Selective laser melting forming method for modified 7075 aluminum alloy composite powder

Abstract

The invention provides a selective laser melting forming method for modified 7075 aluminum alloy composite powder, and belongs to the technical field of special materials for aluminum alloy additive manufacturing. According to the 7075 aluminum alloy, grain refinement is achieved under the action of the Ti2AlNb powder, the cracking tendency of the alloy is greatly reduced, meanwhile, the mechanical property of the 7075 aluminum alloy is further improved, the 7075 aluminum alloy is more suitable for the SLM process, and the material system of the additive manufacturing aluminum alloy is enriched. Experimental results show that the yield strength of the Ti2AlNb / 7075 aluminum alloy prepared after SLM forming can reach 318.28 MPa, the tensile strength can reach 380.96 MPa, the ductility can reach 6.43%, the density can reach 99.21%, and the defects of SLM cracks, pores and the like of the 7075 aluminum alloy are obviously eliminated.

Description

Technical Field

[0001] The invention relates to the technical field of special materials for aluminum alloy additive manufacturing, and in particular to a laser selective melting molding method for a modified 7075 aluminum alloy. Background Art

[0002] Selective laser melting (SLM), as a metal additive manufacturing technology (AM), also known as metal 3D printing technology, is one of the important methods of metal additive manufacturing. In the SLM forming process, metal powder is fully melted and quickly cooled under the action of a high-energy laser beam, and can form dense parts with good metallurgical bonding in one go, which has great advantages in the manufacturing and rapid development of precision and complex special-shaped structural parts. Compared with traditional cutting processing methods, SLM can improve the utilization rate of raw materials. 7xxx series aluminum alloy (Al-Zn-Mg-Cu series) is a typical high-strength aluminum alloy that can be hardened by aging. It has the characteristics of low specific gravity, high specific strength, and strong corrosion resistance. It is one of the indispensable lightweight structural materials in the fields of aerospace, national defense and military industry.

[0003] However, high-strength aluminum alloys including 7075 have high alloying element content, wide solidification temperature range, poor welding performance, and are prone to periodic thermal cracks and coarse columnar crystal poor organization during SLM forming, which greatly limits the application of SLM forming. Even in the Al7075 samples formed under the optimal SLM process conditions, there are still a large number of cracks and pores, and such cracks and pores cannot be eliminated by optimizing process parameters. Cracks will seriously weaken the bearing capacity and corrosion resistance of the formed components, and cause stress concentration during the use of the components, thus becoming a source of fracture; pores will seriously reduce the density of the formed parts and greatly reduce the performance of the formed parts. Inhibiting the formation of thermal cracks and the growth of coarse columnar crystals during solidification by appropriate grain refinement methods to improve the comprehensive mechanical properties of high-strength aluminum alloys and composite materials prepared by SLM has become a research hotspot in recent years. Based on this, the present invention provides a laser selective melting forming method for a modified 7075 aluminum alloy that can effectively suppress the cracking problem of high-strength aluminum alloys during SLM forming and obtain high mechanical properties. Summary of the invention

[0004] The purpose of the present invention is to provide a method for laser selective melting of modified 7075 aluminum alloy composite powder. By in-situ generating Al3(Ti, Nb) and Ti4Nb particles during the selective laser melting process, not only the cracks and pores of 7075 aluminum alloy can be improved, but also its mechanical properties can be improved.

[0005] The solute growth restriction theory believes that in addition to strong nucleating agents, solutes with good segregation ability are also crucial to the grain refinement of aluminum alloys. The growth restriction ability of typical elements in aluminum alloys on the front grains at the solid-liquid interface can be quantified by the growth restriction factor (GFR):

[0006] Where: m is the liquidus slope; k is the solute distribution coefficient; C0 is the solute concentration in the alloy. As shown in Table 1, compared with other elements, Ti and Nb have a strong growth restriction effect.

[0007] Table 1 Growth restriction effect of typical segregation elements in aluminum alloys

[0008] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions: The present invention provides a modified 7075 aluminum alloy. In one or more embodiments, the aluminum alloy is preferably a 7075 aluminum alloy.

[0009] Preferably, the particle size of the 7075 aluminum alloy is 15 to 53 μm.

[0010] In one or more embodiments, the intermetallic compound reinforcement phase is preferably Ti2AlNb powder.

[0011] Preferably, the particle size of the Ti2AlNb powder is 1 to 35 μm.

[0012] In one or more embodiments, the composition of the 7075 aluminum alloy composite powder is as follows: 5.0 wt.% Ti2AlNb powder, and the balance is 7075 aluminum alloy powder. The present invention also proposes a preparation method and a molding method for the composite powder, and the preparation and molding method comprises the following steps: (1) 7075 aluminum alloy powder and Ti2AlNb powder were mixed in a jar mill to obtain composite powder. The rotation speed of the jar mill was 80 r / min and the jar milling time was 3 to 6 hours. The powder jar was vibrated for 10 to 15 minutes every 1 to 2 hours.

[0013] (2) placing the obtained composite powder into a drying oven for drying; the drying oven is evacuated, the drying temperature is 60°C to 80°C, and the drying time is 3 to 6 hours; (3) The dried composite powder is placed in a metal additive manufacturing device for molding to obtain a sample; the preferred molding process parameters are: laser power of 160-280 W, scanning speed of 200-1000 mm / s, scanning spacing of 80-110 mm, powder thickness of 20-30 mm, substrate of aluminum alloy substrate, substrate preheating temperature of 180-200 °C, adjacent powder layer scanning angle rotated 67 °, molding chamber using argon gas, oxygen content less than 100 ppm.

[0014] Preferably, the laser power is 220 W, the scanning speed is 600 mm / s, the powder thickness is 30 mm, the scanning interval is 90 mm, and the substrate preheating temperature is 200°C.

[0015] The beneficial effects of the present invention are: In the present invention, during the selective laser melting forming process, Al3X (X=Ti or Nb) type nucleating agents are generated in situ by Ti2AlNb powder, and Al3X intermetallic compounds are more effective nucleating agents than carbides / borides. Ti2AlNb powder reacts with Al in situ to generate metastable phase L12-Al3Ti, which can promote the non-uniform nucleation of α-Al, further refine the grains to inhibit cracks and improve the mechanical properties. Compared with directly adding Ti nanoparticles, Ti2AlNb powder is not easy to oxidize in the air; at the same time, Al3Nb generated by the in-situ reaction of Ti2AlNb powder and Al has a coherent interface with α-Al, wherein the Nb element in the Al matrix also has a higher high grain growth limiting factor (Q) value, which is easier to improve the mechanical properties of SLM formed 7075 aluminum alloy. In addition to the in-situ generation of Al3X precipitation phase during the printing process, Ti4Nb nanoparticles can also be generated to achieve grain refinement, eliminate thermal cracks, and improve the mechanical properties of 7075 aluminum alloy. Therefore, the present invention provides a SLM7075 high-strength aluminum alloy that not only greatly reduces the cracking tendency of the alloy under the synergistic effect of two precipitation phases and Ti4Nb nanoparticles, but also significantly strengthens the mechanical properties of the 7075 aluminum alloy, making it more suitable for SLM technology and enriching the additive manufacturing aluminum alloy. The experimental results show that the yield strength of the Ti2AlNb / 7075 aluminum alloy prepared by the present invention after SLM molding can reach 318.28MPa, the tensile strength can reach 380.96MPa, the elongation can reach 6.43%, and the density can reach 99.21%, which obviously eliminates the defects of SLM cracks and pores of the 7075 aluminum alloy. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is the powder particle size distribution test diagram of 7075 aluminum alloy; Figure 2The scanning electron microscope powder morphology images of the two powders used in the present invention are as follows: (a) is a scanning electron microscope image of 7075 aluminum alloy powder, (b) is a scanning electron microscope image of Ti2AlNb powder, and (c) is a scanning electron microscope image of Ti2AlNb / 7075 aluminum alloy composite powder; Figure 3 This is an optical microscope cross-sectional morphology image of the Al7075 alloy after SLM forming in Example 1 of the present invention; Figure 4 This is an optical microscope cross-sectional morphology image of the Al7075 alloy after SLM forming in Comparative Example 1 of the present invention; Figure 5 XRD diagrams of Al7075 alloy samples prepared by SLM molding method in Example 1 and Comparative Example 1 of the present invention; Figure 6 It is a stress-strain curve diagram of the Al7075 alloy samples prepared by the SLM forming method in Example 1 and Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0017] In order to make the above features and advantages of the present invention more clearly understood, the following embodiments are given for detailed description. The methods of the present invention are all conventional methods in the art unless otherwise specified.

[0018] The specific composition of the 7075 aluminum alloy powder used in the following examples and comparative examples is shown in the following table.

[0019] Table 2 Chemical composition of 7075 aluminum alloy powder (mass fraction / %)

[0020] Example 1 (1) Weigh out two kinds of powders according to the ratio of 5wt.%TiAlNb+95.0wt.%7075 aluminum alloy for use; (2) The mixed powder weighed in step (1) was subjected to a jar milling treatment, the jar mill speed was 80 rpm, the jar milling time was 6 hours, and the powder jar was vibrated for 10 minutes every 1 hour; then, the mixed powder was subjected to a drying treatment, the drying box was evacuated, the drying temperature was 70°C, and the drying time was 6 hours.

[0021] (3) The composite powder dried in step (2) is subjected to laser selective melting molding: molding process parameters: laser power is 220 W, scanning speed is 600 mm / s, scanning spacing is 90 mm, powder thickness is 30 mm, substrate is aluminum alloy substrate, substrate preheating temperature is 200 ° C, adjacent powder layer scanning angle is rotated 67 °, and the molding chamber uses argon gas with an oxygen content of less than 100 ppm.

[0022] Comparative Example 1 A certain amount of pure 7075 aluminum alloy raw powder is weighed for later use; the remaining technical features are the same as those in Example 1.

[0023] Performance Testing Figure 1 This is the powder particle size distribution test diagram of 7075 aluminum alloy. Figure 1 It can be seen that the powder of 7075 aluminum alloy used in the present invention mainly has D10=22.5 μm, D50=32.6 μm, and D90=51.4 μm.

[0024] Figure 2 SEM powder morphology of the powder used for SLM. Figure 2 (a) is a scanning electron micrograph of 7075 aluminum alloy powder, (b) is a scanning electron micrograph of Ti2AlNb powder, and (c) is a scanning electron micrograph of Ti2AlNb / 7075 aluminum alloy composite powder. Figure 2 It can be seen that the particle size of the SLM powder used in Example 1 of the present invention is relatively uniform, and most of the powder morphology is uniformly spherical.

[0025] Figure 3 This is an optical microscope cross-sectional morphology of the Al7075 alloy after SLM forming in Example 1 of the present invention; the density of the sample prepared by selective laser melting in Example 1 is 99.21%, Figure 3 It can be seen that there are no crack defects inside the prepared Al7075 alloy, only extremely small pores.

[0026] Figure 4 This is an optical microscope cross-sectional morphology of the Al7075 alloy after SLM forming in Comparative Example 1 of the present invention; the density of the sample prepared by selective laser melting in Comparative Example 1 is 95.9%, Figure 4 It can be seen that there are a large number of cracks and pores in the prepared original Al7075 alloy, mainly thermal cracks parallel to the forming direction.

[0027] Figure 5 The XRD diagram of the Al7075 alloy samples prepared by the SLM molding method in Example 1 and Comparative Example 1 of the present invention is obtained by X-ray diffractometer testing. Figure 5 It can be seen that the Al7075 alloy sample prepared by the SLM forming method in Example 1 of the present invention has α-Al phase, Al3Ti phase, Al3Nb phase and Ti4Nb nanoparticles, while Comparative Example 1 has only α-Al phase.

[0028] Figure 6 The Al7075 alloy samples prepared by the SLM forming method in Example 1 and Comparative Example 1 of the present invention were subjected to tensile tests 3 times at room temperature to obtain stress-strain curves. Figure 6It can be seen that the tensile strength, yield strength and elongation of the Al7075 alloy prepared in Example 1 are greatly improved compared with the original Al7075, and its yield strength can reach 318.28 MPa, the tensile strength can reach 380.96 MPa, and the elongation can reach 6.43%; while the tensile strength of the Al7075 alloy prepared in Comparative Example 1 is only 45 MPa, and the elongation is only 0.16%.

[0029] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention should fall within the scope of the present invention.

Claims

1. A laser selective melting method for modified 7075 aluminum alloy composite powder. Features: The components of the 7075 aluminum alloy composite powder are as follows by mass percentage: 5.0 wt.% Ti2AlNb powder and the balance 7075 aluminum alloy powder.

2. The method according to claim 1, Features: The 7075 aluminum alloy powder is in the shape of a solid sphere with a particle size of 15 to 53 μm; the Ti2AlNb powder is in the shape of a solid sphere with a particle size of 1 to 35 μm.

3. The method according to claim 1 or 2, Features: The following steps are involved: The 7075 aluminum alloy powder and the Ti2AlNb powder are put into a pot mill for mixing, the obtained composite powder is put into a drying box for drying, and the dried composite powder is subjected to laser selective melting molding to obtain a molded sample.

4. The method according to claim 3, Features: The rotation speed of the jar mill is 80 r / min, and the jar milling time is 3 to 6 hours; vibrate the powder jar for 10 to 15 minutes every 1 to 2 hours.

5. The method according to claim 3, Features: The drying box is vacuumed, the drying temperature is 60℃~80℃, and the drying time is 3~6 hours.

6. The method according to claim 3, Features: Laser selective melting forming process parameters: laser power is 160-280W, scanning speed is 200-1000mm / s, scanning spacing is 80-110mm, powder thickness is 20-30mm, substrate is aluminum alloy substrate, substrate preheating temperature is 180-200℃, adjacent powder layer scanning angle is rotated 67°, and the forming chamber uses argon gas with an oxygen content of less than 100ppm.

7. The method according to claim 6, Features: The laser power is 220W, the scanning speed is 600mm / s, the powder thickness is 30mm, the scanning spacing is 90mm, and the substrate preheating temperature is 200℃.

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