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18REACTIVE ROUNDED Ti/Al COMPOSITE POWDERS PRODUCED BY HIGH-ENERGY BALL MILLING FOR SELECTIVE LASER MELTING TECHNOLOGY
A. A. Nepapushev", D. O. Moskovskikh", V. S. Buinevich", S. G. VadchenkoA, and A. S. Rogachevfi
aCenter of Functional Nano-Ceramics, National University of Science and Technology MISiS, Moscow, 119049, Russia
bMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, Moscow, 142432, Russia
DOI: 10.24411/9999-0014A-2019-10107
The modern additive manufacturing (AM) is an efficient way to produce complex-shape [1], functionally-graded [2] or non-standard parts [3] for various engineering applications. Currently, the AM is not confined to the rapid printing of prototypes but allows to produce real-scale fully functional parts. For example, selective laser sintering (SLS) was initially developed for the production of polymer prototypes by the point-by-point scanning technique. Afterward, due to the application of more potent heat sources, this approach was widened to include metallic and ceramic powders. Porous parts fabricated by SLS are now widely employed in biomedicine.
Currently, pure metals are rarely used in the AM due do their low mechanical properties and resistance towards oxidation and corrosion. Application of pre-alloyed Ti, Ni, and Fe-based powders increases significantly the properties of produced parts. A new perspective direction for the development of AM lies in the application of reactive composite precursor powders. In this approach, a mixture of components with a relatively low melting point is used as the feedstock, and the mixture reacts during the SLS or SLM to form a more refractory compound. If the reaction is exothermic, the heat produced by the reaction is added to that produced by the laser heating. This widens the possibilities of SLM and allows to produce more refractory materials and products. However, the production of the composite reactive particles with the required spherical shape is a complicated problem due to the difference in the mechanical properties of the reagents and impossibility of application of the different atomization methods. This problem can be solved using the mechanical structuring methods, for example, high-energy ball milling (HEBM), but it necessitates the optimization of the mechanical treatment regime for the production of spherical powders.
In this work, we investigated the possibility of the production of reactive composite powders in the Ti-Al system with various morphology especially with rounded particles with high flowability, which is important for use in SLM technology, by HEBM. This system is interesting because the exothermic reaction in it can be initiated at the relatively low temperature of 650-700°C, moreover, this temperature can be further decreased by using mechanical activation. For the HEBM initial Ti and Al in the chosen compositions were loaded in the jars along with the milling bodies. HEBM treatment was performed using an Activator 2S device in the following conditions: working volume of the vial of 250 ml, ball to mixture mass ratio of 20:1, the rotation speed of the planetary disc of 200 rpm. 2-mm and 6-mm steel balls were used as the milling bodies. HEBM was conducted in the argon atmosphere (4 atm).
During our previous studies of the influence of HEBM regime on the structure and properties of various powder mixtures [4, 5] we discovered that high-velocity milling does not allow to produce spherical particles. Therefore, in this study, we decreased the disc rotation speed to transition from the dynamic impact and attrition regime to a "softer" regime, characterized by
iSHS 2019
Moscow, Russia
rolling of balls against one another and the vial walls. As a result, the treatment of Ti + Al mixture at disc rotation velocity of 200 rpm and K = 1.0 using balls with varied diameter leads to a considerable change in the morphology of produced powders (Fig. 1). After 60-min activation, the particles shape becomes more rounded. This process is largely influenced by the diameter of the milling bodies. If 6-mm balls are used, the treated powders have a size of 300-400 |im (Fig. 1a); a decrease in the balls size to 2 mm leads to the formation of particles with a size of 100-150 ^m. Two fractions of powders can be observed: the first fraction consists of relatively coarse and rounded particles, while the second fraction is comprised of finer (30-50 |im) and irregularly shaped particles (Fig. 1b). The increase in the HEBM duration to 120 min leads to the formation of more pronounced layered structure and more spherical shape of particles (Fig. 2). More detailed study of the internal structure of the powders revealed their composite structure, in which the titanium (Figs. 2c, 2d, bright phase) is embedded in the aluminum matrix (Figs. 2c, 2d, dark phase). Therefore, the adjustment of the HEBM regime, in particular of the disc rotation velocity, allows one to influence to a great extent the morphology of produced composite powders.
(a) (b)
Fig. 1. Ti + Al powder after 60 min of HEBM: (a) 6-mm balls; (b) 2-mm balls.
(c) (d)
Fig. 2. Ti + Al powder after 120 min of HEBM: (a) 6-mm balls; (b) 2-mm balls and (c-d) corresponding cross-sections.
Powder flowability is important for the AM since it defines whether the deposited powder layers will be uniform and reproducible. In this work, the flowability was determined by the direct measurement of the time needed for the 50 g of powder to flow through a funnel with a diameter of 2.54 mm (Table 1). For the mixtures treated using 6-mm balls, an increase in the treatment duration to 120 min leads to the improved flowability due to the more spherical shape of the powders as compared to 60-min treatment. In the case of the mixtures treated using 2-mm balls, the flowability decreases after 120 min, which might be related to a decrease in the particles size and an increase in their specific surface. The Ti + Al mixture prepared in a mortar does not flow (no flowability).
Table 1. Flowability of Ti + Al powder after HEBM treatment._
Treatment duration_2-mm balls_6-mm balls
0 min Does not flow
60 min 56.7 s 87,5 s
120 min 61.7 s 81.25 s
In mixtures after 60 and 120 min of HEBM were initiated the reactions, the ignition temperature of the treated mixture is 640-650°C, which is near to the melting temperature of Al. The microstructure of the powders after 60-min HEBM at 200 rpm and subsequent combustion synthesis was investigated. The morphology and size of the particles after the SHS coincide with the composite particles produced by HEBM. EDS results show that the conversion was incomplete, and the reaction resulted in the formation of a composite comprised of Ti and TiAl3.
Thus, for mixtures of elementary Ti and Al powders regimes of high energy ball milling in a planetary mill, that allow obtaining rounded reactive composite particles of Ti/Al, were found. The produced powders are characterized by flowability sufficient for the application in the SLS and SLM processes. In the produced composite powders, the combustion is initiated at ~ 660°C and Ti-TiAl3 composite is formed as a result of combustion.
This work was supported by the Ministry of Science and Higher Education of the Russian Federation in the framework of the Federal Target Program "Research and Development on Priority Directions of the Scientific and Production Complex of Russia for 2014-2020", agreement no. 14.587.21.0051, project RFMEFI58718X0051.
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