Tensile properties and fracture locations of friction-stir-welded joints of 2017-T351 aluminum alloy

TL;DR: In this article, the relationship between welding parameters and tensile properties of the joints has been investigated and it was shown that the tensile property and fracture locations of the joint are significantly affected by the welding process parameters.

About: This article is published in Journal of Materials Processing Technology.The article was published on 2003-12-10 and is currently open access. It has received 460 citations till now. The article focuses on the topics: Welding & Heat-affected zone.

Summary

1. Introduction

• Heat-treatable aluminum alloys are difficult to fusion weld because some welding defects such as crack and porosity are easily formed in the weld during the solidification of the welding pool [1] .
• Friction stir welding (FSW) is a solid phase welding process in which the metal to be welded is not melted during the welding, thus the crack and porosity often associated with fusion welding processes are eliminated [1, 2] .
• Therefore, the FSW process can be used to weld heat-treatable aluminum alloys in order to obtain high-quality joints [1] [2] [3] [4] .
• Hence, it is important to study the effects of welding process parameters on the mechanical properties of the joints and determine the optimum welding parameters so as to obtain high-quality friction-stir-welded joints.

2. Experimental procedure

• The base material used in this study was a 2017-T351 aluminum alloy plate of 5 mm thick, whose chemical compositions and mechanical properties are listed in Table 1 .
• The plate was cut and machined into rectangular welding samples of 300 mm long by 80 mm wide, and the samples were longitudinally butt-welded using an FSW machine.
• The designated welding tool size and welding parameters are listed in Table 2 .
• The configuration and size of the transverse tensile specimens were prepared with reference to JIS Z2201, and the marked length and width of each specimen were 50 and 12.5 mm, respectively.

3.1.1. Tensile properties of joints

• Fig. 1 shows the tensile properties of the joints welded at different revolutionary pitches.
• When the revolutionary pitch is smaller than 0.13 mm/rev, the ultimate strength and 0.2% proof strength are at comparatively high levels and slightly increase with the revolutionary pitch.
• These results indicate that a softening effect has occurred in the 2017-T351 aluminum alloy due to FSW just as it did in the other heat-treatable aluminum alloys.
• The optimum welding parameters can be determined from the relation between the tensile properties and the welding parameters.
• The ultimate strength of the joint is equivalent to 82% that of the base material.

3.1.2. Fracture locations of joints

• The fracture location of any joint is a direct reflection of the weakest part of the joint.
• Studying the fracture location of the joint is quite important to understand and improve the mechanical properties of the joint.
• Fig. 2 shows the strain distributions of the joints welded at the different revolutionary pitches.
• When the revolutionary pitch is much smaller e.g. 0.02 mm/rev, the fracture location of the joint is only 4.1 mm from the weld center.
• Moreover, it should be noted that all the joints are fractured on the advancing side or at the weld center, but not on the retreating side of the joints.

3.2. Discussion

• A softened region has been formed in the joints of 2017-T351 aluminum alloy due to the effect of friction heat as occurred in the joints of other heat-treatable aluminum alloys [7, [21] [22] [23] .
• The tensile properties and fracture locations of the joints are, to a large extent, dependent on the welding defects and hardness distributions of the joints, and which, in turn, on the welding parameters [6, 11, 14] .
• When the revolutionary pitch is smaller than 0.13 mm/rev, FSW produces defect-free joints (see Fig. 3(a) and (b) ).
• In practice, the reason for the fracture at or near the interface between the weld nugget and the TMAZ is the remarkable difference in the internal structure between the weld nugget and the TMAZ.
• As the revolutionary pitch increases, the heat input to the joint decreases, and thus the weld nugget size becomes small and the distance of the weld center to the interface decreases.

4. Conclusions

• A softened region, composed of a weld and two HAZs, has clearly occurred in the friction-stir-welded joints of the 2017-T351 aluminum alloy, thus the tensile properties of the joints are lower than those of the base material.
• The welding parameters have significant effects on the tensile properties and fracture locations of the joints.
• When the revolutionary pitch is greater than a definite value (e.g. 0.13 mm/rev), some void defects exist in the joints, the tensile properties of the joints are considerably low, and the joints are fractured at the weld center.
• On the other hand, when the revolutionary pitch is smaller than the definite value, no defects are formed in the joints, the tensile properties of the joints are at comparatively high levels, and the joints are fractured near or at the interface between the weld nugget and the TMAZ on the advancing side.
• Under the condition of an optimum revolutionary pitch of 0.07 mm/rev, the ultimate strength of the joint is maximum, equivalent to 82% that of the base material.

Figures

Fig. 4. Microhardness distributions of the typical joints welded at the different revolutionary pitches.

Fig. 3. Cross-sections of the typical joints welded at the different revolutionary pitches: (a) 0.02 mm/rev; (b) 0.07 mm/rev; and (c) 0.27 mm/rev.

Fig. 2. Strain distributions in the typical joints.

Table 1 Chemical compositions and mechanical properties of 2017-T351 aluminum alloy

Table 2 Tool size and welding process parameters used in the experiments

Fig. 1. Tensile properties of the joints.

Frequently asked questions

Q1. What have the authors contributed in "Tensile properties and fracture locations of friction-stir-welded joints of 2017-t351 aluminum alloy" ?

In order to demonstrate the friction stir weldability of the 2017-T351 aluminum alloy and determine optimum welding parameters, the relations between welding parameters and tensile properties of the joints have been studied in this paper. 

Q2. What is the effect of the revolutionary pitch on the tensile properties of the joints?

As the revolutionary pitch increases, the heat input to the joint decreases, and thus the weld nugget size becomes small and the distance of the weld center to the interface decreases. 

Q3. What was the tensile properties of the joints?

The tensile tests were carried out at room temperature at a crosshead speed of 1 mm/min using a computer-controlled testing machine and the tensile properties of each joint were evaluated using three tensile specimens cut from the same joint. 

Q4. What is the tensile strength of the joint?

When a tensile load is applied to the joint, the stress and strain concentration takes place in the lower-strength part or region of the joint, and consequently, the joint is fractured in this region [8]. 

Q5. What are the tensile properties of the joints?

The tensile properties and fracture locations of the joints are, to a large extent, dependent on the welding defects and hardness distributions of the joints, and which, in turn, on the welding parameters [6,11,14]. 

Q6. What is the simplest way to weld heat-treatable aluminum?

After welding, the joints were cross-sectioned perpendicular to the welding direction for metallographic analyses and tensile tests using an electrical-discharge cutting machine. 

Q7. Why are heat-treatable aluminum alloys difficult to fusion weld?

Heat-treatable aluminum alloys are difficult to fusion weld because some welding defects such as crack and porosity are easily formed in the weld during the solidification of the welding pool [1]. 

Q8. What is the purpose of this paper?

This paper aims to demonstrate its friction stir weldability and the emphasis is placed on the relations of the tensile properties and fracture locations of the joints to the welding parameters in order to determine the optimum FSW parameters and find out the weakest locations of the joints. 

Q9. What is the effect of FSW on the aluminum?

These results indicate that a softening effect has occurred in the 2017-T351 aluminum alloy due to FSW just as it did in the other heat-treatable aluminum alloys. 

Q10. How far from the weld center is the fracture location of the joint?

When the revolutionary pitch is much smaller e.g. 0.02 mm/rev, the fracture location of the joint is only 4.1 mm from the weld center.