1. Introduction
Demand for aluminum alloys has steadily increased in industries that require lightweight and high-strength structures, such as aircraft, automobiles, and bridges1-3) .In particular, 7000 series aluminum alloys have been widely used as high-performance structural materials due to their low density, excellent corrosion resistance, and high mechanical strength. Among them, the AL-DGFVE232B-T6 alloy is a high-strength Al-Zn-Mg-Cu alloy. It is highly applicable to high-reliability structures (e.g., aircraft fuselages, vehicle frames, and bridge members) due to its high strength-to-weight ratio4,5).
7000 series aluminum alloys are strengthened mainly through the precipitation hardening mechanism. The mechanism improves mechanical strength by inhibiting dislocation movement through the formation of η(MgZn₂) precipitates during the aging process6,7). Such precipitates, however, are sensitive to heat, and may degrade the mechanical performance of the substrate as they are easily dissolved or redistributed when exposed to high temperatures during the welding process8).
The fusion zone (FZ) is composed of the ER5556A filler wire, a 5000 series Al-Mg alloy. It is a non-precipitation hardened alloy for which strength is secured mainly through solid solution strengthening9,10). On the other hand, AL-DGFVE232B-T6, which is the substrate, is a 7000 series alloy that achieves strength through precipitation hardening. The two materials are different in terms of heat treatment response as well as hardening mechanisms.
Therefore, the weld metal, heat-affected zone (HAZ), and substrate may exhibit different mechanical changes and microstructural responses even under the same heat treatment conditions. This should be considered during the design of heat treatment conditions for FZ strength recovery11).
In the metal inert gas (MIG) welding process, relatively high heat input can be applied depending on the process conditions, which may cause structural softening and degradation of mechanical properties in HAZ and FZ12,13). The width of HAZ is determined by the actual heat input rather than the welding method. For MIG welding, the formation and characteristics of HAZ may also significantly vary depending on process variables, such as the welding current, voltage, and feed rate14). For this reason, in the case of the FZ of MIG welding performed under relatively high heat input conditions, post-weld heat treatment to restore degraded mechanical properties and stabilize the structure is considered an important process15). In this study, 8mm-thick AL-DGFVE232B-T6 alloy plates were subjected to MIG welding in the X and Y groove shapes, and four artificial aging conditions and one natural aging condition were applied.
The degree of mechanical performance recovery was quantitatively assessed through tensile strength and hardness tests, and changes in microstructure were analyzed using optical microscopy (OM). Based on this, an attempt was made to investigate mechanical and structural responses in the FZ of the substrate and weld metal with different strengthening mechanisms according to the heat treatment conditions and to derive the most effective heat treatment conditions for FZ strength recovery.
2. Test Method
2.1 Specimen preparation
MIG welding that applies X-groove and Y-groove shapes was used to perform butt welding for 8mm-thick AL-DGFVE232B-T6 alloy plates. Specimens were prepared after removing weld beads in accordance with the standard as shown in Fig. 1. Since aluminum alloys have high thermal conductivity and electrical conductivity, structural softening and mechanical property degradation may easily occur in HAZ during welding. To minimize this, process conditions were set so that the welding heat input defined by the current, voltage, and welding speed could remain low. The welding conditions applied in this study are based on the process conditions provided by industries, and heat input was calculated based on them. The welding conditions of each groove are summarized in Tables 1 and 2 while the overview and specifications of the geometry are presented in Fig. 2.
Table 1
MIG welding condition of X-groove
Table 2
MIG welding condition of Y-groove
2.2 Experimental conditions
In this study, heat treatment conditions were set by referring to previous studies on heat treatment for 7000 series alloy welds to evaluate the mechanical properties of AL DGFVE232B-T6 alloy welds16,17).
In this study, aging conditions were divided into non-aging (Non), natural aging (Na), and artificial aging. Non-aging is a reference condition that does not apply aging treatment after welding. Natural aging and artificial aging are natural and artificial aging conditions, respectively. In this study, changes in mechanical performance (recovery or degradation) due to the application of aging were compared based on non-aging.
Table 3 shows basic experimental conditions. [A] indicates the substrate and [B] the condition after MIG welding (after welding). The non-aging condition was applied for [B]. The basic experiment aimed to quantitatively examine the level of mechanical performance degradation immediately after welding (non-aging) compared to the substrate.
Table 3
Basic experimental conditions of specimens A, B and C
| No. | Specimen type | Condition |
|---|---|---|
| A | Substrate | AL-DGFVE232B-T6 alloy |
| B | After welding | MIG welding |
Table 4 summarizes the aging conditions of the weld. Natural aging is the condition of three months at room temperature, and artificial aging consists of cases 1 to 7. Case 1(120℃·24h) is a single-step aging condition while cases 2 to 7 are two-step artificial aging conditions. After applying step 1 (100℃·8 h) in the same manner, the temperature (150℃ or 200℃) and time (12 to 48h) were varied at step 2 for comparison. A natural aging (three months at room temperature) condition was also included as a comparison group.
3. Experiment Results
3.1 Hardness test
The Vickers hardness (HV) test was conducted to compare the hardness distribution in AL-DGFVE232B-T6 alloy welds according to the heat treatment conditions. The hardness was measured at seven points, which were located at 3mm intervals within the ±9 mm range on the left and right sides based on the weld center (0 mm). The testing power was 4.9 N and its duration was 10s. The hardness test was conducted three times at each measurement position, and the average value was used. Figs. 3 to 5 show the hardness distribution in the X-groove welds by heat treatment condition and Figs. 6 to 8 the hardness distribution in the Y-groove welds by heat treatment condition. Tables 5 to 10 summarizes the hardness values measured under each condition. The hardness was lowest at the weld center (0 mm) under most conditions and it increased towards HAZ as the distance from the center increased, resulting in the V-shaped distribution. It appears that the presence or reprecipitation of some precipitates in HAZ contributed to the result while the precipitation hardening effect was reduced in FZ by the welding heat cycle. In particular, softening at the weld center was relatively evident under the non-aging and natural aging conditions, and the degree of hardness recovery varied depending on the heat treatment conditions under the application of the artificial aging conditions18,19).
Table 5
Hardness values of X-groove welds (Non, Na, 120℃·24h)
| mm＼HV | Non | Na | 120℃·24h |
|---|---|---|---|
| 9 | 116 | 133 | 142.5 |
| 6 | 112.5 | 136.5 | 138.5 |
| 3 | 102.5 | 104.5 | 94.75 |
| 0 | 77.9 | 104.5 | 85.9 |
| -3 | 103.5 | 102 | 100.6 |
| -6 | 114.5 | 135.5 | 134.5 |
| -9 | 119.5 | 133 | 138 |
Table 6
Hardness values of X-groove welds (150℃ ·12, 24, 36, 48h)
Table 7
Hardness values of X-groove welds (200℃·36, 48h)
| mm＼HV | 200℃·36h | 200℃·48h |
|---|---|---|
| 9 | 89.8 | 89.7 |
| 6 | 90.65 | 93.8 |
| 3 | 87.15 | 92.62 |
| 0 | 91.54 | 98.2 |
| -3 | 88.1 | 94.6 |
| -6 | 91.15 | 92.4 |
| -9 | 88.37 | 92.55 |
Table 8
Hardness values of Y-groove welds (Non, Na, 120℃·24h)
| mm＼HV | Non | Na | 120℃·24h |
|---|---|---|---|
| 9 | 111.5 | 138.5 | 136 |
| 6 | 111 | 127.5 | 135 |
| 3 | 78.3 | 103 | 94.45 |
| 0 | 75.65 | 101.4 | 94 |
| -3 | 86.85 | 111.5 | 107.5 |
| -6 | 89.05 | 127.5 | 133 |
| -9 | 109 | 148 | 139 |
Table 9
Hardness values of Y-groove welds (150℃·12, 24, 36, 48h)
Table 10
Hardness values of Y-groove welds (200℃ ·36, 48h)
| mm＼HV | 200℃·36h | 200℃·48h |
|---|---|---|
| 9 | 100.91 | 98.9 |
| 6 | 101.69 | 98.6 |
| 3 | 102.5 | 98.3 |
| 0 | 97.35 | 94.7 |
| -3 | 104.7 | 98 |
| -6 | 103.7 | 98.5 |
| -9 | 100.7 | 99.12 |
Among the artificial aging conditions, case 1 (120℃· 24h) is T6, a heat treatment condition for 700 series alloys. Under the case 2 to 5 (150℃) conditions, the hardness at the weld center increased compared to non-aging, thereby confirming the strength recovery effect. Under the 200℃ conditions, however, the hardness recovery effect was limited or rather tended to decrease. It appears that the hardness decreased because precipitates were coarsened due to the over-aging caused by long-term exposure to high temperature. The increased uniformity of the precipitate distribution led to hardness recovery at the weld center, which is qualitatively consistent with the tensile strength recovery tendency observed from the subsequent tensile test20,21). The same tendency is also observed from the average hardness values for each condition summarized in Tables 5 to 10.
When hardness levels by groove shape were compared, X-groove generally exhibited higher hardness levels than Y-groove. For X-groove, relatively high hardness recovery was observed under the 150℃·36h condition while the lowest hardness level occurred under the 200℃·48h condition. In the case of Y-groove, hardness recovery also occurred under the case 2 to 5 (150℃) conditions, but the hardness significantly decreased under the 200℃·48h condition. These results indicate that the degree of softening and the hardness recovery behavior for the weld may vary depending on heat treatment conditions and groove geometry22).
3.2 Tensile test
The tensile test was conducted to evaluate the mechanical behavior of AL-DGFVE232B-T6 alloy welds according to the heat treatment conditions. The test speed was 3 mm/min. To ensure the reliability of the specimens, the test was conducted by preparing three specimens for each condition. Average tensile strengths are summarized in Tables 11 and 12. Among all conditions, the 200°C conditions were excluded from the tensile test because significant softening caused by over-aging was observed from the hardness test. The test results revealed that fractures occurred near the interface between FZ and HAZ as shown in Fig. 9.
Table 11
Tensile strength of X-groove welds
| Condition | Tensile strength(MPa) |
|---|---|
| Non | 303.1 |
| Na | 309.5 |
| 120℃·24h | 284.1 |
| 150℃·12h | 294.5 |
| 150℃·24h | 292.8 |
| 150℃·36h | 318.8 |
| 150℃·48h | 302.8 |
Table 12
Tensile strength of Y-groove welds
| Condition | Tensile strength(MPa) |
|---|---|
| Non | 288.4 |
| Na | 309.5 |
| 120℃·24h | 282.6 |
| 150℃·12h | 299.1 |
| 150℃·24h | 307.4 |
| 150℃·36h | 303.5 |
| 150℃·48h | 305.1 |
When the tensile strengths of each condition were compared, the results were found to be 431 MPa for the substrate, 303.1 MPa (approximately 29.7% reduction) after X-groove welding (non-aging), and 288.4 MPa (approximately 33.1% reduction) after Y-groove welding (non-aging). The tensile strength after welding significantly decreased compared to the substrate. This appears to be due to the reduced precipitation hardening effect and microstructural changes in FZ and HAZ.
Fig. 10 and Fig. 11 show the tensile strength results of the X-groove and Y-groove welds according to the heat treatment conditions, respectively. All aging conditions exhibited lower strength than the substrate, but the strength recovery effect compared to the non-aging condition was observed.
For X-groove, the 150℃·36h condition exhibited the highest tensile strength recovery tendency among all artificial aging conditions. In the case of Y-groove, the 150℃·24h conditions showed relatively superior strength recovery characteristics among all artificial aging conditions.
Fig. 12 and Fig. 13 show the correlation between tensile strength and hardness (measured at 0 mm) under different heat treatment conditions for the X-groove and Y-groove welds, respectively. For X-groove, the tensile strength and hardness showed similar tendencies and exhibited the maximum values at 150℃·36h. In the case of Y-groove, strength recovery was also observed under the case 2 to 5 (150℃) conditions. As the aging time increased, however, the strength and hardness did not always increase and they decreased under some conditions. This appears to be due to the combination of changes in the size and distribution of precipitates and non-uniform precipitation behavior in FZ and HAZ.
3.3 Structural analysis and component analysis
To examine changes in the microstructure of AL-DGFVE232B-T6 alloy welds under different heat treatment conditions, observations were performed with focus on FZ and HAZ where fractures occurred during the tensile test. A total of nine specimens were prepared, including the non-aging and natural aging conditions as well as the main heat treatment conditions presented in Table 5 (cases 1 to 7). After cross-sectional polishing and etching, the precipitate distribution and gains were analyzed using OM (200 and 500 magnification). Fig. 14 to 19 show the microstructures of the X-groove and Y-groove welds under different heat treatment conditions, respectively.
Non-uniform distribution of precipitates was observed from both X-groove and Y-groove under the 120℃·24h condition. Under this condition, distribution characteristics that result from insufficient aging were also observed. Under the 150℃ conditions, however, precipitates were more uniformly distributed as the aging time increased. In particular, relatively uniform microstructures were observed under the 150℃·24h and 150℃·36h conditions of X-groove and the 150℃·24h condition of Y-groove.
The coarsening and agglomeration of precipitates were observed under the 200℃·36h and 200℃·48h conditions. Under these conditions, hardness and tensile strength tended to decrease. A coarsening tendency was observed under the high-temperature and long-term aging conditions. Thus, these conditions can be interpreted as conditions in which over-aging behavior becomes dominant.
In addition, EDS component analysis was conducted on the interface to examine the distribution of Mg and Zn, which are the key factors of precipitation hardening. The analysis conditions included a spot size of 3, 30 kV, and an analysis time of 300s. The Mg and Zn contents are summarized in Tables 13 and 14. Relatively high Mg and Zn contents were detected at 150℃·36h for X-groove and 150℃·24 h for Y-groove, which coincided with the conditions that exhibited the uniform distribution of precipitates. This indicates that Mg and Zn contents are related to hardness and tensile strength recovery. Under the long-term conditions at 200℃, on the other hand, hardness and strength tended to decrease due to the reduced strengthening effect caused by precipitate coarsening and agglomeration. Therefore, changes in mechanical properties under different heat treatment conditions were explained based on the microstructural observation and EDS analysis results.
4. Conclusions
In this study, various heat treatment conditions were applied to the X-groove and Y-groove welds of the AL-DGFVE232B-T6 alloy to compare and analyze changes in mechanical properties (tensile strength and hardness) and microstructure. The main conclusions are as follows.
1) Mechanical property degradation after welding
Immediately after welding, the tensile strength decreased by approximately 128 MPa (29.7%) to 303.1 MPa for the X-groove specimens and by approximately 143 MPa (33.1%) to 288.4 MPa for the Y-groove specimens compared to the substrate (431 MPa). This reduction in strength was accompanied by microstructural changes observed after welding.
2) Strength recovery effect by heat treatment
For X-groove, a tensile strength recovery of approximately 15.7 MPa (5.2%) compared to non-aging was observed under the 150℃·36h condition among the artificial aging conditions as 318.8 MPa was measured. In the case of Y-groove, a tensile strength recovery of approximately 19.0 MPa (6.6%) compared to non-aging was observed under the 150℃·24h condition as 307.4 MPa was measured. Under these conditions, relatively high strength recovery tendencies were observed among the artificial aging conditions.
3) Effects of high-temperature and long-term aging conditions
Under the aging conditions of 200℃ or higher, the coarsening or local distribution of precipitates was observed as the aging time increased. Under these conditions, hardness and tensile strength tended to decrease. In particular, the Y-groove specimens exhibited a relatively larger softened zone compared to X-groove.
4) Microstructure and component analysis results
While relatively uniform microstructure distribution was observed under the aging conditions near 150℃, precipitate coarsening and local concentration were observed under the high-temperature (200℃ or higher) and long-term conditions. In addition, the EDS analysis results revealed relatively high Mg and Zn contents under the 150℃·36h condition of X-groove and the 150℃·24h condition of Y-groove. Under these conditions, hardness and tensile strength recovery tendencies were also observed.
Overall, the welding process of the AL-DGFVE232B-T6 alloy decreased tensile strength by approximately 30% (128 to 143 MPa) compared to the substrate. The aging temperature of approximately 150℃ and 24 to 36h artificial aging treatment tended to recover strength by approximately 5 to 7% (16 to 19 MPa) compared to non-aging. The results of this study are expected to be used as reference data for setting heat treatment conditions for AL-DGFVE232B-T6 alloy welds and assessing the mechanical reliability of welded structures.
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