Phase composition and magnetic properties of (Sm,Zr)Fe₁₁Ti magnets produced by selective laser melting

Highlights

• The first (Sm,Zr)Fe₁₁Ti magnets were produced by the Selective Laser Melting.

• The coercivity of 3D-printed magnet based on (Sm,Zr)Fe₁₁Ti phase reached 3.0 kOe.

• Zr stabilize (Sm,Zr)Fe₁₁Ti phase during 3D-printin.

Abstract

Additive manufacturing is a perspective method to create new functional materials, e.g., permanent magnets. This work presents a proof-of-concept of additive manufacturing of (Sm,Zr)Fe₁₁Ti permanent magnet. A way to produce permanent magnets from (Sm,Zr)Fe₁₁Ti powder and its mixture with low-melting additive Sm₇₅(Cu,Co)₂₅ by the selective laser melting is demonstrated. The phase transformations which accompany the liquid phase sintering of hard magnetic particles in low-melting Sm₇₅(Cu,Co)₂₅ additive are discussed. The printing parameters which allow sintering of the hard magnetic alloy particles are found. When the main phase is (Sm,Zr)Fe₁₁Ti, the coercivity of H_c = 3.0 kOe is achieved.

Introduction

Sm(Fe,T)₁₂ compounds (where T = Ti, V, Cr, Mo, Si, Zr, etc.) discovered by Ohashi et al. [1] are considered to be promising for new class of permanent magnets. In the temperature range of 100–400 ℃, such compounds have higher intrinsic magnetic properties (the anisotropy field H_a and saturation magnetization M_s) than those of Nd₂Fe₁₄B [2]. In addition, essential advantage of Sm(Fe,T)₁₂ compounds is the minimum rare-earth metal content in comparison with other rare-earth compounds which are considered for permanent magnets manufacturing, i.e., 7.7 at.% vs 11.8 at.% in R₂Fe₁₄B and 10.5 at.% in Sm₂Co₁₇. The Sm(Fe,T)₁₂ compounds do not contain expensive rare-earth metals and cobalt. However, the industrial production of bulk magnets from the Sm(Fe,T)₁₂ compounds is complicated. The main disadvantage is the instability of the SmFe₁₂ phase in bulk. This phase requires stabilizing elements such as 3d- and 4d- transitional metals, or some other elements [3]. However, such additions significantly decrease spontaneous magnetization. The second problem is high coercivity state. The typical anisotropy field of Sm(Fe,T)₁₂ falls in the range 70–100 kOe [4], [5]; however, the coercivity reaches 4–6 kOe and 6–11 kOe in melt-spun ribbons with T = Ti [6], [7], [8], [9], [10], [11] and T = V [12], [13], respectively. In chemically synthesized single crystalline powders, it reaches 12.6 kOe [14]; the highest value of 13.2 kOe is obtained in Sm-Fe-Ti films which have a paramagnetic interlayer between crystallites [15]. Presently, there is a certain progress in production of permanent magnets with the main SmFe₁₂ phase. Thus, in [16], permanent magnets with the coercivity 4.6–5.4 kOe were produced from (Sm_0.8Zr_0.2)_1.05­1.10(Fe_0.9Co_0.1)_11.3Ti_0.7 powder by the spark plasma sintering. The V addition allowed production of anisotropic sintered magnets with H_c = 8.4 kOe [17]. This trend suggests future competition of these magnets with Nd-Fe-B magnets. The high-resolution transmission-electron microscopy demonstrated that V addition favors formation of the intergranular layer enriched with V and Sm; whereas, in the absence of V, grains are in direct contact [12], [17]. In [18], it has been shown for thin magnetic films that the grain boundary infiltration increases the Sm(Fe,T)₁₂ coercivity. The significant increase in coercivity by 30–80% has been revealed in samples with layers of Cu, Sm-Cu, Cu-Ga, Mg-Zn, Sn-Zn, and Al-Zn.

The appearance of the high-coercivity bulk permanent magnets let us believe in future discovery of new methods of permanent magnets production. Current trend in additive manufacturing is to produce functional materials, such as permanent magnets, rather than constructional ones [19], [20], [21], [22], [23]. Additive manufacturing has a number of advantages over traditional technologies, i.e., any shape of parts in small series production, minimum waste due to reuse of materials [24], shorter production cycle, and local properties tuning. The latter is of the most importance, because no other method is able to provide it even in principle. This includes the intrinsic magnetic properties tuned by varying the chemical composition, degree of crystallographic texture, crystallite spatial orientation, coercivity, etc. Among the disadvantages of additive manufacturing are low production capacity and its high cost. Thus, the approaches proposed complement the existing ones, rather than replace them.

Since the Sm(Fe,T)₁₂ compounds have higher temperature stability than that of Nd₂Fe₁₄B, the use of organic binding with low degradation temperature in additive manufacturing is inappropriate. The selective laser melting (SLM) additive technology is considered to be highly promising for production of permanent magnets and is successfully employed in production of Nd-Fe-B and Sm-Co permanent magnets [19], [20], [22], [25]. One of the essential problems of this technology is that all key printing processes that affect the structure and properties of magnets are non-equilibrium. The cooling rate of the melt in the course of the SLM is lower than that upon melt-spinning and can be considered to be of the same order as the strip-casting cooling rate [26], [27]. The latter, however, results in a qualitatively different structure of the bulk magnet.

To create a high-coercivity state of magnets, the mode of additive manufacturing can be tuned to ensure minimal changes in the structure and coercivity of samples. That is why, in this work, two methods of additive manufacturing of single-layer samples were studied, i.e., from the main SmFe₁₁Ti phase powder and from the mixture of the main-phase and low-melting additive.

The aim of this work is a proof-of-concept of the SLM production of bulk permanent magnets from Sm(Fe,Ti)₁₂.