Metal Injection Molding (MIM) is an advanced powder metallurgy method that enables the production of dense, complex-geometry parts by mixing metal powders with polymer binders in appropriate proportions to form a homogeneous feedstock, injecting this feedstock into molds, then removing the binder (debinding) and finally sintering [1–3]. Because of its advantages such as high dimensional precision, low machining requirements and high material efficiency, the MIM method is widely used especially in the automotive, electronics, medical and defense industries [4,5].
Fe–Ni-based alloys have a wide range of applications in structural and magnetic applications thanks to their low expansion coefficient, high magnetic permeability, excellent thermal stability and good mechanical properties [6,7]. For this reason, studies on the production of Fe–Ni alloys by the MIM method have increased in recent years. However, the effects of parameters such as sintering temperature, atmosphere and time on density, microstructure and mechanical properties in these alloys still require detailed investigation [8].
1. Effect of Sintering Temperature
Sintering temperature is a decisive parameter for the densification kinetics, pore structure and grain growth of MIM parts. As the temperature increases, atomic diffusion accelerates, the porosity ratio decreases and the material density increases. However, above a certain temperature, excessive grain growth occurs, which causes a decrease in mechanical strength [9].
Hu et al. (2018) produced HK30 stainless steel by the MIM method and investigated different sintering temperatures (1270, 1280, 1300 °C). As a result, the highest density (7.61 g/cm³), a tensile strength of 570 MPa and 31% elongation were obtained at 1280 °C. At higher temperatures, a decrease in mechanical properties was observed due to grain growth and boundary weakening [10]. Similarly, Zhang et al. (2011) reported that the sintering temperature directly affects the microstructural homogeneity in Fe–Ni-based MIM samples and that a single-phase austenitic structure was obtained at around 1350 °C [11].
2. Effect of Sintering Atmosphere
The sintering atmosphere affects the oxide removal, surface reactions and chemical composition of the material. Especially in Fe–Ni alloys, the presence of oxides slows down the diffusion kinetics and prevents the closure of pores [12]. Kearns and his colleagues reported that sintering processes carried out in a N₂ atmosphere gave results close to full density in HK30-type MIM parts, and that nitrogen in the atmosphere slightly increased the microhardness [13]. On the other hand, sintering carried out in vacuum or H₂ atmospheres removed oxides more effectively, resulting in higher density and cleaner grain boundaries [14]. Similarly, the choice of atmosphere is critically important in Fe–Ni alloys; it is stated that especially in sintering carried out under vacuum, the tendency of Ni to form oxides on the surface decreases and diffusion takes place more homogeneously [15].
3. Effect of Sintering Time
Sintering time determines the degree of completion of diffusion. While densification is insufficient at short times, grain growth accelerates at long times. Hu et al. (2018) stated that a 7-hour holding time at 1280 °C gave optimum results; lower density was observed at shorter times, and grain growth and embrittlement at longer times [10].
In Fe–Ni systems, a sintering time of 1–2 hours is generally considered sufficient; it is reported that a relative density of up to 97% can be achieved, in particular, after a 60-minute holding time at 1350 °C in a H₂ atmosphere [16].
4. Previous Studies on the MIM Process in Fe–Ni Alloys
There are various studies in the literature on the production of Fe–Ni alloys by the MIM method. Ma et al. (2013) investigated the magnetic properties of a Fe–50% Ni alloy depending on the sintering temperature and reported that at 1350 °C the coercivity decreased and the permeability increased [17].
In another study titled “Sintering MIM Fe–Ni alloys”, the effect of the Ni ratio on sintering behavior was investigated experimentally; it was observed that low Ni ratios support early-stage densification but slow down late-stage diffusion [18]. In addition, it has also been reported that pre-alloyed Fe–Ni powders offer a more homogeneous microstructure and higher density compared with mixed elemental powders [19].
5. Research Gap and the Aim of This Study
When the literature is examined, although there are studies in which the sintering temperature, atmosphere and time parameters are examined individually within the scope of the MIM process in Fe–Ni alloys, it is seen that a systematic investigation that considers these three parameters together is limited.
For this reason, in this study, the effects of variations in sintering temperature, atmosphere and time on the density, microstructure and mechanical properties of Fe–Ni-based samples produced by the MIM method will be examined with a holistic approach. The results obtained will contribute to determining the optimum sintering conditions in Fe–Ni alloys and to making the MIM process more efficient.
References
[1] Xometry, “All About Metal Injection Molding (MIM),” 2024.
[2] Power-of-Powder Design Guide, ATW Companies, 2021.
[3] German, R. M., Powder Metallurgy and Particulate Materials Processing, MPIF, 2005.
[4] Upadhyaya, G. S., Powder Metallurgy Technology, Cambridge Int. Science, 2002.
[5] Ma, J. et al., Magnetic Properties of Fe–50% Ni Alloy Fabricated by MIM, JMMM, 2013. (This source is also cited in the Fe–Ni properties section.)
[6] Zhang, C., Luo, J., Xiang, R., An Investigation on a New Type of Fe–Ni Alloy in MIM Process, Adv. Mater. Res., 2011.
[7] Hu, Y., Li, Y., Lou, J., He, H., Zhang, X., Effects of Sintering Temperature and Holding Time on Densification and Mechanical Properties of MIM HK30 Stainless Steel, Int. J. Metall. Metal Phys., 2018.
[8] Kearns, M. A., Evaluation of Microstructure and Mechanical Properties of Metal Injection Molded HK30, Sandvik Report.
[9] German, R. M., same source [3], MIM sintering principles section.
[10] Hu, Y. et al., Int. J. Metall. Metal Phys., 2018.
[11] Zhang, C. et al., Adv. Mater. Res., 2011.
[12] German, R. M., or Upadhyaya, G. S., sintering atmosphere section.
[13] Kearns, M. A., Sandvik Report, Metal Powder Report series.
[14] Davies, P. A., Metal Injection Moulding of Heat Treated Alloy 718, Sandvik.
[15] ATMSh, MIM of Soft Magnetic Materials (Fe–Ni), 2022.
[16] Sintering MIM Fe–Ni Alloys, ResearchGate Conference Paper, 2020.
[17] Ma, J. et al., Magnetic Properties of Fe–50% Ni Alloy Fabricated by MIM, JMMM, 2013.
[18] Sintering MIM Fe–Ni Alloys, ResearchGate Conference Paper, 2020.
[19] ATMSh, MIM of Soft Magnetic Materials (Fe–Ni), 2022.