This issue of 3D Science Valley reveals that the crack growth curve of Scalmalloy is similar to that of aluminum alloys AA5754 and AA606 1-T6 which are widely used in automobile industry and marine ships.
3D science valley
Developing high strength aluminum alloy
According to Academician Wu Xinhua, the application of high-strength 3D printed aluminum alloy in aerospace manufacturing is particularly important. The main goal is to reduce the weight of spacecraft and shorten the delivery cycle, thus reducing the comprehensive production cost and improving the comprehensive benefits. High-strength 3D printed aluminum alloy can reduce the weight of spacecraft parts by 20-90% and shorten the processing cycle by 3- 12 months. Typical applications include: satellite RF array antenna bracket, coupling shock absorber, various bracket components in space station, such as guide rail bracket, measurement and control antenna bracket, etc.
Whether welding process or selective laser melting process, the causes of thermal cracks are similar. In both cases, process parameters will cause thermal stress, which is the key factor to cause cracks. However, it is difficult to control thermal stress by controlling process parameters. In order to significantly reduce the thermal stress, it is necessary to significantly reduce the temperature gradient, but it is impossible to achieve this goal by changing the process parameters or environment in the selective laser melting process. In the process of heat treatment, the alloying elements used to produce strengthening phase usually increase the solidification temperature range, which is also very unfavorable in previous studies.
In recent years, the research focus of materials science has gradually turned to the development of new high-performance alloys suitable for the unique conditions of LPBF process. The alloy design concept is usually based on high cooling rate (105- 106 K/s) and extremely high temperature gradient (G ~ 106 K/m), which improves the solid solution limit of the alloy and promotes the formation of metastable phase. However, in the process of 3D printing, high temperature gradient usually causes the columnar crystal structure to elongate along the structural direction, thus promoting the occurrence of thermal cracks.
Control crack
The known Scalmalloy has mechanical properties and crack growth curve similar to AA7075-T735 1 (an aluminum alloy widely used in traditional aircraft). In this paper, the crack propagation behavior of 3D Science Valley will be compared with that of aluminum alloy AA5754 widely used in ships, automobile bodies and infrastructure related to chemical plants, and AA606 1-T6 widely used in light aircraft, bicycle frames and motors.
See table 1 for the comparison of mechanical properties of the three alloys:
Table 1. Comparison of yield stress, ultimate strength and failure strain between Scalmalloy and two widely used aluminum alloys; σy and σult values of AA5754 vary with tempering procedures.
After heat treatment, the mechanical properties of LPBF and Scalmalloy based on powder layer selective laser melting metal are better than those of AA5754 and AA606 1-T6. But compared with AA5754 and AA606 1-T6, what is the crack propagation performance of da/dN and δ K curves related to Scalmalloy?
It has been seen that the crack propagation in Scalmalloy is similar to that in AA7075-T735 1 alloy. We can see the consistency between the long crack and the "short crack" curve of AA5754. We also see the similarity between R = 0. 1 curves related to Scalmalloy, AA5754 and AA606 1-T6.
Figure 1. R = 0. 1 da/dN and δ K curve of Scalmalloy, short crack propagation.
Figure 2. R = 0.7 da/dN and δ k curve of scalmalloy.
Fig. 2 shows the curves of high R ratios da/dN and δ k for cases d) to g), in which the similarity between the curve of R = 0.7 related to Scalmalloy and the high R ratio tests related to AA5754 and AA606 1-T6 is seen again. Figures 1 and 2 also show that the R = 0. 1 and 0.7 da/dN and δ k curves of small cracks related to AA7050-T745 1 are also consistent with the corresponding long cracks related to crack propagation in Scalmalloy. This means that the small crack curve related to AA7050-T745 1 can be (approximately) regarded as an extension of the power relationship between da/dN and δ K..
Figure 3 shows that various da/dN curves are very similar to δ κ curves when considering different fatigue thresholds and toughness. Table 2 shows the constant values used in various tests of AA5754 and AA606 1-T6 in Figure 3. To help compare Scalmalloy with these different aluminum alloys, Figure 3 also contains the trend line of Scalmalloy.
The growth of cracks, that is, the fastest growing crack that determines the service life of aircraft, can be estimated by equation. It is found that the crack growth curve of Scalmalloy made of additives is similar to that of commonly used aluminum alloys AA5754 and AA606 1-T6, which are recognized as having good fatigue properties. This discovery enhances the potential of Scalmalloy. Compared with AA5754 and AA606 1-T6, Scalmalloy has excellent mechanical properties, which can be used to manufacture additive parts for ships, light aircraft and automobiles. Manufacturers can replace parts and military aircraft parts with aluminum alloys, as well as light aluminum parts for satellites and space structures.
New materials and technologies
For a long time, only a few Al-Si-based casting alloys have achieved crack-free processing in 3D printed aluminum alloys. For forged aluminum alloy with poor weldability, high thermal gradient will promote the growth of columnar crystals, resulting in thermal cracks, so the application of additive manufacturing of forged aluminum alloy is greatly limited.
According to the market observation of 3D Science Valley, this restriction is being broken. Since 20 19, the commercialization of high-strength aluminum alloy 3D printing materials has opened a brand-new door for parts processing that must be realized by forging. Combined with the design freedom of 3D printing, the additive manufacturing technology of forged aluminum alloy will gain huge market space in the fields of pressure vessels, hydraulic manifolds, supports and high-strength structural parts.
YSZ+606 1 aluminum alloy
At present, adding a certain amount of yttrium stabilized zirconia (YSZ) can induce grain refinement, change the microstructure of 3D printed 606 1 aluminum alloy, and thus eliminate hot cracks.
There are two ways to reduce cracks in forged aluminum alloy products processed by additive manufacturing technology. The first method is to control thermal stress in the printing process. The second method is to enhance heterogeneous nucleation by changing the alloy composition or adding nucleating agent directly into the basic powder.
Zirconium-based nano-grain nucleating agents +7075 and 606 1 aluminum alloy
According to the market observation of 3D Science Valley, there is also a high-strength 3D printing forged aluminum alloy material which also adopts the method of adding zirconium-based nucleating agent to realize grain refinement and eliminate cracks. This material is a high-strength 7A77.60L aluminum powder developed by HRL Laboratory for 3D printing, and has been put into the market. Zirconium-based nano-particle nucleating agent was selected in HRL laboratory and compounded into 7075 and 606 1 series aluminum alloy powders. The formed material is crack-free and equiaxed (that is, the length, width and height of the grain are roughly equal), and the fine-grained microstructure is realized, and the strength of the material is equivalent to that of forged material. The average yield strength, ultimate strength and average elongation of this 3D printed aluminum alloy material are as high as 580 MPa, 600 MPa and 8% respectively.
Al-Mn-Ti-Zr alloy
In the previous sharing of 3D Science Valley, a low-cost, Sc-free and widely used Al-Mn-Ti-Zr alloy specially developed for LPBF process was also proposed in the scientific research field. The alloy is intended to be used as a substitute for AlSi 10Mg, and has a similar wide range of applications. By using high solidification rate, a large amount of unconventional Mn(3.7±0.5 wt%) was metastably solidified in α-Al matrix, which significantly promoted solution hardening (about 104 MPa 37% yield strength share). The yield strength, ultimate tensile strength and elongation at break of the final samples are 284±3 MPa, 320±65438±0 MPa and 65438 0.2% respectively. This new alloy has a bimodal microstructure, which consists of alternating fine equiaxed and coarse columnar grain regions.
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Only by knowing deeply can we go far. 3D Science Valley provides the industry with an in-depth observation of additive and intelligent manufacturing from a global perspective. For more detailed analysis of 3D printing in application fields and in-depth understanding of supply chain analysis of aluminum metal market, 3D printing of aluminum metal, printing technology, modeling, simulation, patents, aluminum alloy and high-strength aluminum alloy composite materials, please go to the White Paper on 3D Printing of Aluminum Metal 1.0 published by 3D Science Valley.
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