0　Introduction

Titanium and its alloys have high specific strength and excellent corrosion resistance; however, it has a high cost as the use of structural parts. The titanium steel composite plate, which has the excellent mechanical properties of steel and the corrosion resistance of titanium alloy, can save a lot of titanium material as well as reduce costs. The titanium steel composite plate produced by explosive welding method has high bonding strength, less elements diffusion and brittle compounds at the interface. After explosive welding, the composite plate experiences an explosive hardening and existing residual stress due to the plastic deformation, melting and diffusion of atoms, which affects its performance. Annealing treatment can eliminate residual stress produced by explosive welding, at the same time it will improve the plastic toughness and reduce hardness at a large extent[1]. The heat treatment process shall take full account of the thermal physical properties of the two metals to ensure its comprehensive performance. Therefore, this paper took a different heat treatment process to TA1/Q345 composite plate and the microstructure and mechanical properties are analyzed, thus provided a reference for practical Engineering application.

1　Material and heat treatment process

The test materials in this paper are TA1/Q345 composite plate, the size of Q345 steel (the supply status is controlled rolling state) is 550 mm × 280 mm × 41 mm and the size of TA1 industrial pure titanium (the supply state was annealed) is 550 mm × 280 mm × 5.5 mm. The chemical component of the two kinds of base materials are shown in Table 1 and Table 2, and their mechanical properties are shown in Table 3. The TA1/Q345 composite plate is produced by explosive welding method with anfo explosive, the detonation velocity is 2 100 m/s, and the clearance value of base plate and flyer plate was 11 mm.

The TA1/Q345 composite plate is treated by box type resistance furnace, which can be maximum heated up to 850 ℃. The recrystallization temperature of TA1 is generally at 500-650 ℃, and the recovery temperature is lower than the recrystallization temperature. The recrystallization temperature of Q345 steel is about 650 ℃, so the heat treatment process developed in this study is shown in Table 4.

Table 1　Chemical composition of TA1 (wt%)

FeCNHOOthersTi0．0230．0150．0050．0010．07≤0．4Balance

Table 2　Chemical composition of Q345 steel (wt%)

CSiMnPSFe0．150．361．40．0130．009Balance

Table 3　Mechanical properties of TA1 and Q345 steel

BasemetalTensilestrength/MPaYieldstrength/MPaDuctility(%)TA133024552．0Q34555037531．0

Table 4　Heat treatment process parametersof explosive welding

TestplatenumberHeattreatmentprocessCoolingtype1——2480℃×4hFurnacecooling3600℃×4hFurnacecooling4700℃×4hFurnacecooling

2　Test result

2.1　Residual stress analysis

In this paper, the residual stress of the composite plate after explosive welding was measured by the blind hole method. The residual stress and its direction angle are calculated as follows:

(1)

(2)

(3)

Where σ1 and σ2 are the principal stress, ε1, ε2 and ε3 are the value of release strain measured by strain gauge 1, 2 and 3 respectively, A and B are strain release coefficient, θ is the angle between the maximum principal stress and the reference axis of the 1 strain gauge in the strain gauge, E is the modulus of elasticity of the material.

Table 5 shows that the shear strength of explosive welding composite plate can reach 416MPa. After the heat treatment, the shear strength of the interface is reduced with the heat treat temperature increase. The literature show this changes is related with the diffusion of C element and formation of TiC and Fe-Ti intermetallic compound[8]. The shear fracture surface was scanned by SEM, and the shear fracture morphology of the samples under different heat treatment was shown in Fig.5. Fig.5a is the shear fracture of No.1 sample (explosion state). There are convex and concave groove distributions at fracture surface periodically, which are the fracture morphology characteristics of TA1 and Q345 welded by explosive. It is the surface debond phenomenon since the peak and trough is clearly visible. It is the wave combination, which increases the interface combination of area, so as to im-prove the bonding strength of composite plate. Fig.5c and Fig.5d are the fracture morphology of No.3 and No.4 shear sample. We can find the fracture morphology is laminar microstructure. The fracture is usually attributed to cleavage fracture with brittle fracture characteristics. It can be seen that the higher the temperature, the more obvious lamellar morphology, greater brittle fracture tendency and lower shear strength.

The mechanical properties of the composite plate are tested with reference to the GB/T 6396-2008 “Composite steel plate mechanical and process performance test” and GB/T 228-2002 “Metal material tensile test method at room temperature”.

Fig.1　Diagram of hole-drilling position

2.2　Metallographic analysis

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Fig.3 is the microstructure of the interface of TA1/Q345 explosive welding composite plate under different heat treatment processes. The interface is wavy combination, and at the crest of the front end looks like a "trunk". In explosive welding, the metal is formed in high temperature and high pressure environment, especially at the interface zone. TA1 invades into the Q345 side and takes shape of a trough shaped structure. This structure uplifts on the barrier and captures the jet, thus forming a regular “trunk” structure[3], increasing the interfacial bonding strength as well.

Fig.2　Residual stress of TA1/Q345 composite plate (a) Residual stress at explosion state (b) Residual stress after heat treatment

Fig.3　Microstructure of TA1/Q345 clad plate under different heat treatment process (a) No.1 sample (explosive state) (b) No.2 sample (480 ℃ × 4 h) (c) No.3 sample (600 ℃ × 4 h) (d) No.4 sample (700 ℃ × 4 h)

Fig.3a is the interface microstructure at explosive state of TA1/Q345 composite. The interface wavelength is about 0.83 mm and the wave height is about 0.2 mm. The base plate Q345 steel side is pearlite and ferrite bidirectional microstructure. The flyer side TA1 is bright, hard to corrode, and there are several adiabatic shear lines, which are special plastic deformation lines[4]. The microstructure at composite interface on both sides has a large deformation. Grains are elongated like fiber texture, especially near the interface and the “trunk” structure.

After 480 ℃ heat treatment in Fig.3b, the microstructure is not changed significantly compared with that of the explosive state, and the microstructure near the binding zone was fibrous, and no obvious changes were found in either side. This is because at low temperature the material only experience a recovery process, the recovery only involve the movement of point defects but have no affecting on the microstructure[5].

The metallographic specimen of the composite plate is cut along the direction of explosive detonation. The etchant solution used in TA1 is 3HF-6HNO3-91H2O mixed solution, and the corrosion solution of steel layer is 4% alcohol solution of nitric acid[2]. The corrosion samples are observed under OLYMPUS PME3 metallographic microscope.

It can be seen from Table 5 that the tensile strength of the composite plate decreases with the increase of heat treatment temperature, which is related to the release of the explosive harden during the heat treatment. The ductility of TA1/Q345 composite plate at explosive state is lower than that of other heat treat-ment process. The ductility of No.3 specimen is largest, this is because after 600 ℃ heat treatment, the microstructure have a recrystallization process. The grains are fine and the plasticity was improved.

2.3　Micro-hardness

The micro hardness test is carried out on the TUKON 2100 Vivtorinox hardness tester at load of 0.98 N, holding time of 15 s. The total of 12 points are tested, the distance between the two points is 200 μm. The distribution curves of micro-hardness (HV) results are shown in Fig.4.

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It can be seen from the graph that the micro-hardness at the interface is obviously larger than that of at the two sides, and the hardness decreases gradually from the interface to the base plate. This is because the plastic deformation at the interface is the largest, there are a number of deformation twins and dislocations at the interface, the work hardening effect is significant by the time[6-7].

Fig.4　The Vickers hardness result of TA1/Q345 clad plate

Micro-hardness at the side tissue increases as far away from the interface. The hardness of No.2samples (480 ℃ × 4 h heat treatment) is 231 HV0.1, almost has no change compared with that of No.1 sample (explosion state). After heat treatment of 600 ℃ × 4 h, the hardness decreased significantly at the interface and both sides. After 700 ℃ × 4 h heat treatment, the element diffusion area increases at high temperature. Therefore, Ti, Fe and C form an intermetallic brittle compound, resulting in a high hardness at the interface of No.4 samples.

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2.4　Mechanical properties

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The tensile test and shear test was carried out on the HT-2402 type universal hydraulic tensile testing machine. Bending test was used WE-10 hydraulic universal material testing machine. Test results are shown in Table 5.

Fig.3c and Fig.3d are the microstructure of the TA1/Q345 composite board after the heat treatment process of 600 ℃ and 700 ℃, respectively. It can be seen from the chart that after annealing at 600 ℃ and 700 ℃, the deformation microstructure disappears and the grains near the interface become bulky. The grains after 700 ℃ heat treatment are more bulky than 600 ℃. Recrystallization and grain growth occurr in the interface with the increase of heat treatment temperature. At the same time, the microstructure near the interface of Q345 steel occurs seriously decarburization phenomenon, which appears as a white tissue layer close to the interface.

Table 5　Mechanical properties of TA1/Q345 clad plate

SampleTensilestrength/MPaYieldstrength/MPaDuctility(%)Shearstrength/MPaBendingtest90°180°154445921．0416TA1crackIntact251839926．0336IntactIntact350139329．5310IntactIntact445532727．5266IntactIntact

The diagram of hole-drilling position is shown in Fig.1 and the test results are displayed in Fig.2. The results show that the residual stress in TA1/Q235 explosive welded composite plate is tensile stress state. The value of residual stress around detonation point is bigger than that of at the periphery edge of the composite plate, which is attributed to the actual boundary effect. The stress state turns into compressive stress state after the composite plate going through the anneal process and spreading with a sine wave along detonation wave propagation direction. The residual stress value after 600 ℃ heat treatment is smaller than other temperature.

Fig.5　Fracture morphology of TA1/Q345 clad plate after shear test under different heat treatment (a) No.1 sample (explosive state) (b) No.2 sample (480 ℃ × 4 h) (c) No.3 sample (600 ℃ × 4 h) (d) No.4 sample (700 ℃ × 4 h)

Fig.6 is the TA1/Q345 composite plate bending specimens after different heat treatment process. No.1 sample (explosion state) has a break in TA1 side when reach a certain excurvature bend angle, while other samples have no cracks. When take an incurvation bend test to the specimens, all samples show good bending property.

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Fig.6　The bending samples of TA1/Q345 clad plate under different heat treatment (a) The samples of out-of-plane bending (b) The samples of in-of-plane bending

3　Conclusions

(1)The residual stress at explosive state is tensile stress state. The stress state turns into compressive stress state after the anneal process and the residual stress value after 600 ℃ heat treatment is smaller than other temperatures.

(2)There is a large plastic deformation in the interface of TA1/Q345 composite plate. After annealing at 600 ℃ and 700 ℃, the deformed microstructure disappears. The grain size after 700 ℃ annealing is relatively larger than that of 600 ℃.

(3)The micro-hardness at the interface was higher than that of both sides. And the micro-hardness reduces with the increase of heat treatment temperature. However, the micro-hardness of the interface becomes abnormally large after 700 ℃ annealing, which is related to the formation of Ti, Fe and C intermediate compounds at high temperature.

变压器是由两个主要部分组成，第一是绕组，第二是铁心。这两个组成部分主要关注的是绕组，在一般情况下，绕组都是运用绝缘的铝线或铜线所制成的。与电源相连接的绕组，将其称为初级绕组或原绕组。将与负载相相连接的绕组称之为次级绕组或者是副绕组。在使用变压器的过程中可将其分成低压变压器器和升压变压器，如果低压侧绕组匝数较少，会使得导线较细。如果高压侧绕组匝的数量增多，会促使导线变粗。[2]

(4)With the increase of heat treatment temperature, the tensile strength and shear strength of the composite plate are reduced. There is a best ductility after 700 ℃ annealing. To sum up, it is considered that 600 ℃ is more suitable as a reasonable annealing temperature to TA1/Q345 explosive welding composite plate.

References

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[2]　Li Z W. Study on microstructures and properties of Titanium/mild steel interface bonded by diffusion. Dalian: Dalian Jiaotong University, 2005. (in Chinese)

[3]　Wang X X. Study on explosive welding of Rare Metals and heat treatment method. Nanjing: Nanjing University of Science & Technology, 2014. (in Chinese)

[4]　Zheng Y M. Effect of annealing on fly line structure and properties of titanium-steel explosive composite sheet. The Chinese Journal of Nonferrous Metal, 2001, 11(S1): 154-157. (in Chinese)

[5]　Pang J M, Li L M, Li M Q, at al. Effect of annealing temperature on microstructure and properties of TA1 tube. Titanium Industry Progress, 2011, 28(2): 26-28. (in Chinese)

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[7]　Nizamettin K, Behcet G. Joining of titanium/stainless steel by explosive welding and effect on interface. Journal of Materials Processing Technology, 2005, 169(2): 127-133.

[8]　Jiang H T, Yan X Q, Liu J X, at al. Effect of heat treatment on microstructure and mechanical property of Ti-steel explosive-rolling clad plate, Transaction of Nonferrous Metals Society of China. 2014, 24(3): 697-704.