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LF21 aluminum alloy sheet microstructure and properties of friction stir weldingAbstract: Tests about friction stir butt-welds of LF21 aluminum sheet with 1.4 mm in thickness were made in a single pass. The experimental results show that when rotating speed is 1 500 r / min and welding speed is 100 - 180 mm / min, the good welds can be achieved. The tensile strength of welds is 125 - 134 MPa, which is between 78% and 83% of that of base metal. It shows that the welding adaptability of LF21 aluminum alloy is good. The micro-hardness result shows that the weld is softened, and the soften zone of weld is about 25 mm in width. The brittle MnAl6 compounds are precipitated along the rolling direction with the heat of weldi ng, so the tensile strength of weld decreases.Key words: friction stir welding; sheet; LF21 aluminum alloy; precipitateFriction stir welding (referred FSW) is a high-speed rotation of the mixing head and the material to be welded between the combined effects of intense heat to achieve solid phase connection method. It is suitable for fusion welding method for welding difficult to weld light alloy in aluminum, titanium, magnesium alloy welding areas, with no pores and cracks, welding deformation is small, weld strength, fatigue and good performance, display have a broad prospect for industrial application. LF21 aluminum manganese alloy is a typical rust-proof aluminum alloy, is a non-heat treatable aluminum alloy reinforced. The alloy is characterized by a strong corrosion resistance, weldability, and has a moderate strength, high ductility in the annealed state, while in the low work hardening plasticity but when corrosion resistance is a most widely used anti-rust aluminum. Used as a liquid or gas medium in the low load parts, such as tanks, pipes and various shaped container material.Currently, LF21 rust aluminum welding generally use fusion welding, but in the actual welding is easy to produce a variety of welding defects such as excessive weld porosity, slag, the mechanical properties of welded joints up to standard requirements, the welding deformation is too large, etc. . In addition, there is a fusion welded thin LF21 hardening effect, the plate residual stress, prone to warping and cracking phenomena welding. Since there is corrosion LF21 sheet sealing requirements with low load working environment is widely used, in this, the author of the selected thickness of 1.4 mm alloy sheet FSW experiments for solving the alloy welding applications in engineering problems has important significance.Materials and methodsExperimental material thickness of 1.4 mm is measured for LF21 aluminum alloy rolled plate, sizes (length × width × thickness) of 300 mm × 70 mm × 1.4 mm, the main chemical composition and mechanical properties are shown in Table 1 and Table 2 shown.LF21 rust-proof aluminum main strengthening phase MnAl6, impurity phase (FeMn) Al6 or (Fe, Mn, Si) Al6, etc., but the presence of impurities reducing the segregation of Mn, to promote grain refinement of a certain effect. Friction stir welding experiment in self-developed experimental friction stir welding machine for welding along the length of the sample in the form of single-pass butt welding direction. As the spiral-shaped mixing head helps to friction stir weld forming dense, so the present study spiral stir welding head, the mixing head parameters are: shaft shoulder diameter of 10 mm, stirring rod diameter 3 mm, length paddles 1.2 mm. Before welding, butt welding parts no special surface treatment to remove the oxide film. In clear pending butt welders surface dirt and dust on the surface, will be two parts to be welded after the split, fixed on the table. Welding, high-speed rotary drill into the mixing head plates to be welded seams, when the mixing head shoulder compacted aluminum, along the seams forward to complete the welding process. Welding process parameters used in the: mixing head rotational speed of 1 500 r / min, the seam along the direction of friction stir welding speed of 100 ~ 180 mm / min. LF21 aluminum alloy sheet for a single channel friction stir butt weld, the weld pieces were sampled along the transverse welds, preparation of metallographic specimens and tensile specimens in POL YV AR-MET optical microscope to observe the district after welding microstructure, the CSS -44100 electronic tensile machine tensile tested in the HMV-2 on a Vickers hardness tester microhardness.2 Results2.1 Mechanical properties of the weldIn the mixing head rotation speed 1500 r / min, the welding speed is 100 ~ 180 mm / min of the welding process, the thickness of 1.4 mm of the LF21 aluminum FSW weld mechanical properties which in Figure 1. Seen, the welding of the weld 1.4 mm thick tensile strength between 125 ~ 134 MPa, and Table 2 shows the intensity comparison shows the substrate, the substrate strength of weld strength factor 78% to 83% Thus, with the FSW welding method when the alloy sheet has good welding adaptability. As the speed increases seam welding, weld strength increased and then showed a decreasing trend, this is because the rotating speed of 1500 r / min, if the welding speed is too slow, then the effect on the weld heat flow per unit length is large, the welding temperatureis too high, the weld zone of heat, so that the weld strength is reduced; other hand, if the weldingspeed is fast, the role of the weld heat flow per unit length less, the welding temperature is too low, the thermoplastic weld metal flow variation, the weld metal is difficult to diffusion bonding into a whole, may be weakly bound to weld tissue, thereby reducing the weld strength. Therefore, when the welding speed change, there will be a weld strength peaks.2.2 weld microstructure analysisAt a speed of 1500 r / min, the welding speed is 160 mm / min of the welding process, the thickness of 1.4 mm of the LF21 friction stir welds Figure 2 shows a cross-sectional morphology.LF21 sheet friction stir welds also exist four zones: weld nugget zone (referred to WNZ), heat affected zone (referred TMAZ), heat affected zone (referred HAZ) and substrate area (referred to BM), its magnification microstructure shown in Figure 3.Seen from Figure 3, LF21 friction stir weld good, did not produce holes, loose tissue, thermal cracking and other common welding defects. In addition, from Figure 3 also shows that the welding process because the combined effects of heat in different ways, the weld microstructure and properties of the district there is a big difference. In welding frictional heat local temperatureunder the action of nugget zone by mixing head direct mechanical agitation, something completely dynamic recrystallization, forming a fine equiaxed; heat affected zone by mixing head indirect mechanical stirring action, and its Rolling tissue a greater degree of bending deformation and incomplete recrystallization; heat affected zone not subject to mechanical agitation mixing head, but in the welding heat cycle, the occurrence of different degrees of recovery, static recrystallization or grain coarsening phenomenon, in addition, in the heat affected zone, precipitation of manganese in the rolling direction and gathered into coarse brittle MnAl6 compounds.2.3 Weld microhardnessLF21 antirust friction stir welding aluminum alloy sheet microhardness cross section shown in Figure 4.Seen, the weld occurs in the welding process softening softening Chief weld length of approximately 25 mm. Nugget zone average hardness of 56 or so. The minimum weld hardness value appears in the heat-affected zone (TMAZ) and the heat affected zone (HAZ) of the transition zone, the minimum hardness of 52. By the precipitation of manganese, hardness of the weld to fluctuations. In addition, the forward side of the weld hardness change relatively flat, while the back side is more severe changes in hardness.LF21 rust-proof aluminum alloy friction stir weld tensile fracture surface. Section on the distribution of a large number of non-equiaxed dimples, the size of 15 ~ 40 μm. This is becausethe grains are welded rolled sheet after rolling, the occurrence of a large deformation, making dimples having a non-equiaxed be imposed. In addition, a large number of dimples is also distributed small tiny dimples, nearly equiaxed shape, its size is about 5 μm, revealed that the fracture position in the welding process the grain recrystallization occurs. Larger cross section deeper.3 Analysis and discussionLF21 Mn is an element of the main components of the alloy, as the Mn content increased, the tensile strength increases. When the Mn content is 1.0% to 1.6%, the alloy not only has high strength and good ductility and corrosion resistance. From Al-Mn binary phase diagram, the phase (L) between the solidus line and the crystallization interval is small, only a difference of 0.5 ~ 1.0 ℃; Mn in the maximum solid solubility of aluminum is 1.8%, with temperature drop, Mn solid solution in the aluminum matrix in the sharp reduction, which makes the Mn content of 1.0% to 1.6% of the LF21 alloy at room temperature to form a supersaturated solid solution. Also, Mn in the aluminum alloy and because the diffusion coefficient is small, so LF21 alloy prone to precipitation annealing process, to form coarse grains. LF21-rust aluminum above characteristics, determine its organization and friction stir welding performance. The welding thermal cycle under the effect of the weld zone of Mn in the aluminum matrix formed during casting is easy to fine precipitates nucleation, the rolling direction and gathered into a coarser precipitate brittle MnAl6 compound (shown in Figure 3 ( c)), the tensile strength of the weld tensile strength than the base material.4 Conclusionsa. LF21 aluminum alloy sheet using friction stir welding method for welding adaptability. When the rotational speed of 1 500 r / min, the welding speed is 100 ~ 180 mm / min, the availability of better welding performance, the weld tensile strength of 125 ~ 134 MPa, weld strength factor of 78% of the substrate ~ 83%.b. LF21 aluminum alloy sheet FSW organizations exist four areas, namely weld nugget zone, the heat-affected zone, heat affected zone and the base area. 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