Journal Information
Vol. 8. Issue 5.
Pages 4318-4332 (September - October 2019)
Share
Share
Download PDF
More article options
Visits
0
Vol. 8. Issue 5.
Pages 4318-4332 (September - October 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.07.043
Open Access
Microstructures and mechanical properties of Al-Mg2Si-Si alloys resistance spot welded with Al-Si interlayers
Visits
0
Qingdong Qina,
Corresponding author
58124812@qq.com

Corresponding authors.
, Honglong Zhaoa,
Corresponding author
2669142163@qq.com

Corresponding authors.
, Yingzhe Zhangb, Juan Lib,c, Zhenglong Wangb
a Key Laboratory of Light Metal Materials Processing Technology of Guizhou Province, Guizhou Institute of Technology, Guiyang 550003, PR China
b Department of Materials & Metallurgy Engineering, Guizhou Institute of Technology, Guiyang 550003, PR China
c 2011 Special Functional Materials Collaborative Innovation Center of Guizhou Province, Guiyang 550003, PR China
This item has received
0
Visits

Under a Creative Commons license
Article information
Abstract
Full Text
Bibliography
Download PDF
Statistics
Figures (17)
Show moreShow less
Tables (1)
Table 1. Compositions of the Al-Mg2Si-Si alloy and Al-Si interlayer.
Abstract

This study investigates Al-Mg2Si-Si alloy joints produced via resistance spot welding (RSW) with and without the use of Al-Si interlayers. The joint microstructures produced both with and without Al-Si interlayers consist of three zones: the base material (BM), the heat affected zone (HAZ) and the weld nugget (WN). The WN is oval, and increases in the welding current increase the WN size in both welding methods. The joints produced via RSW with an Al-Si interlayer have larger WNs than those produced without an interlayer under the same welding current conditions. Relationships between the welding current and the WN size are established by Gaussian fitting and Lorentz fitting. A comparison of the two welding approaches shows that a successful weld can be obtained at a low welding current when an Al-Si interlayer is used, whereas a larger welding current is required to obtain the same result without the use of an Al-Si interlayer. The transition layer (interface between the HAZ and the WN) in the joints welded without an interlayer is a single eutectic phase structure; however, in the joints welded with an interlayer, the transition layer generates an equiaxed-columnar crystal zone. The formation of the equiaxed-columnar crystal zone and the change in composition of the WN (according to the results of differential scanning calorimetry tests) improve the tensile-shear loads (TSLs). The equations of the peak TSLs as a function of the welding current and WN size are established by Gaussian fitting.

Keywords:
Al-Mg2Si-Si alloy
Resistance spot welding
Interlayer
Microstructure
Mechanical properties
Full Text
1Introduction

The Al-Mg2Si-Si alloy is a hypereutectic Al-Si alloy with a high Mg content, which generates many intermetallic compounds of Mg2Si. According to the literature [1], Mg2Si exhibits high hardness (0.45×1010N/m2), low density (0.199×104kg/m3) and high melting temperature (1358K). The Al-Mg2Si-Si alloy can be used as a wear-resistant material due to the presence of the Mg2Si phase. It is well known that the primary Mg2Si phases generated in an Al melt during a common casting process have coarse dendritic structures, and the Al-Mg2Si-Si materials with such structures exhibit poor mechanical properties. A substantial number of modification works have been performed and achieved some good results, such as modifications with P [2], Ca+Sb [3], and Sb [4]. These beneficial results accelerate the application process of Al-Mg2Si-Si alloy. Welding processing is indispensable for material engineering applications. However, fewer studies have been performed on welding and joining this alloy.

Currently, welding research on this material is focused on friction stir welding (FSW). Our previous works studied FSW of this alloy [5] and found that the coarse primary Mg2Si dendrites were transformed to polygonal particles in the weld nugget (WN); the segregation was reduced for the WN. The ultimate tensile strength of the friction stir welded joints was 5% greater than that of the base material (BM). Nami et al. [6] studied the weldability of Al-15Mg2Si alloys via FSW, and a successful connection was achieved for these alloys. Their results show that FSW is a good weld technology for joining Al-Mg2Si-Si alloys. Additionally, some works have focused on the welding of dissimilar Al-Mg2Si-Si alloys using FSW. For example, Sharifitabar et al. [7] studied the microstructures of dissimilar FSW between 2xxx Al alloy and Al-Mg2Si alloy. Their results showed that some defects were found for both one-pass and two-pass welds.

According to the above mentioned studies, FSW enables good joining for Al-Mg2Si-Si alloy. However, not all shapes of Al-Mg2Si-Si alloy can be welded by FSW; for instance, FSW cannot be used for thin sheet joining of this alloy. Hence, it is necessary to develop more welding technologies. Resistance welding, which includes resistance spot welding (RSW) and resistance seam welding [8–10], is a common technology for sheet joining. A substantial amount of work has been done on RSW of Al alloys, while many challenges remain. For example, expulsion is a common phenomenon in RSW, and expulsion is particularly problematic when RSW Al alloys due to the existence of an alumina film [8]. To eliminate the influence of the surface oxide, Luo et al. [11] investigated an approach that combined a preheating treatment with RSW for 5052 aluminum alloy. Their results showed that the weld quality was improved by the preheating treatment. Pore formation is another common phenomenon observed when RSW Al alloys due to several factors, such as hydrogen rejection, shrinkage strain, and surface contamination [12]. Therefore, RSW Al alloys is still subject to many challenges. In particular, RSW Al-Mg2Si-Si alloys with composite structures present many challenges.

The present work investigates the weldability of Al-Mg2Si-Si alloy via RSW and studies the microstructure and mechanical properties of the joint produced via RSW with and without an Al-Si interlayer addition.

2Experimental methods2.1Welding parameters of RSW

The preparation process of the Al-Mg2Si-Si alloy has been described in the literature [5]. Table 1 shows the composition of the Al-Mg2Si-Si alloy in the present work.

Table 1.

Compositions of the Al-Mg2Si-Si alloy and Al-Si interlayer.

Alloys  Fe  Ti  Cu  Mn  Zn  Mg  Si  Al 
Al-Mg2Si-Si  0.22  0.12  2.03  <0.10  0.08  8.55  9.41  Bal. 
Al-Si interlayer  <0.80  <0.05  <0.30  <0.15  <0.20  <0.10  11.12  Bal. 

The Al-Mg2Si-Si materials were cut into thin sheet samples with dimensions of 2×20×100mm3. Commercial 4073 Al-Si alloy foils with a thickness of 200μm were used as the interlayer; the composition of these foils is shown in Table 1. The surfaces of the BMs and the Al-Si interlayer were polished with silicon carbide metallographic abrasive paper. Then, all the samples were further cleaned with ethylene glycol. The RSW was conducted using a 250kW welding machine. The electrodes were made of copper with a radius of 20mm on both sides, as shown in Fig. 1. For comparison, the samples were subjected to RSW without and with an Al-Si interlayer addition, as shown in Fig. 1a and b, respectively. The current, electrode force and time were set to 11–14kA, 3.6kN and 200ms for the RSW according to preliminary tests of the previous works.

Fig. 1.

Schematic illustration of RSW (a) without and (b) with an Al-Si interlayer addition.

(0.15MB).
2.2Mechanical property tests

The hardness tests of the resistance spot welded Al-Mg2Si-Si joints were conducted on a microhardness machine with a load and holding time of 0.2kg and 20s, respectively. To ensure the accuracy of the microhardness value, four measurements were taken for each sample under the same conditions. Tensile-shear samples were assembled with an overlap distance of 25mm. Fig. 2 shows the sketches and dimensions of the tensile-shear samples. The tensile-shear tests were performed using an MTS universal testing machine at room temperature. The crosshead speed was set to 0.25mmmin−1. Five measurements were taken for each joint produced with the same welding technology under the same conditions.

Fig. 2.

Sketches and dimensions of the tensile-shear samples.

(0.06MB).
2.3Microstructural observation and DSC testing

The samples were prepared using standard routines, and all the samples were cleaned with ethylene glycol. The sample microstructures were examined with transmission electron microscopy (TEM), scanning electron microscopy (SEM) and optical microscopy (OM). The etchants used for the microstructural observation via OM and SEM were NaOH solution (20%) and HF solution (0.5%), respectively. Differential scanning calorimetry (DSC) tests were performed on a DSC machine with a temperature ramp rate of 10Kmin−1; the samples were subjected to a temperature range of 500–650°C in an Ar atmosphere. Three measurements were taken for each sample under the same conditions.

3Results and discussion3.1Effect of the welding currents on the WN

A joint produced via RSW is composed of three zones: the BM zone, the heat affected zone (HAZ) and the WN. Fig. 3 shows images of the macroscopic structures of the resistance spot welded joints, where (a), (c), (e) and (g) are the joints produced via RSW without an interlayer, and (b), (d), (f) and (h) are the joints produced via RSW with an Al-Si interlayer. These images show that the WN is an oval, and as the welding current increases, this shape does not obviously change. However, the increase in welding current causes an increase in the WN size for both welding methods, i.e., RSW with and without an Al-Si interlayer. The change in current generates a different effect on the quality in the two methods. After RSW without an Al-Si interlayer, an effective weld cannot be obtained at a low current of 11kA or 12kA, as shown in Fig. 3a and c, respectively. A penetrating crack appears from the center to the edge in the WN. As the welding current increases to 13kA, the penetrating crack disappears, and a good weld is obtained (Fig. 3e). When the current further increases to 14kA, the features of the WN do not obviously change, as shown in Fig. 3g. In comparison, a good weld is obtained after RSW with an Al-Si interlayer under a low welding current of 11kA or 12kA, as seen in Fig. 3b and d, respectively. The size of the WN at a welding current of 12kA is larger than that at 11kA. However, as the current further increases to 13kA or 14kA, many large holes appear at the center of the WN. This phenomenon is especially prevalent when using a welding current of 14kA, wherein some interconnected holes appear in the WN.

Fig. 3.

Macroscopic structures of Al-Mg2Si-Si joints resistance spot welded under different welding currents: (a, b) 11kA, (c, d) 12kA, (e, f) 13kA and (g, h) 14kA, wherein (a), (c), (e) and (g) were welded without an interlayer, and (b), (d), (f) and (h) were welded with an Al-Si interlayer.

(1.7MB).

Based on Joule's law (Q=I2Rt, in which t is time, R is resistance, I is current and Q is heat input) [13], the heat released during RSW depends on the values of I, R and t. When the welding time is kept constant, Q is affected only by I and R. It is well known that bulk resistance and contact resistance are the main resistances in RSW [8], as shown in Fig. 1a and b. The bulk resistances are R2+R3 and R2+R3+R7 for RSW without and with Al-Si interlayers, respectively. Contact resistance exists at the faying interface (R8 for RSW without an Al-Si interlayer and R5+R6 for RSW with an Al-Si interlayer) and at the sheet/electrode interfaces (R1+R4). Of all the resistances, the most substantial are the contact resistances (R8 and R5+R6) since the WN formation initiates here [14]. In comparison, the two RSW welding processes in the present work show that R5+R6 is larger than R8; accordingly, a greater amount of heat is released during RSW with an Al-Si interlayer than in RSW without an Al-Si interlayer at the same welding current. Thus, a better weld is obtained under low welding current conditions through the use of an Al-Si interlayer.

3.2Characteristics of microstructure

The microstructure of the BM is shown in Fig. 4. The primary Mg2Si phases are identified as polygonal particles with an average size of 20μm. The eutectic Mg2Si phases are identified as having Chinese script morphologies.

Fig. 4.

OM microstructure of the Al-Mg2Si-Si BM.

(0.62MB).

OM microstructures of the welding joints produced via RSW without an interlayer are shown in Fig. 5, wherein (a)–(d) correspond to welding currents of 11–14kA. Fig. 6 shows the microstructures of the welding joints produced via RSW with Al-Si interlayers, wherein (a)–(d) correspond to welding currents of 11–14kA. The characteristics of the OM microstructures are consistent with the macroscopic structures in Fig. 3. There exist penetrating cracks and defects in the WNs of the samples produced without an interlayer at low currents of 11–12kA, and as the current increases, these cracks and defects disappear. In contrast, for the samples produced via RSW with Al-Si interlayers, better welding joints were obtained at lower currents (11–12kA) than at higher currents. Fig. 5a shows that there is no primary Mg2Si in the WN of the sample produced without an interlayer at low welding current conditions. As the current increases, the primary Mg2Si phase begins to appear in the WN (Fig. 5b), and the volume fraction of this phase increases with increasing current (Fig. 5c and d). Fig. 6a shows that there is also no primary Mg2Si phase generation in the WN of the sample produced with an interlayer under a welding current of 11kA. When the current increases to 12kA, some primary Mg2Si particles appear in the WN (Fig. 6b).

Fig. 5.

OM microstructures of the Al-Mg2Si-Si joints produced via RSW without Al-Si interlayers: (a) 11kA, (b) 12kA, (c) 13kA, and (d) 14kA.

(1.41MB).
Fig. 6.

OM microstructures of the Al-Mg2Si-Si joints produced via RSW with Al-Si interlayers: (a) 11kA, (b) 12kA, (c) 13kA, and (d) 14kA.

(1.4MB).

As the welding current further increases, defects and holes begin to appear, as shown in Fig. 6c and d. Enlarged images of Figs. 5d and 6b are shown in Fig. 7a and b, which show the welds produced without and with an Al-Si interlayer, respectively. The enlarged images show the characteristics of the interface and transition layer. Fig. 7a shows that the interface of the welds produced without an interlayer consists of the BM and eutectic phase zones (zones I and II are the BM and eutectic phase zones, respectively). The transition layer is a single eutectic phase structure of zone II in Fig. 7a. Fig. 7b shows that the interface of the welds produced with an Al-Si interlayer is composed of an equiaxed crystal zone, a columnar crystal zone and the BM zone (zones I, II and III are the BM, columnar crystal and equiaxed crystal zones, respectively). The transition layer is no longer a single eutectic phase but consists of an equiaxed crystal region and a columnar crystal zone.

Fig. 7.

Enlarged images of the welding joints: welds produced (a) without an interlayer and (b) with an Al-Si interlayer.

(1.14MB).

Fig. 8 shows SEM microstructures of the Al-Mg2Si-Si joints produced via RSW without an interlayer under a 14kA welding current. Fig. 8a shows that a good interface is formed between the HAZ and the WN. The number of eutectic Mg2Si phases with Chinese script morphologies is reduced, and these are replaced by a point-like eutectic Mg2Si phase in the HAZ (Fig. 8b), which is similar to the microstructures of Al-Mg2Si-Si alloys after solution heat treatment [2].

Fig. 8.

SEM microstructures of the Al-Mg2Si-Si joints produced via RSW without an interlayer: (a) interface of the HAZ and the WN; microstructures of (b) the HAZ and (c) the WN.

(0.69MB).

The primary Mg2Si phase decreases or disappears, and the eutectic Mg2Si phase presents a finer point-like morphology with a size of ∼1μm in the WN, as shown in Fig. 8c.

Fig. 9 shows SEM microstructures of the joints produced via RSW with an interlayer under a welding current of 12kA. Fig. 9a shows that the joint produced via RSW with an interlayer under a 12kA current has a better combination between HAZ and WN. The microstructure of the HAZ does not obviously change (Fig. 9b). Fig. 9c shows the microstructure of the Al-Si interlayer, in which the primary Si phase presents fine particles. Fig. 9d shows that the fine primary Si particles are replaced by a grid-like eutectic Si phase after RSW, and some finer eutectic Mg2Si particles are generated inside the eutectic Si grid.

Fig. 9.

SEM microstructures of the Al-Mg2Si-Si joints produced via RSW with an interlayer: (a) interface between the HAZ and the WN, and the microstructures of (b) the HAZ, (c) the Al-Si alloy foil and (d) the WN.

(1.01MB).

During RSW without an interlayer, the Al-Mg2Si-Si alloy is not completely melted due to insufficient heat release under the low welding current of 11kA. Therefore, the alloy does not have a good joint, and penetrating cracks occur. Under partial melting conditions, only the low melting point phase of eutectic Mg2Si melts and bonds to each other. This phenomenon explains why the WN does not have a primary Mg2Si phase at low welding currents. As the current increases, the amount of heat also increases. This increase in heat results in the primary Mg2Si phase melting, which produces a better combination of the two materials. Therefore, as the welding current increases, more primary Mg2Si phases are produced in the WN solidification process. However, the use of the interlayer causes two changes: one is a change in the resistance value of the joint system, and the other is a change in the local melting point. The change in resistance has been discussed above. Regarding the local melting temperature, because of the use of an interlayer with a low melting point, the contact surfaces are melt more easily. Under the effect of these two factors, the welding of the material can be completed at a low current of 12kA. This connection is dominated by the capillary force of the Al-Si alloy interlayer. As the welding current increases, the amount of heat released increases, and the BM begins to melt. Therefore, the microstructures containing eutectic Mg2Si, eutectic Si and primary Mg2Si are formed in the WN at a welding current of 12kA, as shown in Figs. 6b and 9d. The transformations of the local microstructures of the WN will result in changes in the mechanical properties. As the welding current further increases, some pores are generated because of the expulsion phenomenon. Expulsion is a common phenomenon in the RSW process [8]. Expulsion usually occurs at either the electrode-workpiece interface or the faying surface (i.e., surface expulsion or interfacial expulsion) because of inappropriate welding parameters, such as a short period of time with a high welding current [15]. In the present work, expulsion is more likely to occur when RSW with an Al-Si interlayer than without an interlayer. The reasons for this increased likelihood of expulsion are that the Al-Si alloy interlayer has a lower melting point than the BM, and the use of this interlayer increases the interfacial resistance and releases more heat.

As mentioned above, the increase in the welding current increases the size of the WN. According to Figs. 3, 5 and 6, the relationship between the WN area and the welding current is shown in Fig. 10. The results show that the WN sizes when the interlayer is used are all larger than those when the interlayer is not used, and the addition of the interlayer produces a higher rate of size increase as the welding current increases. Quantitative relationships between the WN area (S) and the welding current (i) can be obtained by using Gaussian fitting (Eq. (1)) and Lorentz fitting (Eq. (2)) based on the two curves in Fig. 10, where Eqs. (1) and (2) are for RSW without and with interlayer addition, respectively.

Fig. 10.

Relationship between the WN area and the welding current.

(0.18MB).
3.3Solidus temperatures of the WN and the BM

The DSC curves for the WN and the BM are shown in Fig. 11, wherein Fig. 11a is the heating process and Fig. 11b is the solidification process. These DSC curves show two endothermic peaks (starting at 537°C and 562°C) in the curve during the melting process of the BM in Fig. 11a. Based on a ternary phase diagram of Al-Mg-Si (Fig. 12) and those in previous works [1], the two endothermic peaks correspond to the melting temperatures of CuAl2 (537°C) and eutectic phase (562°C). Since the heat absorbed by the eutectic phase melting is excessively large, the endothermic peak on the primary Mg2Si phase melting is not shown on the curves. However, the DSC curves of the WN present different characteristics than those of the BM, regardless of whether an Al-Si interlayer was used in the RSW process. All the curves of the WN have only one endothermic peak, which corresponds to the eutectic phase melting. The change in the curve indicates a change in the composition. The disappearance of the CuAl2 endothermic peak may be due to high-temperature burning during the welding process. The microstructure of the WN (Figs. 8c and 9d) also verified the reduction in CuAl2. Furthermore, the addition of the Al-Si alloy interlayer significantly reduces the melting temperature of the WN (553°C and 569°C). This reduction makes it easier to achieve proper welds at low currents during RSW.

Fig. 11.

DSC curves for the WN and the BM during the (a) heating and (b) cooling processes.

(0.48MB).
Fig. 12.

Vertical section of the Al-Mg-Si ternary phase diagram (Al-15Mg2Si to Si).

(0.15MB).

The DSC curves of the solidification process are shown in Fig. 8b, wherein three exothermic peaks exist (starting from 632°C, 583°C and 534°C) in the BM curve. The three exothermic peaks correspond to the crystallization temperature of the primary Mg2Si, the eutectic phase and CuAl2. The solidification temperatures of the WN and the BM are significantly different in Fig. 11b. In the solidification stage of the WN for the direct welding process, the crystallization temperature of the primary Mg2Si is decreased to 618°C, whereas the crystallization temperature of the eutectic phase is increased to 593°C. Similar to the melting process, the exothermic peak for CuAl2 solidification does not appear. A comparison of the DSC curves from the melting and solidification processes shows that the composition of the WN changed as a result of RSW without an interlayer. As mentioned in the phase diagram of Al-Mg-Si, the WN composition moves from b to a, in which the crystallization temperatures of the primary Mg2Si and eutectic phases decrease and increase, respectively. Obviously, the composition of the WN when using the Al-Si alloy interlayer inevitably changes due to the addition of Al and Si elements. This change in composition causes simultaneous reductions in the crystallization temperature and melting temperature of the eutectic phase, which promotes the welding quality improvement of RSW.

3.4Mechanical properties of the resistance spot welded joints

The microhardness values of the resistance spot welded joints are shown in Fig. 13. The hardness values of the joints produced by the two welding methods (RSW without and with an interlayer) exhibit the same characteristics. The hardness values gradually decrease from the BM (102 HV) to the central area of the WN (93 HV). The appearance of joint softening is caused by two reasons. The first reason for joint softening is the change in composition of the WN, as mentioned above in the DSC tests; the reduction in the primary phase and the increase in the eutectic phase lead to softening of the joint. For RSW with interlayer addition, the addition of the Al-Si interlayer reduces the reinforcing phase, such as eutectic Mg2Si and Si, which affects the hardness of the joint. The other reason for joint softening is the effect of heat. The resistive heat of the RSW process causes Mg and Si atoms to solidify into the matrix, which is similar to the solution heat treatment process; this phenomenon softens the WN and the HAZ. The TEM microstructures of the BM and WN in the joint produced without an interlayer are shown in Fig. 14. Some needle-shaped β″ precipitates are present in the BM, as shown in Fig. 14a. The presence of these precipitates maintains the hardness of the BM. After RSW, however, these β″ precipitates are dissolved by the resistive heat; hence, the hardness decreases and the welding joint softens. The TEM results confirmed some of the reasons for the softening of the welding joint. The phenomenon where the dissolution of needle-shaped β″ precipitates softens welding joints is common when welding Al alloys. Pereira et al. [16] studied the effect of welding technology on the mechanical properties of 6082-aluminum joints produced via RSW. Their results showed that the hardness of the WN significantly decreased under all tested welding currents, even under low welding currents. They believed that the decrease in hardness was attributed to the dissolution of the β″ strengthening precipitates. Zhao et al. [17] also observed the dissolution of β″ strengthening precipitates during friction stir processing of 6063 Al alloy.

Fig. 13.

Microhardness values of the RSW joint under different welding conditions.

(0.34MB).
Fig. 14.

TEM microstructures of the joint produced via RSW without an interlayer: (a) the BM and (b) the WN.

(0.43MB).

Fig. 15 shows the peak tensile-shear loads (TSLs) of the Al-Mg2Si-Si alloy joints produced via RSW under different welding currents. The results show that as the welding current increases, the TSL increases in the joints produced via RSW without an interlayer. When an interlayer is used during RSW, as the current increases, the TSL rapidly increases to a maximum and then begins to decrease. These results of the TSL test are consistent with the microstructural changes. The maximum TSL when using an interlayer is slightly higher than that without using an interlayer. The change in the TSL is mainly affected by welding defects. As mentioned above, as the current increases, the defects in the welding joint gradually disappear when an interlayer is not used during RSW. When an interlayer is used during RSW, a better microstructure can be obtained at low welding currents. The change in TSL shows this response.

Fig. 15.

Peak TSL of the Al-Mg2Si-Si with respect to the welding current.

(0.19MB).

The typical SEM morphologies of the fracture surfaces of the welding joints produced via RSW are shown in Fig. 16a and b, where (a)–(b) and (c)–(d) are welded at a welding current of 14kA without an interlayer and at a welding current of 12kA with an Al-Si interlayer, respectively. The smooth fracture surfaces of the joints produced without an interlayer indicated that the fracture path was along the welding interface, as shown in Fig. 16. Although no obvious defects were found in the metallographic microstructure, as mentioned in Fig. 5, some shrinkage cavity was shown in the enlarged image of the fracture surface (Fig. 16b). After the interlayer addition, obvious cleavage facet characteristics were observed, as shown in Fig. 16c and d. Hence, the change in the fracture path was caused by the Al-Si interlayer addition. The addition of an Al-Si interlayer with a low melting point provides a better fusion of the BM for welding. Zhang [18] and Dai [19] also reported similar fracture surfaces in Mg/steel joints produced via RSW and Mg/Al joints produced via gas tungsten arc welding. The former used RSW to weld Mg to 22MnB5 boron steel coated by an Al-Si eutectic layer. The presence of the coating reduced the welding defects and improved the welding appearance. The molten Si and Al diffused into the BM of the Mg alloys, forming a columnar-equiaxed dendrite structure. The fracture of this columnar-equiaxed dendrite structure formed fracture surfaces with cleavage facets. The latter used gas tungsten arc welding to join AZ31B-Mg/6061-Al alloys.

Fig. 16.

Typical SEM morphologies of the fracture surfaces: (a) joints produced under a 14kA welding current without an interlayer; (b) enlarged image of (a); (c) joints produced under a 12kA welding current with an Al-Si interlayer; (d) enlarged image of (c).

(0.8MB).

The mechanical property tests showed a brittle fracture mode (cleavage facet fracture surfaces). The existence of the columnar crystal region (Fig. 7b) generates the cleavage facet fracture surfaces.

According to the relationship between the TSL and welding current (i) of the resistance spot welded Al-Mg2Si-Si joints, as shown in Fig. 15, quantitative relationships between the TSL and welding current can be obtained by using Gaussian fitting. Eqs. (3) and (4) are for the joints produced via RSW without and with interlayer use, respectively.

Then, Eqs. (1) with (3) and (2) with (4) are combined to obtain Eqs. (5) and (6), respectively, as follows.

The relationships between the welding current, WN size and mechanical properties are established, as shown in Fig. 17, which presents 3D surface plots of the TSLs as a function of the welding current and WN size. The results show that for the samples produced via RSW without an interlayer, the welding current must be increased to obtain good mechanical properties (Fig. 17a). However, for the samples produced via RSW with an Al-Si interlayer, a moderate welding current can obtain good mechanical properties.

Fig. 17.

3D surface plots of the TSLs as a function of welding current and WN size: (a) joints produced without an interlayer; (b) joints produced with an Al-Si interlayer.

(0.68MB).
4Conclusions

  • (1)

    Al-Mg2Si-Si alloy joints produced via RSW contain three zones (the BM, HAZ and WN), regardless of the use of an Al-Si interlayer during welding. The WN is oval, and as the welding current increases, the WN size increases for both welding methods. The welds produced with an Al-Si interlayer have larger WNs than those produced without an interlayer under the same welding current conditions.

  • (2)

    The relationships of the welding current and WN size for the joints produced with and without an Al-Si interlayer are consistent with the Gaussian equation and the Lorentz equation, respectively.

  • (3)

    When an interlayer is used, a successful weld can be obtained at a low welding current, whereas a larger welding current is required to obtain a successful weld when an interlayer is not used. The addition of an interlayer can significantly reduce the welding current.

  • (4)

    The addition of the Al-Si interlayer results in changes in the composition and microstructure of the transition layer (interface between the HAZ and the WN), wherein an equiaxed-columnar crystal zone is formed.

  • (5)

    The changes in microstructure and composition of the joints produced with an interlayer lead to improvements in the maximum TSL. The maximum TSL of the joints produced with an interlayer is slightly higher than that of the joints produced without an interlayer. The relationships of peak TSLs as a function of welding current and WN size for the two weld technologies are all consistent with the Gaussian equation.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgments

This work was supported by the Program for the Distinguished Young Scientific Talents of Guizhou, China [Qian Ke He Platform and talent (2016) 5633], the National Natural Science Foundation of China [51564005], and the Guizhou Provincial Higher Education Engineering Research Center, China [Qian Jiao He KY (2017) 021]], and the Technological Innovation Talent Team of Guizhou Province, China [Qian Ke He Talent Team (2015) 4008].

References
[1]
Q.D. Qin, Y.G. Zhao, C. Liu, P.J. Cong, W. Zhou, Y.H. Liang.
Effect of holding temperature on semisolid microstructure of Mg2Si/Al composite.
J Alloy Compd, 416 (2006), pp. 143-147
[2]
Q.D. Qin, W.X. Li, K.W. Zhao, S.L. Qiu, Y.G. Zhao.
Effect of modification and aging treatment on mechanical properties of Mg2Si/Al composite.
Mater Sci Eng A, 527 (2010), pp. 2253-2257
[3]
H.C. Yu, H.Y. Wang, L. Chen, M. Zha, C. Wang, C. Li, et al.
Spheroidization of primary Mg2Si in Al–20Mg2Si–4.5Cu alloy modified with Ca and Sb during T6 heat treatment process.
Mater Sci Eng A, 685 (2017), pp. 31-38
[4]
H.Y. Wang, F. Liu, L. Chen, M. Zha, G.J. Liu, Q.C. Jiang.
The effect of Sb addition on microstructures and tensile properties of extruded Al–20Mg2Si–4Cu alloy.
Mater Sci Eng A, 657 (2016), pp. 331-338
[5]
Q.D. Qin, B.W. Huang, Y.J. Wu, X.D. Su.
Microstructure and mechanical properties of friction stir welds on unmodified and P-modified Al-Mg2Si-Si alloys.
J Mater Process Technol, 250 (2017), pp. 320-329
[6]
H. Nami, H. Adgi, M. Sharifitabar, H. Shamabadi.
Microstructure and mechanical properties of friction stir welded Al/Mg2Si metal matrix cast composite.
Mater Des, 32 (2011), pp. 976-983
[7]
M. Sharifitabar, H. Nami.
Microstructures of dissimilar friction stir welded joints between 2024-T4 aluminum alloy and Al/Mg2Si metal matrix cast composite.
Compos B Eng, 42 (2011), pp. 2004-2012
[8]
S.M. Manladan, F. Yusof, S. Ramesh, M. Fadzil, Z. Luo, S. Ao.
A review on resistance spot welding of aluminum alloys.
Int J Adv Manuf Technol, 90 (2017), pp. 605-634
[9]
S.M. Manladan, F. Yusof, S. Ramesh, Y. Zhang, Z. Luo, Z. Ling.
Microstructure and mechanical properties of resistance spot welded in welding-brazing mode and resistance element welded magnesium alloy/austenitic stainless steel joints.
J Mater Process Technol, 250 (2017), pp. 45-54
[10]
Q. Chu, W.Y. Li, H.L. Hou, X.W. Yang, A. Vairis, C. Wang, et al.
On the double-side probeless friction stir spot welding of AA2198 Al-Li alloy.
J Mater Sci Technol, 35 (2019), pp. 784-789
[11]
Z. Luo, S. Ao, Y.J. Chao, X. Cui, L. Yang, Y. Lin.
Application of pre-heating to improve the consistency and quality in AA5052 resistance spot welding.
J Mater Eng Perform, 24 (2015), pp. 3881-3891
[12]
J. Bi, J.L. Song, Q. Wei, Y. Zhang, Y. Li, Z. Luo.
Characteristics of shunting in resistance spot welding for dissimilar unequal-thickness aluminum alloys under large thickness ratio.
Mater Des, 101 (2016), pp. 226-235
[13]
Y. Wang, Z. Mo, J. Feng, Z. Zhang.
Effect of welding time on microstructure and tensile shear load in resistance spot welded joints of AZ31 Mg alloy.
Sci Technol Weld Join, 12 (2007), pp. 671-676
[14]
N. Williams, J. Parker.
Review of resistance spot welding of steel sheets part 1modelling and control of weld nugget formation.
Int Mater Rev, 49 (2004), pp. 45-75
[15]
R.F. Qiu, H.X. Shi, H. Yu, K.K. Zhang, Y.M. Tu, S. Satonaka.
Effects of electrode force on the characteristic of magnesium alloy joint welded by resistance spot welding with cover plates.
Mater Manuf Process, 25 (2010), pp. 1304-1308
[16]
A.M. Pereira, J.M. Ferreira, A. Loureiro, J.D.M. Costa, P.J. Bártolo.
Effect of process parameters on the strength of resistance spot welds in 6082-T6 aluminium alloy.
Mater Des, 31 (2010), pp. 2454-2463
[17]
H.L. Zhao, Q. Pan, Q.D. Qin, Y.J. Wu, X.D. Su.
Effect of the processing parameters of friction stir processing on the microstructure and mechanical properties of 6063 aluminum alloy.
Mater Sci Eng A, 751 (2019), pp. 70-79
[18]
K. Zhang, L. Wu, C. Tan, Y. Sun, B. Chen, X. Song.
Influence of Al-Si coating on resistance spot welding of Mg to 22MnB5 boron steel.
J Mater Process Technol, 271 (2019), pp. 23-35
[19]
X. Dai, H. Zhang, J. Liu, J. Feng.
Microstructure and properties of Mg/Al joint welded by gas tungsten arc welding-assisted hybrid ultrasonic seam welding.
Mater Des, 77 (2015), pp. 65-71
Copyright © 2019. The Authors
Journal of Materials Research and Technology

Subscribe to our newsletter

Article options
Tools
Cookies policy
To improve our services and products, we use cookies (own or third parties authorized) to show advertising related to client preferences through the analyses of navigation customer behavior. Continuing navigation will be considered as acceptance of this use. You can change the settings or obtain more information by clicking here.