known as TC4 titanium alloy, is a typical α+β phase titanium alloy.

It has high strength, low density, high fracture toughness, excellent corrosion resistance and biocompatibility, and is widely used in aerospace, shipbuilding, automobile, energy, medical, chemical and biological medicine industries.

Selective Laser Melting (SLM) technology, as a typical laser additive manufacturing technology based on computer aided design (CAD) model manufacturing parts, provides a series of market competitive advantages for some manufacturing enterprises, including near net shape production without molds and tools, high material utilization efficiency and horizontal flexibility.

Laser printing technology in SLM technology has higher temperature gradient and faster cooling rate, which is one of the most promising additional manufacturing technologies for producing TC4 titanium alloy parts with complex shapes.

The typical structure of TC4 titanium alloy produced by SLM technology is columnar β grain, ultra-fine non-equilibrium metastable martensite α ‘phase and a large number of dislocations. This structure is different from the equiaxed α phase+intergranularβ phase obtained after conventional annealing and forging.

The existence of ultra-fine grain size and a large number of dislocations makes the material harder and stronger.

The non-equilibrium metastableα’ is unfavorable to the ductility and fatigue resistance of the material.

Therefore, its tensile properties always show high strength (tensile strength limit can reach 1320MPa) and low plasticity (plastic strain is 2%~7%).

The formed parts produced by SLM technology have low elongation after fracture and large residual stress, so they need to be heat treated.

In general, all kinds of thermomechanical treatment can not change or control the microstructure of titanium alloys, and heat treatment is the only way to improve the microstructure and mechanical properties of titanium alloys.

At present, there are many studies on the effect of heat treatment on the properties of TC4 titanium alloy by selective laser melting.

However, the existing reports have only studied one plane (side) of the sample, without considering that selective laser melting TC4 titanium alloy sheet has two forming surfaces.

According to the existing research, and considering the α-phase transition temperature, researchers from Shanghai University of Engineering and Technology studied the influence of different heat treatment temperatures on the microstructure and properties of different forming surfaces of TC4 titanium alloy sheet selectively melted by laser, in order to provide a theoretical basis for the development and application of selective laser melting TC4 titanium alloy.

1. Test materials and methods

1.1 Test materials

Table 1 Forming Process Parameters of Selective Laser Melting TC4 Titanium Alloy Sheet

Fig. 1 Schematic Diagram of Printing Scheme for Selective Laser Melting TC4 Titanium Alloy Plate

The test material is spherical TC4 titanium alloy powder.

The gas phase atomization method is adopted.

According to the forming process parameters shown in Table 1 and the printing scheme shown in Fig. 1, the layer by layer rotation 67 ° scanning strategy is adopted.

The XY axis is used as the bottom Z axis for printing. The printed TC4 titanium alloy plate is shown in Fig. 2.

Fig. 2 Macro morphology of TC4 titanium alloy plate melted by selective laser

1.2 Test method

Use the wire cutter to cut small pieces with the size of 20mm×20mm×8mm in the right area of the plate as shown in Fig. 2, and then divide them into 16 block samples.

When cutting, mark the top and side of the sample, the top surface is XOY, and the side surface is XOZ.

Four side samples and four top samples are selected from the 16 block samples, and they are divided into four groups. Each group includes a top sample and a side sample.

One group is used as the original sample, and the other three groups are heat treated according to the process parameters shown in Table 2.

Table 2 Heat Treatment Process Parameters

After heat treatment, the sample is inlaid, polished and then etched for 25s with a solution of HNO3, HF and H2O mixed in a volume ratio of 10 ∶ 5 ∶ 85;

Then the micro morphology was observed by optical microscope and scanning electron microscope (SEM), and the phase composition was analyzed by X-ray diffraction (XRD).

The Vickers microhardness tester is used to test the hardness of the selected laser melted TC4 titanium alloy plate samples after heat treatment.

20 test points are selected for each sample, and the average value is taken.

2. Results and discussion

2.1 Phase composition

Fig. 3 XRD Spectrum of Original Specimen and Top and Side Specimens at Different Heat Treatment Temperatures

As shown in Fig. 3, the crystal structures of phase α and phase α ‘are the same, and the positions of diffraction peaks are the same, so all diffraction peaks of phase α and phase α’ in the original sample can be marked as phase α ‘;

Compared with the β phase diffraction peak in the original sample, the β phase diffraction peak of the top surface sample does not increase significantly with the increase of heat treatment temperature.

When the heat treatment temperature rises to 950 ℃, the β phase diffraction peak increases more, indicating that the β phase content in the top surface sample increases.

The change rule of XRD spectrum of side specimen is the same as that of top specimen;

At different heat treatment temperatures, the diffraction peak heights of the top and side samples have little difference.

2.2 Microstructure

Fig. 4 Microstructure of original sample and top interview sample under different heat treatment processes

Fig. 5 SFM Morphology of Original Sample and Top Sample under Different Heat Treatment Processes

It can be seen from Fig. 4 and Fig. 5 that there is columnar β phase in the original top surface sample and a large number of acicular martensite α ‘phase in the crystal.

With the increase of heat treatment temperature, the grain of the top sample gradually coarsens and the grain of β gradually decreases;

The surface of the original top surface sample shows a chessboard shape, which is formed by 67 ° filling angle generating cross scanning paths between adjacent layers.

There are ultra-fine layered acicular martensite α ‘phase in the columnar grains, and the long axis orientation of most martensite α’ phase is about ± 45 °, because there is a strict Burgess orientation relationship between α and β, namely, (0001) α/{110} β and<1120>α/<111>β;

After 750 ℃/2h+air cooling treatment, compared with the original sample, the grain size of the top sample has no obvious change, and a part of the acicular α ‘phase in the columnar β phase grain boundary transforms into a layered α phase, which indicates that the structure is a Widmanstatten structure;

After 850 ℃/2h+air cooling treatment, the grains on the top surface of the sample are coarsened, and columnar β crystals are still visible.

The acicular α ‘phase is completely transformed into layered α and β phases, and the β grains are small, and the layered α phase is still in the previous columnar β phase crystal, so it can be judged that the structure is a basket structure;

After 950 ℃/2h+air cooling treatment, the grains on the top surface of the sample are coarsened obviously, forming a spherical phase α, and basically no columnar phase β can be seen. The β grains aggregate, grow, and become thin rods, forming a layered β phase native transformation structure.

From this, it can be judged that the structure is a bimodal structure.

Fig. 6 Microstructure of original sample and side sample under different heat treatment processes

Fig. 7 SEM Morphology of Original Sample and Side Sample under Different Heat Treatment Processes

It can be seen from Figure 6 and Fig. 7 that:

The scanning trace of the side sample is deeper than that of the top sample, and the columnar β phase is clearer;

With the increase of heat treatment temperature, the grain of the side sample gradually coarsens, and the columnar β phase grain boundary gradually blurs, which is the same as that of the top sample;

At different heat treatment temperatures, there are columnar β phases in the side specimen, which is different from the top specimen.

2.3 Hardness test

Fig. 8 Hardness of Original Specimen and Top and Side Specimens at Different Heat Treatment Temperatures

It can be seen from Fig. 8:

The average hardness values of the original top surface sample and the original side surface sample are 320HV and 317HV respectively;

With the increase of heat treatment temperature, the hardness of the top surface sample decreased from 308HV (750 ℃) to 291HV (850 ℃), and then increased to 309HV (950 ℃);

The hardness change rule of the side specimen is the same as that of the top specimen.

Its hardness decreases from 311HV (750 ℃) to 297HV (850 ℃), and then increases to 303HV (950 ℃).

The results of XRD and SEM analysis show that:

During heat treatment at 750~850 ℃, the sample mainly undergoes the transformation from α ‘phase to α phase.

α’ phase is a supersaturated solid solution, and its hardness is significantly higher than that of α phase;

There are a lot of acicular martensite α ‘phase in the top and side samples.

After 750 ℃/2h+air cooling treatment, α’ phase transforms into α phase, and the hardness of the samples decreases;

After 850 ℃/2h+air cooling treatment, the acicular α ‘phase was completely transformed into α phase and β phase, and the microstructure was mainly layered a phase and small block β phase, and the hardness of the sample decreased;

After 950 ℃/2h+air cooling treatment, the hardness of the top and side samples increased, because the heat treatment temperature exceeded the transformation temperature of phase α (882 ℃), and recrystallization occurred, forming spherical phase α and layered phase β transformation structure.

Compared with spherical α phase, the existence of layered α phase will reduce the elongation after fracture of titanium alloy.

There are many interlaced and fine secondary α phases in layered β phase, which hinder the progress of sliding.

It is difficult for titanium alloy to deform. The content of layered β phase in bimodal structure is high, leading to the increase of hardness of titanium alloy.

3. Conclusion

(1) With the increase of heat treatment temperature, the acicular martensite α ‘phase on the top and side of TC4 titanium alloy plate melted by selective laser decreases continuously.

When the heat treatment temperature is 850 ℃, the acicular α’ phase completely transforms into α phase and β phase.

When the heat treatment temperature (950 ℃) exceeds the transformation temperature of α phase, the content of β phase increases.

After heat treatment at 950 ℃, the top surface of TC4 titanium alloy plate melted by selective laser has almost no columnar β phase, and equiaxed β phase is formed, while columnar β phase still exists on its side.

(2) The hardness of TC4 titanium alloy plate melted by selective laser without heat treatment is the highest, and the hardness of its top surface and side surface are 320HV and 317HV respectively.

After heat treatment at different temperatures, the hardness of the top and side surfaces of titanium alloy plates decreases first and then increases with the increase of temperature.

The hardness of the top surface of the titanium alloy plate decreases from 308HV (750 ℃) to 291HV (850 ℃), and then increases to 309HV (950 ℃). The hardness of the side surface decreases from 311HV (750 ℃) to 297HV (850 ℃), and then increases to 303HV (950 ℃).