Mechanical properties and strengthening mechanisms in laser beam welds of pure titanium

Mechanical properties of laser beam welds

The relationship between the mechanical properties and the grain orientation of the base metal (BM) is given in Table 1. The RD, TD and ND are the rolling, transverse and normal directions respectively. The tensile strength and hardness of BM obviously depend on the test direction, which is associated with the grain orientation or texture distribution. The tensile strength, elongation, fracture position and photographs of the tested specimens of the laser beam welds under various welding conditions are present in Table 2. The WD was parallel to the RD when a laser power of 8 kW is adopted, while the WD was perpendicular to the RD under the condition of 10 kW laser power for the tensile test. The tensile strength and elongation of the laser beam welds are approximately identical with those of BM irrespective of the welding conditions in the present work. Moreover, as seen from Table 2, all the fractures of the welds occur at the BM, far from the heat affected zone (HAZ) and fusion zone (FZ), implying that HAZ and FZ are stronger than BM.

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Table 1

Relationship between mechanical properties and grain orientation of BM

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Table 2

Tensile strength, elongation, fracture position and photographs of tested specimens of laser beam welds under various welding conditions

The hardness distributions in the laser beam welds at the laser power of 10 kW under various welding speeds are shown in Fig. 1. The fusion lines cannot be accurately identified due to the similar microstructure formed in HAZ and FZ, and thus, they were approximately distinguished according to the location of the weld root and weld toe.5 From Fig. 1a–d, the location of the hardness analysis is shown in the top right corner, and the average hardness of the different zones is given in the top left corner. It should be pointed out that the WD was parallel to the RD in the samples for the hardness test and microstructural observation in this work. This is to say that both the indentation direction and the observation direction are parallel to the RD. The average hardness value of the BM is ∼132 HV, quite consistent with the result shown in Table 1. Additionally, the weld width gradually becomes narrower with the increase in welding speed due to the decrease in welding heat input. The average hardness in the HAZ is almost the same under various welding speeds, but the average hardness in the FZ seems to be low at slow welding speed (compared with the fast welding speeds) because of the coarse microstructures formed at the comparatively slower cooling rate. Although an obvious scatter is detected in the HAZ and FZ as well as the BM, the average hardness in the HAZ and FZ is clearly higher than that in the BM. It can also be seen from Fig. 1 that the scatter extent of hardness in HAZ and FZ is larger than that in the BM. This phenomenon may be explained according to the textures in these regions and will be discussed in the following section.

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Figure 1

Hardness distributions in laser beam welds at laser power of 10 kW under various welding speeds

Grain structure and orientation in laser beam welds

In order to investigate the relationship between the hardness and the grain orientation, several coarse grains with different orientations were chosen from the grain boundary map in the FZ at 10 kW laser power and 2 m min⁻¹ welding speed (as shown in Fig. 2, where the indentation marks within the grain C are displayed in the top left corner). The HABs (solid black line) are the α grain boundaries, while the LABs (solid grey line) depict the substructures within the α grains. It was found that the selected grains show similar grain size and have no distinct substructures. The relationships between the hardness and the grain orientation of the analysed grains are detailed in Table 3. The relatively high hardness was detected when indenting into the basal plane, whereas the low hardness was found when indenting into the prismatic plane. Thus, the significant scatter in the hardness distribution can be ascribed to the crystallographic anisotropy, which is nearly related to the hexagonal close packed structure of pure titanium. 5 16 5,16,17

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Figure 2

Grain boundary map and analysed grains of hardness test with different orientations in FZ at 10 kW laser power and 2 m min⁻¹ welding speed

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Table 3

Relationships between hardness and grain orientation of analysed grains shown in Fig. 2

The grain boundary maps of the various regions in the weld at 10 kW laser power and 4 m min⁻¹ welding speed are shown in Fig. 3 as well as a low magnification overview. Similarly, the LABs and HABs are indicated by solid grey and solid black lines respectively. As seen from the grain boundary maps in Fig. 3, the plentiful HABs and the sporadic LABs are present in the BM because of the recrystallised structure of the BM. The microstructure of the BM is a mix structure with fine and coarse grains, and the coarse grains indicate the occurrence of grain growth during the manufacturing process of pure titanium sheet. In contrast, the LABs in the HAZ and FZ are remarkably abundant, although the grain sizes in these regions are larger than that in the BM. The LABs, i.e. the substructure boundaries, are important to the strength of the welds, as discussed later.

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Figure 3

Low magnification overview and grain boundary maps of various regions in weld at 10 kW laser power and 4 m min⁻¹ welding speed

The lengths of LABs and HABs are given in Table 4, together with the grain size, hardness, (0002) pole figure and rotated (0002) pole figure of various regions in the weld. The rotated pole figures were acquired by rotating the real pole figures 90° around the ND in order to understand the textures in these regions more clearly. An evident concentration of grain orientation with the basal plane is close to the centre part of the rotated pole figure in the BM. However, the grain orientation with the basal plane is not found to concentrate near the centre part of both the real pole figures and the rotated pole figures in the HAZ and FZ. This is different from the result observed in the friction stir weld of pure titanium. 18 18,19 The texture in the BM may explain why the scatter extent of hardness in the BM is smaller than that in the HAZ and FZ. It can also be seen from Table 4 that the length of the LABs markedly increases in the HAZ and FZ, and the increased length of the LABs indicated that a large number of substructures are formed in the HAZ and FZ. The formation of substructure is mainly attributed to two aspects: the adjacent α variants inherited from one β grain may represent the close orientation due to the non-equilibrium transformation caused by fast cooling rate,20 and the neighbouring α variants transformed from different β grains possibly show the same orientation because of the close orientation between the two β grains.21

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Table 4

Grain size, hardness, lengths of LABs and HABs, (0002) pole figure and rotated (0002) pole figure of various regions in weld shown in Fig. 3

Strengthening mechanisms in laser beam welds

As shown in Table 4, the mean grain size in the BM is ∼22 μm, while the grain sizes in the HAZ and FZ are ∼43 and 52 μm respectively (i.e. grain coarsening has occurred in the weld). The coarse grains usually result in a decrease in hardness and/or strength according to the Hall–Petch relation described by the following equation (1) where σ_grain is the yield stress, σ_o is the resistance to dislocation movement, k_o is the hardening contribution from the grain boundary and d_grain is the grain diameter.

However, the results of the hardness test shown in Table 4 cannot be explained according to the grain size, suggesting that other factors cause the strengthening of the HAZ and FZ. In fact, the increment of yield stress resulting from substructure strengthening Δσ_substructure should be taken into account considering the existence of abundant substructures in the HAZ and FZ dissimilar with the BM. The substructure strengthening is associated with the fact that the substructure boundaries (LABs) act as a barrier to dislocation motion.22^– 24 Thus, the total yield stress in the HAZ σ_HAZ can be described as follows (2) Although the grain size clearly increases in the HAZ compared with the BM, the increment of yield stress produced by substructure strengthening may lead to the final increase in total yield stress in the HAZ.

However, the active titanium element will inevitably absorb a small quantity of other elements from the surrounding environment during welding, although welds with silvery white surfaces were obtained after welding. It is well known that the strength of pure titanium depends on the microelement contents, especially on the N and O elements. The microelement contents in the weld under the condition of 10 kW laser power and 4 m min⁻¹ welding speed are shown in Table 5. The microelement contents in the FZ are actually higher than those in the BM, and the increment of yield stress generated by solute solution strengthening Δσ_solute should also be taken into account in the FZ. The solute solution strengthening is mainly due to the fact that the solute atoms can impede dislocation motion by the interacting between the solute atoms and the dislocations. 24 24,25 Therefore, the total yield stress in the FZ σ_FZ can be put in the form as follows (3) Although the coarsest grain is found in the FZ, the substructure strengthening and the solute solution strengthening probably result in the highest yield stress in the FZ compared with the BM and HAZ.

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Table 5

Microelement contents in weld at 10 kW laser power and 4 m min⁻¹ welding speed

Region	Microelement contents/ppm
	C	H	O	N
BM	37	10	601	25
FZ	41	11	651	29
Method	C: combustion infrared absorption method;
	H, N: inert gas fusion thermal conductivity method;
	O: inert gas fusion infrared absorption method.