Effects of Laser Shock Processing on Impact Toughness of Ti-17 Titanium Alloy

Test sample

A 5-mm thick Ti-17 titanium alloy plate was used in the experiments. Ti-17 titanium alloy materials, with chemical compositions (see Table 1) of Ti-5Al-2Sn-2Zr-4Cr-4Mo (wt%), were annealed at 55 °C for 2 h and air cooled. The mechanical and physical properties of the Ti-17 titanium alloy are listed in Table 2. The microstructures of the Ti-17 titanium alloy are shown in Figure 2. The figure illustrates that Ti-17 titanium alloy is an α+β two-phase alloy with large grains, consisting of fine αplates embedded in the β matrix [20]. Prior to the LSP, the intended peening surfaces of the samples were ground with 1200 grit sandpaper followed by final polishing to achieve a surface roughness of 0.05 μm.

Figure 2:

Ti-17 titanium alloy microstructure.

A sketch of the impact sample, fabricated according to the national standard of the People’s Republic of China GB/T 229-2207, is shown in Figure 3. The Charpy impact test was performed with a SANS-2000 (MTS) pendulum bob impact testing machine at room temperature. One operation mode of this apparatus is to place a Charpy V-notch specimen (shown in Figure 3) across the parallel jaws of the machine. In the impact test, a heavy pendulum released from a known height strikes and fractures the sample on its downward swing. The energy absorbed by the fracture can be calculated by determining the mass of the pendulum and the difference between its initial and final heights.

Figure 3:

Schematic of the impact toughness sample.

Laser shock processing

In this research, laser shock processing was performed with an Nd: glass laser operating at 1064 nm with a pulse duration of 10 ns and a spot diameter of 3 mm. The laser energy was approximately 5 J and the repetition rate of the laser was 2 Hz; 0.1 mm of black tape was used to protect the titanium alloy from the heating effects of the laser. To increase the peak pressure of the laser shock wave, a layer of flowing water with a layer thickness of approximately 2-mm was used as the transparent confining layer. The overlap rate of the laser spot was 50 %. The 25-mm×10-mm impact zone was subjected to two passes of LSP on both sides of the impact sample, as shown in Figure 3.

Characterization

After the LSP treatment, the tape coating was removed and the workpiece surface was cleaned with ethanol. The residual stress was measured by X-ray diffraction with the sin²φ method, using an X-ray diffraction tester (X-350A Handan Stress Technology Co. Ltd., China). The X-ray source was CuKα and the beam diameter was approximately 1 mm. The voltage and current of the X-ray pipe were 26 kV and 6.0 mA, respectively. Measurements were performed at different locations across the LSP region. The microhardness of the base metal and the samples subjected to LSP were measured with a microhardness tester (DHV-1000) under 100 g of load for a holding time of 15 s. The microhardness of a section of the sample before and after LSP was tested at 0.05 mm intervals along a straight line on the sample.

The microstructures of the untreated and LSP-treated samples were examined with a transmission electron microscope (TEM) (JEM-2100, JEOL). The fracture morphologies of the samples were examined before and after LSP using a scanning electron microscope (SEM) (JSM-6010LA, JEOL).

Surface characteristics

The surface morphology of a single spot subjected to the laser pulse energy is shown in Figure 4. When the laser pulse was 5 J, a circular dent was created after the LSP treatment because of plastic deformation at the shock wave loading region. The plastic deformation produced by the laser pulse energy is a consequence of the shock wave pressure, which is proportional to the root of the laser pulse energy [21]. Line profiles of the peened surface treated at 5 J are shown in Figure 4; the depth of the microdents was about 15.3 μm. After multiple LSP treatments, the surface exhibited a regular array of microdents, which were caused by the plastic deformations induced by successive laser irradiations at different locations [22, 23, 24]. The dent spacing was controlled by the laser shot-to-shot offset.

Figure 4:

Microdents on material surface after LSP treatment.

The residual compressive stress distributed on the sample before and after LSP is shown in Figure 5. The plasma generated in the breakdown water layer prevented the laser from directly influencing the material surface. Multiple impacts could produce a higher surface residual stress than a single impact because of increased plastic strain and cyclic hardening [25]. In many metal materials, plastic deformation can enhance microhardness of the base material by inducing high-density dislocations or grain refinement [10]. The microhardness distribution on the Ti-17 titanium alloy was measured after the LSP treatment (see Figure 6). The depth of the gradient hardened layer (GHL) was determined by calculating the microhardness distribution along the cross-section of the laser-peened sample. According to Chen et al. [26] and Osamu et al. [27], induced residual compressive stress can enhance the hardness of materials. A residual stress field was produced by the uneven plastic deformation of the deformed layer. After LSP, the surface layers of the samples featured severe elastic deformations. Residual stress fields were introduced and the domain sizes were refined. In addition, dislocations and dislocation cells were formed. These effects could increase the microhardness of the samples. The impact toughness is also related to the compressive residual stress [28, 29]. Thus, we hypothesized that the gradients of hardness and impact toughness had similar distributions. Hence, the hardness along the cross-section of the laser-peened sample can be used to determine the GHL depth. The GHL depth of the Ti-17 titanium alloy treated with 5-J laser pulse energy was measured with the microhardness test, as shown in Figure 6. At a laser energy of 5 J, the cross-section showed that the microhardness increased at the surface of the GHL, and then gradually decreased to the value of the untreated region at a depth of (1.00 ± 0.05) mm, as shown in Figure 6. This result indicates that the GHL thickness was (1.00 ± 0.05) mm at laser pulse energy of 5 J because the laser-induced shock pressure is proportional to the square root of the laser energy according to Fabbro’s model [30]; namely, higher laser energy produces a higher laser-induced shock pressure. According to Ballard et al. [31], the plastically affected depth is proportional to the shock pressure. Thus, a higher shock pressure induces a deeper plastically affected depth.

Figure 5:

Residual compressive stress on material surface before and after LSP.

Figure 6:

Microhardness of the sample before and after LSP.

Impact energy

Table 3 shows the impact energies of the Ti-17 titanium alloy before and after LSP treatment. The average impact energies of the base material and LSP-treated titanium alloy were 123.225 and 133.175 J, respectively. Thus, the average impact energy of the Ti-17 titanium alloy increased approximately by 10 J after the LSP treatment.

We used statistical analysis to verify to what the degree LSP affected the impact toughness of Ti-17 titanium alloy. The average impact toughness of the LSP treated Ti-17 titanium alloy was Xˉ.

where m and n represent the numbers of test samples.

The sample variance of Xi is S1.

The average impact toughness of the Ti-17 titanium alloy after LSP is Yˉ.

The sample variance of Yi is S2.

Supposing that X∼N(μ1,σ12), Y˜Nμ2,σ22, the original hypothesis H0 that μ1≤μ2 can be proposed.

Therefore, the alternative hypothesis H1 that μ1<μ2 can be proposed.

Under the conditions of the original hypothesis, the statistical power T0 was constructed.

The rejection region:

According to the principle of the statistical test:

At the α=0.05 significance level the t test value was .

The original hypothesis was rejected because T0=3.948>1.943=t0.056. Thus, LSP can improve the impact toughness of Ti-17 titanium alloy at the significance level α=0.05.

Fracture analysis

As shown in Figure 7(a), the untreated impact samples all completely fractured into two parts, whereas the LSP samples were not fractured and remained connected at the reverse of the impact site, as shown in Figure 7(b). Figure 8 shows a photograph of the impact sample after fracture. The crack source of the base material and the sample subjected to LSP are marked by circles. The location of the crack source of the impact fracture shifted from the center to the side of the Charpy-V notch region after the LSP treatment. The LSP treatment caused an increase in the microhardness and uneven plastic deformation of the material, which induced stress concentration on the laser-irradiated surface. The crack source was primarily determined by the stress concentration. In addition, under an external force, the expansion of the crack path in the LSP samples (Figure 8b) featured more twisting than that of the original samples (Figure 8a). This suggests that more energy was consumed to initiate cracking for the LSP samples [32]. Figure 9 shows SEM images of the crack source of the untreated and LSP samples. The untreated material fracture did not show any notable plastic deformation, and the fracture cracks were of an intercrystalline type, where the intergranular binding force is weak (Figure 9a). Figure 9(b) shows the many dimples on the LSP sample fracture, which displayed features of plastic fracture.

Figure 7:

Photographs of samples after impact test. (a) Impact sample before LSP; (b) Impact sample after LSP; (c) Sample marked in Figure 7(a).

Figure 8:

SEM images of impact sample fracture. (a) Fracture of impact sample before LSP; (b) Fracture of impact sample after LSP.

Figure 9:

SEM images of crack source. (a) Fracture of impact sample before LSP; (b) Fracture of impact sample after LSP.

Microstructures

Typical TEM images of the base material and the LSP-treated samples are shown in Figure 10. The density of dislocations in the sample subjected to LSP treatment (Figure 10[b]) increased considerably compared with that in the base material. The laser shock wave displaced materials and plastic deformation occurred through dislocation slip, shock wave reflection, and refraction at grain boundaries. The shock waves had various effects on grains. Thus, dislocation after complex slip, agglomeration, and annihilation led to formation of new grain boundaries and small grains, and also improved the dislocation density [33].

Figure 10:

TEM images of impact sample. (a) Untreated material; (b) Impact sample after LSP.

The higher dislocation density hindered the plastic flow of metal and improved the microhardness of materials. The relationship between the microhardness and the density of dislocations can be defined as [34]:

where HV is the microhardness of the material, HV0 is the microhardness of the matrix, a is a constant related to the material, G is the shear modulus, b is the Burgers vector, and ρ is the dislocation density.

Formula (1) shows that Hv is proportional to ρ^1/2. Furthermore, increasing the dislocation density ρ helps to improve the microhardness of the material.

In addition, Hall–Petch found the relationship between the rupture stress and the diameter of the material grain [35].

where σ is the rupture stress, σ0 is the basic rupture stress, K is the constant related to the material, and d is the diameter of the material grain.

Formula (2) shows that rupture stress is inversely proportional to the diameter of grains in the material. After LSP treatment, the grain diameter became smaller than that of the original material. Therefore, LSP improves the rupture stress of metals and consequently, the samples subjected to LSP required more energy to fracture.

The stress was released through dislocation slip and through propagation of stacking faults under stress because of the fine plastic nature of the titanium alloy. Therefore, the uniform plastic deformation had a high capacity as indicated by the macroscopic analysis. Furthermore, the titanium alloys had a high work hardening capacity. Deformation and microvoids were formed because of the stress concentration at the intersection of the slip bands at the different slip surfaces and along the different slip directions. The microvoids expanded into microcracks, which then fractured [8, 36]. Determination of optimal grain sizes has attracted research attention because of the profound effects of grain size on the impact toughness of materials. In general, greater plastic deformation allows for more energy to be absorbed at turning sites [37], meaning that more energy can be absorbed during impact leading to better impact toughness. Hence, the improved impact toughness in our samples can be attributed to the more effective small surface grains induced by the LSP treatment.