Mechanical Properties of Explosion-Welded Titanium/Duplex Stainless Steel under Different Energetic Conditions

Abstract

In this study, the energy deposited at the welding interface was controlled by changing the stand-off between the flyer and base plates. Pure titanium (TP 270C) and duplex stainless steel (SUS 821L1) were welded under 5- and 15-mm stand-offs, respectively. When the stand-off was 5 mm, the average wavelength and average amplitude of the welding interface were 271 and 61 μm, respectively; at 15 mm stand-off, the average wavelength and average amplitude of the welding interface were 690 and 192 μm, respectively. The differences between the two welding conditions were compared using a tensile test, fracture analysis, a 90° bending test, Vickers hardness, and nanoindentation related to the mechanical properties of materials. The experimental results indicated that the sample with a 5-mm stand-off had better mechanical properties.

1. Introduction

Explosive welding is a processing technology that uses the energy of an explosion to make two dissimilar metal plates dissolve together [1,2,3]. Based on numerous experiments, researchers have proposed the hypothesis of an explosive weldability window [4,5,6,7]. The welding can be achieved when the welding parameters are in this window. When the parameters are outside the window, the welding may fail. Currently, it is believed that a good welding result will be obtained when the welding parameter is a little above the lower boundary of the window. Features slightly above the boundary are the formation of a small wavy interface and a small vortex area; when the parameter is raised above, the wavy interface and vortex area increase due to increased energy deposition at the interface. Many studies have been conducted on the interface morphology under different energy conditions. For example, Manikandan et al. studied explosion-welded titanium–stainless steel [8,9], Habib et al. studied titanium–magnesium alloy [10], Shi et al. studied steel–stainless steel [11], Zhou studied tungsten–copper [12], and Carvalho et al. studied aluminum–copper [13]. In our previous research, we studied the interface morphologies of explosion-welded aluminum alloy high-strength-duplex stainless steel [14,15], titanium-duplex stainless steel [16,17], and magnesium-duplex stainless steel [18] under different energy conditions.

However, there have been few studies evaluating the various mechanical properties of weldments under different energy conditions [19]. Stand-off is the distance between two plates in explosive welding. Under the same detonation velocity, changing the stand-off can change the energy deposited at the interface [17]. In this study, two welding results of pure titanium (TP 270C) and duplex stainless steel (SUS 821L1) were obtained by changing the stand-off. One was with relatively small energy (the welding parameters were near the lower limit of the weldability window) and the other was with a higher energy (the welding parameters had a certain distance above the lower limit of the weldability window). Finally, the differences in the macro- and micro-mechanical properties of weldments under the two welding conditions were compared.

2. Materials and Methods

JIS TP 270C and SUS 821L1 (austenite and ferrite) were used in the study. These two materials have very good corrosion resistance. The structural parts of titanium and stainless steel are used in aerospace engineering, chemical and petrochemical industries, heat exchangers, nuclear reactors and nuclear fuel reprocessing. The size of the plates was 200 mm (length) × 100 mm (width) × 3 mm (height). The chemical composition of the joined materials was provided in [16]. The main explosive used in the experiment was ANFO-A (a mixture of ammonium nitrate and fuel oil), with a density of approximately 530 kg/m³; the detonation velocity was approximately 2575 m/s [8]. A schematic of explosive welding is shown in Figure 1. The thickness of the explosive used in the study was 48 mm. The gaps between titanium and duplex stainless steels were set at 5 and 15 mm, respectively.

The part 15 mm from both sides was used for mechanical property testing. The lengths of the tensile and 90° bending test specimens were the same as those of the original plates. The Vickers hardness and nanoindentation samples were taken from positions about 150 mm from the initiation point and 50 mm from the side. The mechanical properties of the two weldments were comprehensively evaluated using a tensile test, 3D full-field strain measurement and analysis system, fracture analysis, 90° bending, Vickers hardness analysis and nanoindentation analysis. The wavy interface was observed by optical microscopy; fracture analysis was performed using an optical microscope (Nikon, Tokyo, Japan) and a scanning electron microscope (Zeiss, Oberkochen, Germany). After 90° bending, Vickers hardness analysis and nanoindentation analysis results were examined by optical microscopy.

3. Results and Discussion

3.1. Morphology of the Welding Interface

The sizes of the interface waves are shown in Figure 2. When the stand-off was 15 mm, the average wavelength and average amplitude of the welding interface were 271 and 61 μm, respectively; when the stand-off was 15 mm, the average wavelength and average amplitude of the welding interface were 690 and 192 μm, respectively. There was a small difference in wavelength and wave height in the same sample. Before welding, 600-grit SiC paper was used to polish the welding surface, resulting in an uneven welding surface, affecting the process of jetting, the shapes of waves and the melting zone; the different shapes of the waves and melting zone may also be related to the vibration caused by the collision.

3.2. Tensile Test

Tensile tests were conducted on the original materials and welded plates. The results are shown in Figure 3 and

Table 1. Tensile strength of samples.

Sample	TP 270C	SUS 821L1	5-1	5-2	5-3	15-1	15-2	15-3
Yield strength (MPa)	550	520	680	670	700	710	690	705
Tensile strength (MPa)	602	729	703	688	717	747	706	713

. Theoretically, the tensile strength of the composite plate should be between the strengths of the two original plates [20]. Sometimes, the tensile strength of weldments is higher than that of two original metals [21], which is due to the work hardening effect of explosive welding. Here, the strength of the composite plate was close to that of SUS 821L1. With a 15-mm stand-off, the tensile strength of both samples was slightly higher than that of the sample with a 5-mm stand-off, which was due to the more severe work hardening of the weldment with a large interfacial wave. From the perspective of stability, the tensile strength of weldments with small waves was more stable. It can be seen from Figure 3 that, compared with the original materials, the plasticity of the weldments decreased; the greater the wave, the poorer the plasticity, which was related to the degree of work hardening.

A tensile test of the composite plate is a way to evaluate the mechanical properties of explosive welding materials [22]. In this study, a 3D full-field strain measurement and analysis system (digital image correlation method) were used to record the changing process of strain during stretching. The maximum strain on the duplex stainless steel side of the sample with a 5-mm stand-off was 0.48, and no data was obtained on the titanium side due to paint falling off. The maximum strain on the duplex stainless steel side of the sample with a 15-mm stand-off was 0.42, and the maximum strain on the titanium side was 0.39. It can be seen from the data on the duplex stainless-steel side that the sample with a 5-mm stand-off had greater ability to resist plastic deformation, which is consistent with the results reflected in the tensile curve in Figure 3. The photos of the 3D full-field strain measurement and analysis process are shown in Figure 4. Figure 5 shows the samples after the tensile test.

3.3. Fracture Analysis

In Figure 6a, a fracture crack in the sample with a 15-mm stand-off was introduced into the welding interface, resulting in the separation of the welding interface. This was related to excessive residual stress and excessive melting. No obvious macro-crack was found on the welding interface of the sample with a 5-mm stand-off. According to Figure 6b, the macro-fracture of the sample with a 5-mm stand-off was inclined, and the macro-fracture of the sample with a 15-mm stand-off was concave to the welding interface.

As shown in Figure 7, the fracture of the original SUS 821L1 and TP 270C samples was full of dimples and cavities, characteristic of plastic material fracture. The cracks between the composite plates with a 15-mm stand-off were larger than those between composite plates with a 5-mm stand-off. Although both titanium and duplex stainless steel are ductile when stretched alone, the fracture of the composite plate shows that the duplex stainless-steel side remained ductile, and the fracture was full of dimples and cavities. However, the titanium side evidenced the characteristics of brittle fracture, and the fracture mainly presented a quasi-cleavage morphology.

3.4. Bending Test

A bending test is often used to evaluate the deformation resistance of explosive weldments [23,24]. In this study, the specimen was subjected to a 90° bending test. The size of the sample was 200 mm × 20 mm × 6 mm. The test results are shown in Figure 8. When stand-off equaled 5 mm, in region A, no crack appeared at the welding interface; when the stand-off equaled 15 mm, in region B, a crack appeared at the welding interface—the crack was induced by a large deformation, meaning a small wavy interface had stronger resistance to large plastic deformation.

3.5. Vickers Hardness

Results of the Vickers hardness tests are shown in Figure 9. A loading of 0.2 kgf was used to measure the hardness. The hardness of duplex stainless steel increased near the interface, which was caused by large plastic deformation. Whether there was an increase in the hardness of titanium near the interface was unclear. The hardness increase of titanium near the interface is not obvious.

3.6. Nanoindentation

A nanoindentation experiment was performed to further understand the hardness change near the explosive welding interface [25,26]. The loading was 1000 μN. The test results are shown in Figure 10 and

Table 2. Nanoindentation hardness (Unit: GPa).

Sample 1 (Stand-off = 5 mm)	No.1	No.2	No.3	No.4	No.5	No.6	No.7	No.8	No.9
	3.1	3.6	3.2	13.8	5.1	7.0	15.1	6.1	5.5
	Ti	Ti	Ti	SUS 821L1	SUS 821L1	SUS 821L1	SUS 821L1	SUS 821L1	SUS 821L1
	No.10	No.11	No.12	No.13	No.14	No.15	No.16	-	-
	7.5	4.9	6.0	6.4	4.9	4.6	5.0	-	-
	SUS 821L1	SUS 821L1	SUS 821L1	SUS 821L1	Stainless steel	SUS 821L1	SUS 821L1	-	-
Sample 2 (Stand-off = 5 mm)	No.1	No.2	No.3	No.4	No.5	No.6	No.7	No.8	No.9
	3.1	3.3	4.0	12.4	13.5	5.9	15.5	6.4	5.1
	Ti	Ti	Ti	Alloys	Alloys	Alloys	Alloys	Alloys	Alloys
	No.10	No.11	No.12	No.13	No.14	No.15	No.16	No.17	-
	13.0	14.0	14.1	4.4	3.1	6.1	5.4	5.8	-
	Alloys	Alloys	Alloys	Ti	Ti	SUS 821L1	SUS 821L1	SUS 821L1	-

. In Sample 1, the interval in the titanium was approximately 15 um, the interval in the vortex area was approximately 5 um, and the interval in the duplex stainless steel was approximately 15 um. The dot order was from left to right. The interval in Sample 2 was approximately 15 μm. There were three test points in the titanium and duplex stainless steel, respectively. Little difference was observed between the hardness values of the titanium and duplex stainless steel in the two samples, while a large difference was observed in the hardness in the vortex region. The increase in the hardness in the vortex region of Sample 1 was caused by the large plastic deformation of the duplex stainless steel. Except for Nos. 4 and 7, the values of other points in the vortex region were slightly larger than those of the duplex stainless steel. The increase in hardness in the vortex region of Sample 2 was caused by large plastic deformation and generation of the alloy phase. The wide range of hardness changes in the vortex region was due to its complex composition [16]. The maximum hardness of the vortex region of the two samples was more than four times that of titanium and 2.5 times that of duplex stainless steel.

4. Conclusions

The mechanical properties of titanium/duplex stainless steel made by explosive cladding under two energy conditions were compared, and the conclusions are as follows:

• DIC and 90° bending tests indicated that the interface with small waves had greater ability to resist plastic deformation. Overall, the mechanical properties of the small wavy interface were better.
• Little difference was found in the tensile properties of the composite plates obtained under the two welding energy conditions; the tensile stress of the large wavy interface was slightly higher. The crack propagation along the interface of the large waves after fracture was more obvious than that of the small wavy interface. The tensile fracture of pure titanium was ductile; however, the fracture of the titanium side was brittle in the composite plate.
• The hardness change near the interface was caused by large plastic deformation and alloy compounds. The hardness of the micro-area improved by only 1.5 times by large plastic deformation. Under the condition of high-energy welding, the hardness of the vortex region was complex due to the existence of many components and large plastic deformation.