Added Manufacturing (AM), creates digitally designed parts by continuously adding materials. However, due to the inherent thermal cycling characteristics of additively manufactured metal parts, it is almost inevitably affected by the spatially dependent inhomogeneity of phase and mechanical properties, which can lead to unpredictable failures.
Here, researchers from Nanjing University of Technology , Chongqing University , University of Queensland, Australia and other units have demonstrated a collaborative alloy design method to overcome the problem of laser powder bed fusing to make titanium alloys. The relevant paper was published in Nature Communications under the title "Designing against phase and property heterogeneities in addiTively manufactured titanium alloys". This issue of the Valley column will share the new alloy design methods reported in this scientific research result.
Paper link:
https://www.nature.com/articles/s41467-022-32446-2
Research background
Unlike traditional metal manufacturing processes (such as casting and processing), additive manufacturing (AM) builds digitally designed parts layer by layer by layer by melting raw materials (such as powder or wires) using high energy (such as lasers, electron beams or plasma arc ). This unique feature of the additive manufacturing process in
is a double-edged sword. On the one hand, it provides the possibility to produce ideal shapes, microstructures and properties that cannot be achieved by traditional manufacturing methods. On the other hand, the steep thermal gradients inherent in additive manufacturing, high cooling rates, and complex thermal history often lead to pores, elemental segregation, uneven phase distribution in columnar grains and tissues—whether during solidification or subsequent solid-state phase transition—this results in uneven mechanical properties at different locations of the construction of metal parts.
controls and/or alloy compositions of process parameters and/or problems related to pores, elemental segregation and columnar grains have been effectively solved. However, as phase inhomogeneity occurs almost inevitably in alloys that undergo solid phase transitions after solidification of additive manufacturing, achieving uniform mechanical properties remains a long-term challenge. This phenomenon is more evident in additive manufacturing metal parts with complex geometry , which contain different areas of response to mechanical loads, resulting in unpredictable service failures.
Ti-6Al-4V alloy is a typical alloy that exhibits phase spatial variation along the construction direction during additive manufacturing. During the additive manufacturing process, such as the melting of the laser powder layer (L-PBF) (Fig. 1a), after the first layer solidifies, Ti−6Al−4V undergoes a solid state β (body-center cubic tissue)→α′ (hexagonal closed-row tissue) martensite transformation occurs due to the fast cooling rate. With the addition of the continuous layer, the initially formed needle-shaped α'martensite is decomposed into sheet-like (α+β) tissue under thermal cycle (Fig. 1a). Therefore, the microstructure of Ti−6Al−4V prepared by L-PBF is spatially dependent in the construction direction, forming needle-shaped α′ martensite on the top surface, while partial or completely stable lamellar (α+β) tissue is formed in the lower part. This gradient phase distribution was also confirmed by scanning electron microscope (SEM) (Fig. 1b) and X-ray diffraction (XRD).
In order to reveal the influence of phase inhomogeneity on mechanical properties, the researchers conducted tensile tests in the vertical and horizontal directions of the Ti−6Al−4V sample prepared by L-PBF. The printed state Ti−6Al−4V exhibits similar strength but highly dispersed plasticity in both directions (Fig. 1c). In particular, the tensile ductility (from tensile extension to failure) in the horizontal direction varies significantly, ranging from 9.4% to 17.6%, with the top surface being the smallest. Microstructure analysis reveals that spatial phase distribution is the most likely cause, where highly dispersed ductility is observed. This observation is also consistent with the usual view that needle-shaped α'martensite has a lower ductility than the sheet tissue (α+β) due to its inability to resist cracks.
Figure 1 Comparison of the structure and tensile properties of Ti−6Al−4V and newly developed laser bed molten (L-PBF) alloy (25Ti−0.25O).
In the past decade, a large number of studies have been conducted from both process control and alloy design to address the elimination of unwanted α′martensite in the preparation of Ti-6Al-4V in L-PBF additives.The former strategy usually involves manipulating the thermal cycle of L-PBF to trigger constitutive heat treatment (IHT), which facilitates in situ decomposition of martensite. However, due to the limited or lack of thermal cycles experienced by the top layer, needle-shaped α'martensite can only partially decompose or even retain. Therefore, phase inhomogeneity along the construction direction cannot be eliminated. Although heat treatment often homogenizes the tissue after additive manufacturing, unfortunately it extends the production cycle , affecting the effectiveness of the additive manufacturing process. Therefore, it is very desirable to first eliminate phase inhomogeneity. In addition, in situ alloying of Ti-6Al-4V with a beta-stabilizing element (such as Mo) can form a complete beta phase by a single powder, thereby achieving high ductility (although at the expense of strength loss). However, the resulting unmelted particles or significant elemental segregation may lead to problems of uneven mechanical properties and non-repeatability.
alloy design method for eliminating non-homogeneity
Here, researchers demonstrated a collaborative alloy design method. By combining the addition of commercial pure titanium (CP-Ti) powder and Fe2O3 nanoparticles to the Ti-6Al-4V raw materials, phase inhomogeneity can be eliminated in situ in titanium alloys produced by L-PBF. In a strong contrast to Ti−6Al−4V (Fig. 1b), it shows significant phase changes along the building direction, and the newly designed alloy, for example, added 25 wt % CP−Ti and 0.25 wt % Fe2O3 alloy (hereinafter referred to as 25Ti−0.25O, other newly developed alloys are represented in the same way) show uniform sheet (α+β) microstructure throughout the processing section at the intensity level compared to Ti−6Al−4V (Fig. 1d). This uniform microstructure leads to uniform tensile properties in both vertical and horizontal directions (Fig. 1e). Further research shows that this alloy design method is suitable for geometrically complex components, where uniform sheet (α+β) structure can also be achieved.
Figure 2 Raw material preparation and characterization.
Figure 3 Mechanical properties of newly developed L-PBF alloy.
Figure 4 atomic probe chromatography (APT) characterization of newly developed alloys.
summary
In summary, the researchers designed and manufactured a series of titanium alloys with excellent tensile properties without significant mechanical inhomogeneity. Researchers have shown that through reasonable alloy design, typical and undesirable phase inhomogeneities (related to the thermal cycle inherent in additive manufacturing) can be eliminated. The key to this method is the distribution of alloy elements in phase decomposition, which is a common feature of solid phase transition in metal materials.
Researchers expect that the newly developed titanium alloy can become a candidate material that requires uniform mechanical properties of titanium alloys. This requires a comprehensive evaluation of other mechanical properties (such as fatigue and creep resistance) and corrosion resistance. Furthermore, unlike previous studies, which focused primarily on grain refinement (through alloy design) and/or defect control (through machining optimization), the work shows that solving phase inhomogeneity is equally important in obtaining the desired uniform mechanical properties. Due to phase inhomogeneity caused by solid-state thermal cycling, it has been reported in a variety of metal materials prepared by different additive manufacturing techniques, and this design strategy is expected to help develop other additive manufacturing metal alloys with uniform mechanical properties.
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