A study of molybdenum addition on W-Ni-Fe based heavy alloys sintered with spark plasma sintering

BULLETIN OF THE POLISH ACADEMY OF SCIENCES TECHNICAL SCIENCES, Vol. 67, No. 2, 2019 DOI: 10.24425/bpas.2019.128609 A study of molybdenum addition on W-Ni-Fe based heavy alloys sintered with spark plasma sintering B.S.L. PRASAD and R. ANNAMALAI * Vellore Institute of Technology, Vellore Campus, Tamil Nadu 632014, India Abstract. Tungsten heavy alloys comprising tungsten, nickel and ferrous were modified, where molybdenum was added in varying weight proportions keeping the ratio of Ni: Fe (8:2) constant. The powders were mixed in a high-energy ball mill and were further fabricated using the spark plasma sintering (SPS) method at a peak temperature of 1000°C with heating rate of 100°C/min. The details of the microstructure and mechanical properties of these various alloy compositions were studied. With the increasing weight composition of the Mo in the alloy, the relative density of the alloy increased with a significant improvement in all the mechanical properties. The yield strength (YS), ultimate tensile strength (UTS) and hardness improved significantly with increase in the proportion of Mo; however, a reduction in elongation percentage was observed. The maximum strength of 1250 MPa UTS was observed in the alloy with a Mo proportion of 24%. The heavy alloy unmixed with Mo has shown distinct white and grey regions, where white (W) grain is due to tungsten and grey region is a combinatorial effect of Ni and Fe. Upon addition of Mo, the white and gray phase differences started to minimize resulting in deep gray and black ‘C’-phase structures because of homogenization of the alloy. The main fracture mode found during this investigation in the alloys was inter-granular mode. Key words: tungsten heavy alloys, sintering, mechanical properties, fracture modes. 1. Introduction The current research is focused on developing the heavy alloys with enhanced mechanical properties, inter-relationship, and microstructures. There is a need for alloys with improved and enhanced ballistic properties, and with kinetic energy penetration ability for defeating the armor materials which secure and protect the battle tanks [1]. Tungsten heavy alloys are generally adopted in parts production, i.e. in production of parts which are typically exposed to high temperatures such as in welding electrodes, and to withstand heavy wear damages such as in bearing bars, extension shacks, grinding and turning spindles etc. The other advantages of the tungsten heavy alloys include their ability of shielding and collimating X-rays and gamma radiations, high electric and thermal conductivity, vibration dampening and resistance to corrosion. The alloys that are used for commercial purposes such as manufacturing of above mentioned parts, generally contain about 90‒98% tungsten and other remaining 2‒10% as nickel in combination with either iron or copper as the binding phase. Other applications of the tungsten alloys include usage in making inertia kits, in producing counter-balance weights for platforms in air-crafts, in production of cranks of race cars, making ammunition, building armor with anti-piercing capabilities, producing medical and sports equipment etc [2‒6]. Tungsten exhibits highest melting point of all the heavy metals at 3422°C. Hence, the production of alloys containing tungsten has become essential and are syn*e-mail: Manuscript submitted 2017-12-06, revised 2018-04-06, 2018-05-02, 2018-05-14 and 2018-05-19, initially accepted for publication 2018-05-24, published in April 2019. Bull. Pol. Ac.: Tech. 67(2) 2019 thesized using the technique of powder metallurgy. The desired density in the alloy can be achieved by applying liquid phase sintering (LPS) technique in synthesizing tungsten heavy alloys [5, 7‒10]. The production of parts with larger cross-sectional area usually prefers copper (Cu) containing alloys [11]. The hardness of the Cu containing alloys is higher compared to Fe added alloys. The main difficulty is sintering the alloy to full density. The significant advantage of alloys which contain copper is that they are non-magnetic in nature. Using any of the mixing method of powder the elemental metal powders can be easily mixed [12]. During the process of sintering, when the powders of metals are compressed under heat, the temperature of hydrogen for the cleaning purpose is raised to 1000°C not considering the effect of atmosphere. The time taken for sintering varies from few minutes to several hours depending upon size of the final product. When the alloys are shaped in molds, the sintering procedure helps the alloys to be free of carbon remnants and lubricants. The most significant role in production of alloys is played by sintering [13, 14]. In an ideal W-Ni-Fe alloy, the binding rate between the Ni and Fe should be in the ratio of 1.5 and 4. The decrease in the Ni content from this ratio (Ni:Fe – 1.5:4) would make the alloys brittle and not fit for commercial use. Although heating of the metals involved in sintering can compress the metallic powders, it is not enough to bind them properly into one alloy. An increase in the binding rate of metals during sintering will result in proper binding of different metals into one alloy [15, 16]. The tungsten heavy alloy synthesized by liquid phase sintering technique has two phases, particulate body centered cubic (BCC) lattice and continuous face centered cubic (FCC) lattice matrix. Therefore, the chemistry and mechanical properties of the alloy are influenced by thermo-mechanical processing [17]. To improve the 167 B.S.L. Prasad and R. Annamalai mechanical properties of a tungsten heavy alloy, the addition of different metals is done. For example, metals like cobalt (Co), molybdenum (Mo), tantalum (Ta), rhenium (Re) are chosen to improve the mechanical and structural properties of a classical tungsten heavy alloy; resulted in increased strength of the alloy [18‒23]. However, not at all times, the addition of metals has influenced in a positive way. For example, addition of chromium (Cr) had a negative effect on the properties of the conventional tungsten heavy alloy. The reason being, Cr increased the porosity of the alloy by forming inter-metallic barriers. The addition of the Co has been widely used in researches as its addition improves both the strength and the ductility [24]. Upon analyzing the available data and information, Mo and Co were found ideal in making new tungsten based heavy alloys (W-Ni-Fe) with 90% W. The production of the alloys was done using the liquid phase sintering (LPS), followed by swaging for attaining greater strengths exceeding 1500 MPa. These alloys undergone further heat treatment involving the treatment of solution in atmosphere and in partial vacuum at 100‒1200°C finally followed by quenching. The main purpose of the heat treatment here was imparting ductility by decreasing the formation of inter-metallic barriers and eliminating the segregation of the various interfaces leading to the impaired p (...truncated)