Effect of pulse frequency on microstructure and ... - Springer Link

Int J Adv Manuf Technol (2013) 68:19–31 DOI 10.1007/s00170-013-4822-3

ORIGINAL ARTICLE

Effect of pulse frequency on microstructure and properties of Ti-6Al-4V by ultrahigh-frequency pulse gas tungsten arc welding Mingxuan Yang & Bojin Qi & Baoqiang Cong & Fangjun Liu & Zhou Yang

Received: 30 September 2012 / Accepted: 29 January 2013 / Published online: 16 February 2013 # Springer-Verlag London 2013

Abstract An experimental program of welding a titanium alloy, Ti-6Al-4V, was carried out using ultrahighfrequency pulse gas tungsten arc welding (UHF-GTAW). The characteristics of the welds were investigated, such as the defection, microstructure, and mechanical properties. The experimental results show that the high pulse frequency reduces the heat input from the UHF-GTAW process, while gaining one-side welding with backing as a precondition. Basketweave and long acicular a′ martensite only existed in some areas of the FZ (fusion zone) with both low uniformity and distribution density (8.4 %) as a result of conventional GTAW processes. With a pulsed current, basketweave and short acicular a′ martensite were distributed in the FZ evenly. Short acicular a′ martensite could be detected within the parallel long acicular a′ martensite in the CGR when f >30 kHz. Plastic weld joints were characterized by both the elongation, A, and the percentage of the area reduction, Z, and were optimized with a high pulse frequency. Ideal mechanical properties of the joints were achieved with an A of 68 % and a Z of 150 % with f=30 kHz. The integrated effects of the pulse frequency and the heat input are the key factors for determining the microstructure morphology and the mechanical properties.

M. Yang : B. Qi : B. Cong (*) : F. Liu : Z. Yang School of Mechanical Engineering and Automation, Beihang University, Xueyuan Road No.37, Haidian District, Beijing 100191, China e-mail: [email protected]

Keyword Ti-6Al-4V . Ultrahigh-frequency pulse GTAW . Microstructure . Mechanical properties

1 Introduction Titanium alloy is widely used in the aeronautics industry, the astronautics industry, vehicle engineering, and other such fields due to its plasticity, toughness, low density, high strength-to-weight ratio, desirable welding properties, corrosion resistance, and fatigue resistance [1–4]. Ti-6Al-4V occupies more than 50 % of the applications in the field. Compared with other models, the Ti-6Al-4V alloy and its weldments, with a mixed a- and β-phase microstructure, have a higher strength and microhardness [5]. There would be obvious grain growth if the problem of overheating in the welds and the HAZ was realized [6]. A large heat input during the welding would result in appreciable β grain growth in the HAZ, which is directly adjacent to the weld fusion plane where peak temperatures ranged between solidus and β-phase transition [7]. Poor weld metal ductility of the Ti-6Al-4V from the gas tungsten arc welding (GTAW) process was attributed to larger β grain sizes and large heat inputs, which are crucial to the grain size [8]. This indicates that control of the grain size or the microstructure morphology is the key for improving the quality of the welds. The most important parameter of the microstructure is the a colony size for a+β titanium alloys, which determines the maximum slip length of dislocations. By decreasing the a colony size with thermo-mechanical

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Int J Adv Manuf Technol (2013) 68:19–31

processing, the yield strength, the ductility, and the fatigue crack nucleation resistance were improved [9]. The weld fusion zone of the a+β titanium alloys are known to have poor ductility because of the acicular a′ martensite distribution in the intragranular microstructure and a large β grain size caused by conventional direct current arc welding. Pulsed current GTAW developed in 1950s. In contrast to conventional GTAW, heat energy required to melt the base material is supplied only during peak current pulses for brief intervals of time allows the heat to dissipate into the base material leading to a narrower heat-affected zone (HAZ) [10]. Some scholars indicated that a pulsed current at low frequencies for a GTAW process of titanium alloys would not result in measurable grain refinement [11, 12]. However, N. Kishore Babu [13] indicated that the heat energy required to melt the base metal was supplied only during peak pulsed current for brief intervals; it then dissipated into the base metal during the off-peak background current via the pulsed current GTAW process, which could result in a narrower HAZ [14]. The experimental results showed that pulsed current welding could enhance the convective forces in the molten pool, impacting the refinement of the β grain structure. Some others [15, 16] found that frequency of inverter welding power source had a significant impact on the refinement of weld grain. The higher pulse frequency, the better effect on grain refinement. There were also other results confirming that the pulsed current GTAW process refined the fusion zone microstructure [17] by influencing weld pool solidification [18–20]. Several results indicated that a reduction in the β grain size improved the hardness, strength, and ductility in the as-welded condition via pulsed current welding. By [17], the ductility cannot be improved under usual welding conditions by merely altering the heat input, and post-weld heat treatment thus becomes necessary to improve the weld metal ductility to an acceptable level. Some study indicated that post-weld heat treatment at a temperature below the β-transus is often necessary [21]. But pulsed welds showed greater tensile elongation even in the heat-treated condition with [17]. The paper

Table 1 Chemical composition of Ti-6Al-4V plate

Fig. 1 Schematic of pulse and background currents

focused on the effect of pulse welding process on microstructure of Ti-6Al-4V alloy, and would not refer to comparison between as-weld and post-weld heat treatment. The ultrahigh-frequency pulse gas tungsten arc welding (UHF-GTAW) experiment was carried out using a Ti-6Al-4V titanium alloy. The weld appearance, microstructure, and joint mechanical properties were studied in order to explore the correlation between them and the pulse frequency, which would in turn optimize the microstructure morphology and the mechanical properties.

2 Experimental method Ti-6Al-4V titanium alloy was used as a workpiece with dimensions of 200×80×1.5 mm. The chemical composition (wt.%) of base material is shown in Table 1. The UHF-GTAW process is a novel welding technology with a current switch frequency of 10∼50 kHz and a current upslope/downslope rate of di/dt≥50 A/μs. The schematic of the weld current from the experiment is illustrated in Fig. 1. In the figure, Ib/Ip is the background/pulsed current and f the pulse frequency. The times of the background and pulsed currents are tb and tp, respectively, and the duty cycle of the pulse duration is deduced to be d ¼ tp tb þ tp . The parameters of the pulsed current are shown in Table 2. The 160-mm length welds were obtained along the welding direction without a wire. The parameters were as follows: the electrode was 2.4 mm in size and made of 2 % cerium and 98 % tungsten; the distance

Al

V

N

C

H

O

Fe

Ti

5.82

3.99

0.023

1.83

0.0007

0.063

30 kHz, the reduction of ΔQUHF was 20 kJ/m. All of the above caused large differences in the microstructure and the properties of the weld joints between the conventional and UHF-GTAW processes. According to the results of Ahmed and Elmer, a smaller heat input during the UHF-GTAW process led to gain refinement and better joint properties.

3.1 Heat input of the UHF-GTAW process

3.2 Pore sensitivity

Using Eq. 2, the background/pulsed arc voltages were used to calculate the heat input. The waveform of the voltage and the current is illustrated in Fig. 5.

Previous work showed that pore sensitivity of welds from the conventional GTAW process was higher than that of the UHF-GTAW process [25]. The conventional GTAW

voltage, background/pulsed current, duration, and welding speed. Heat input was calculated with integration of time in one cycle T. There is correlation between cycle T and background/pulse time tb/tp which could be expressed by T=tb +tp and δ=tp/(tb +tp). So time factor T, tb, and tp could be divided as shown by Eq. 2. Z T 1 ut it dt vTZ 0 Z tp tb 1 ¼η Ub Ib dt þ Up Ip dt vT 0 tb tp 1 tb ¼η Ub Ib þ Up Ip v tb þ tp tb þ tp 1 ¼ η ð1 d Þ Ub Ib þ d Up Ip v

QUHF ¼ η

Fig. 4 Specimen dimensions for tensile tests

ð2Þ


23

I

a

U Weld surface

b Fig. 5 Waveform of voltage and current by UHF-GTAW process, where 1—welding current and 2—arc voltage (f=20 kHz)

process more easily forms pores without storage in a drycleaning utensil. Hydrogen is the most important factor in creating weld gas pores for Ti-6Al-4V titanium alloys [26, 27]. With the same pre-weld surface contamination, X-ray detection was shown in Fig. 7. There are several independent gas pores distributing at both sides of weld toe by conventional GTAW process. The solubility of hydrogen in titanium decreases with larger temperature, which would cause hydrogen spread to edge of pool as low temperature. Thus, gas pore could be more found at weld toe. By contrast, no gas pores could be found by UHFGTAW process. In addition, all the specimens for mechanical test were eligible by X-ray detection. The gas bubble generation is divided into three stages: nucleation, growth, and escape. In welding, a large number of ready-made surfaces exist that would promote nucleation; after transitory growth in a molten pool, the gas bubble would escape to the surface of the pool. If the escape speed (as shown by Eq. 3) is larger than the crystallization rate, the generation of pores is lower. v¼

2 ðρ1 ρ2 Þgr2 9 η

Table 3 Background and pulse arc voltages and heat input

ð3Þ

No. of gourp 1 2 3 4 5

Weld back

Fig. 6 Weld appearance (f=20 kHz, δ=50 %) a weld surface, b weld back

where, v escape speed, ρ1 density of liquid, ρ2 density of gas, g acceleration of gravity, r radius of gas bubble, η liquid viscosity. The ability of a gas bubble to escape a ready-made surface depends on the surface tension of the liquid metal, the gas phase and the ready-made surface, which could be characterized by the soakage angle, θ (as shown by Eq. 4). The possibility of gas escape increases with decreasing θ. cos θ ¼

σ1;g σ1;2 σ2;g

ð4Þ

Previous results showed that the radius, r, increases as the soakage angle, θ, decreases with the UHF-GTAW process, which reduces the pore sensitivity of Ti-6Al-4V titanium alloys by [22]. The main reason for this is the effect of the arc force of the pulsed current on the increased mobility of the molten pool [28].

Ib/A

Ip/A

f/kHz

Ub/V

Up/V

QUHF/kJm−1

90 45 45 45 45

– 115 115 115 115

– 10 20 30 40

10 5.4 5.3 5.6 5.9

0 12.5 12.4 10.7 10.4

202.5 189.3 187.8 167.2 163.9

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a

a

Conventional GTAW process

b

Conventional GTAW process UHF-GTAW process ( f=10 kHz, =50%)

b

Fig. 7 X-ray detection a conventional GTAW process, b UHF-GTAW process (f=10 kHz, δ=50 %)

3.3 Base metal microstructure The optical microstructure of the base material, as shown in Fig. 8, consists of a grains with areas of the β phase at the grain boundaries; the average grain size of the base metal is approximately 5 μm. UHF-GTAW process ( f=10 kHz, =50%)

3.4 Microstructure of the as-weld condition After the thermal cycle of the GTAW process, grain growth of the Ti-6Al-4V was significant, as shown in Fig. 9. The microstructure of the welds is divided into the following regions: fusion zone (FZ), coarse-grained region (CGR), fine-grained region (FGR), transition zone (TZ), and base metal (BM). Obviously, the CGR

c

UHF-GTAW process ( f=40 kHz, =50%)

Fig. 9 Microstructure morphology of joints (×25). a Conventional GTAW process, b UHF-GTAW process (f = 10 kHz, δ = 50 %), c UHF-GTAW process (f=40 kHz, δ=50 %)

Fig. 8 Microstructure of base metal

and the FGR are considered to comprise the heataffected zone.

Int J Adv Manuf Technol (2013) 68:19–31 Table 4 Measurement of average intercept of FZ

No. of group

25

Average intercept/mm

1 2 3 4 5

Reducing rate/% (compared by no. 1) – 6.2 28.6 14.3 24.8

0.210 0.197 0.156 0.189 0.158

a Comparing the microstructure morphology of the UHF-GTAW process with the conventional GTAW process in Fig. 9, the FZ decreased in size by UHF-GTAW

a


b Conventional GTAW process

b



c

c


Fig. 10 Image processing for distribution density of FZ (×25). a Conventional GTAW process, b UHF-GTAW process (f= 10 kHz, δ=50 %), c UHF-GTAW process (f=40 kHz, δ=50 %)


Fig. 11 Microstructure morphology of FZ (×200). a Conventional GTAW process, b UHF-GTAW process (f = 10 kHz, δ = 50 %), c UHF-GTAW process (f=40 kHz, δ=50 %)

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process that could be confirmed from previous work [29], while the CGR, FGR, and the TZ remained approximately the same. The scale of the FZ decreased with increasing pulse frequency. By intercept measurement mentioned in Section 2, the grain size of FZ could be shown by average intercept in Table 4. The results indicated that the grain size tended to reduce by increasing pulse frequency in FZ, the maximum reached to 28.6 % when f=20 kHz. However, the difference of grain size from macrostructure was not obvious enough to comparing UHF-GTAW with conventional GTAW, which made further study on microstructure morphology more necessary.

to those in the FZ of UHF-GTAW. However, parallel acicular a′ was distributed regularly and the growth of secondary acicular a′ was limited. As a result, basketweave only existed in a small portion of the FZ and had both low uniformity and distributed density. To measure distributed density of such area with basketweave, and long/short acicular a′, image processing software could be used with FZ × 25, and then density of such morphology could be calculated as that shown by Fig. 10. By conventional GTAW process, distributed density of basketweave in FZ could be known at about 8.4 %. From Fig. 11b and c, β grain size is smaller than that of the conventional GTAW process by at least 14.3 %. The a′ phase in the FZ was a short acicular martensite, which was less distributed than the more significant basketweave that owned a larger distribution density at 39.3 and 42.3 %, respectively. Based on the analysis in Section 3.1, the heat input decreases with an increasing pulse frequency, slightly reducing the grain size by 6.2∼28.6 %. When the pulse frequency f=10 kHz, the microstructure displayed a majority of short acicular a′ distributed evenly. For an f up to 30 kHz, basketweave caused by short acicular a′ is present as along with parallel acicular a′; short acicular a′ appeared intermittently. The grain types were distributed alternately between the basketweave (short acicular a′) and long acicular a′. This phenomenon is also present when f=40 kHz.

3.4.1 Fusion zone

3.4.2 Heat-affected zone

The microstructure morphology is shown in Fig. 11. Figure 11a illustrates that the β grains are not comparable

For the GTAW process on Ti-6Al-4V titanium alloys, HAZ is composed of the coarse-grained region and the finegrained region. Figure 12 shows the microstructure of the CGR from the UHF-GTAW process. When the pulse frequency f>30 kHz, a large distributed density of long acicular a′ martensite could be found in the CGR by measurement at 59.8 %. Zooming into the acicular a′ portion of Fig. 12, it shows that short acicular a′ is dispersed among the parallel long acicular a′ as a crossing distribution (Fig. 13). The microstructure morphology of the CGR composed of long and short acicular a′ is promising for joint mechanical properties. The microstructure morphology of the FGR is shown in Fig. 14. In this region, the difference of the microstructure between the conventional and UHF-GTAW processes was not significant in terms of the grain size, distribution of acicular a′, and so on.

Fig. 12 Microstructure of CGR (×200, f=30 kHz)

Fig. 13 Microstructure of CGR (×500, f=30 kHz)

Int J Adv Manuf Technol (2013) 68:19–31 Fig. 14 Microstructure morphology of FGR (×100). a Conventional GTAW process, b UHF-GTAW process (f=40 kHz, δ=50 %)

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a

b


3.5 Mechanical properties 3.5.1 Microhardness Following the method discussed in Section 1, the microhardness of the welds was measured. The result of the cross-section of the welds is shown in Fig. 15, where d is the distance from the measurement point to the weld center. Figure 15 shows that the average microhardness in the welds was more than 340 HV. The peak value was in the CGR for the conventional GTAW process. The fluctuation of the microhardness in the welds was small, except for f=10 kHz, which had a higher microhardness in the CGR and the FGR for the UHF-GTAW process. When f=30 kHz, the same value of hardness was found in the FZ and increased for the CGR. Groups no. 1 and 2 in Table 2 gained bigger microhardness in CGR than other regions in the welds. The

Fig. 15 Microhardness of weld cross-section


hardness in the welds was at least 30 HV more than the hardness in the BM. By the method that was described in Fig. 3, the microhardness gradient could be illustrated in Fig. 16, where d again means the distance between the test point and the weld center. Microhardness gradient means the difference between the points nearby, which could show uniformity of microstructure. It can be concluded that the fluctuation of the microhardness gradient decreases with an increasing pulse frequency. When f > 30 kHz, the uniformity of the weld microstructure improved significantly; the trend of the microhardness became exceptionally stable. 3.5.2 Tensile test The method discussed in Section 1 was used for the tensile tests; the results are shown in and Fig. 17. Figure 18 shows the tensile specimens. The tensile strength maintained a relatively stable state with both the conventional and UHF-GTAW processes and was approximately equal to that of the BM. The important symbols of the plastic weld joints were measured: elongation, A, and the percentage of the reduction of area, Z. Compared with the conventional GTAW process, A increased by more than 32 % and Z increased by more than 60 % from the UHF-GTAW process. The elongation was stable with the changing pulse frequency; however, the area reduction percentage increased in large increments with increasing f below 20 kHz and achieved a balance in the range of 20∼40 kHz. The fracture location of conventional GTAW was in FZ, which owned large microhardness when distance was more than 1 mm in Fig. 15. With

28


Fig. 16 Microhardness gradient. a Conventional GTAW, b f=10 kHz, c f=20 kHz, d f=30 kHz, e f=40 kHz

10 kHz pulse frequency, the area of high hardness was transferred at more than 2.5 mm that could be recognized in CGR by Fig. 9b, where fracture was found. For others, fracture location approximately at the interface between the FZ and the CGR. When the pulse frequency, f, reached 30∼40 kHz, the plastic weld joints were optimized for both the elongation and percentage of the reduction of the area. When the pulse frequency f=30 kHz, the best mechanical properties were achieved with an A of 68 % and a Z of 150 %. This can also be confirmed by the microstructure shown in Figs. 11 and 14. With the effect of the highfrequency pulse current, the heat input declined step-bystep. As a result, basketweave and short acicular a′ martensite were found to distribute with uniformity in the FZ, which was illustrated in Figs. 11b and 9c. And the former could decrease the possibility of crack propagation due to microstructure-induced crack deflection, branching, and closure [30]. For a frequency up to 30 kHz, long acicular a′ martensite with a large density of 59.8 % in CGR was

found. With increasing frequency, short acicular a′ martensite was detected among the parallel long acicular a′ martensite that also increased the difficulty of crack propagation in β grain as the crack has a tendency to propagate in a straighter manner in a grains and along a/β boundaries [31]. Based on the results of the mechanical properties tests, the microstructure morphology of basketweave, short acicular a′ in the FZ, and parallel long acicular a′ with short acicular a′ in the CGR would maintain the tensile strength and enhance the plastic welds from the UHF-GTAW process, especially for a pulse frequency of 30–40 kHz. The correlation between the heat input and the area reduction percentage is shown in Fig. 19. As described in Sections 3.1 and 3.5, the heat input, QUHF, decreased as the Z maintained a stable trend while the pulse frequency changed from 20–40 kHz. Analyzing this phenomenon indicated that, in the region of 20–40 kHz, increasing the frequency did not result in the same trend for the plastic welds with the best match point at f=30 kHz, which implied that the


a

29

b

Tensile strength

Elongation

c

Percentage of the reduction of area Fig. 17 Mechanical properties of joint. a Tensile strength, b elongation, c percentage of the reduction of area

pulse frequency affected the mechanical properties of the welds more than the heat input. The reduction of the pulse time interval of the current (half of cycle time T) from 25 to 12.5 μs may be the reason for this. With such a short time interval, the pulsation of the macrocurrent was weakened, which was detrimental to the microstructure morphology and the plastic welds, similar to conventional direct current arc welding. Therefore, weld properties are not enhanced with a lower heat input in all frequencies, although that could affect the microstructure morphology. The integrated effect of the pulse frequency and the heat input are the main reason

for reduce of grain size and desirable mechanical properties.

4 Conclusions 1. Compared to the conventional GTAW process, the heat input decreased overall with the UHF-GTAW process and decreased with increasing pulse frequency. 2. As an effect of the pulse frequency, basketweave and short acicular a′ martensite distributed in the FZ evenly. Short acicular a′ martensite was detected

30


Fig. 18 Tensile specimens

among the parallel long acicular a′ martensite in the CGR when f>30 kHz. 3. Comprehensive and optimized mechanical properties were achieved with an A rising rate of 68 % and a Z of 150 % with f=30 kHz.

4. The integrated effects of the pulse frequency and the heat input are the key reason for reduce of grain size and desirable mechanical properties. Acknowledgments This work was supported by the National Natural Science Foundation of China under grant nos. 50975015 and 51005011. The authors acknowledge Beihang University for supporting our research. All forms of support are greatly appreciated.

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Fig. 19 Correlation between pulse frequency and technique parameters

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