Investigation of rheological behaviour of 316L stainless steel–3 wt-%TiC powder injection moulding feedstock
Authors/Creators
Description
Highlights of the Study
This study investigated the rheological behavior and moldability of 316L stainless steel–3 wt.% TiC composite feedstocks for powder injection molding (PIM). Using capillary rheometry, the effects of TiC addition, solid volume fraction, shear rate, and temperature on viscosity, shear-thinning behavior, flow activation energy, and overall moldability were systematically evaluated. The results demonstrated that incorporating fine TiC particles can improve the rheological characteristics and moldability of 316L stainless steel feedstocks when appropriate solid loading and processing temperature are selected.
Key findings:
🟢 Composite feedstock design: 316L stainless steel powder was combined with 3 wt.% TiC to prepare composite PIM feedstocks at solid volume fractions of 55 and 60%.
🟢 Particle characteristics: The gas-atomized 316L stainless steel powder was nearly spherical with a median particle size (D50) of 12.6 μm, whereas the TiC particles were angular and considerably finer, with a D50 of 4 μm.
🟢 Binder formulation: The multicomponent binder consisted of 53 vol.% paraffin wax, 30 vol.% ethylene vinyl acetate (EVA), 15 vol.% carnauba wax, and 2 vol.% stearic acid.
🟢 Rheological evaluation: An Instron 3211 capillary rheometer was used to investigate feedstock viscosity over different shear rates and at temperatures of 60, 70, 80, and 90 °C.
🟢 Pseudoplastic behavior: Both the base 316L stainless steel and SS–3TiC composite feedstocks exhibited pseudoplastic or shear-thinning behavior, with viscosity decreasing as shear rate increased.
🟢 Practical flowability: At shear rates around 1000 s⁻¹, the investigated feedstocks exhibited viscosities suitable for injection molding, confirming their practical pseudoplastic behavior within the normal PIM processing range.
🟢 TiC-dependent viscosity: The influence of TiC on feedstock viscosity depended strongly on temperature, shear rate, and solid loading rather than following a single uniform trend.
🟢 Low-temperature flow improvement: At 60 °C and relatively low shear rates, incorporation of 3 wt.% TiC decreased feedstock viscosity, with the effect becoming more pronounced at the higher solid loading of 60%.
🟢 Particle-packing mechanism: Fine TiC particles occupied interstitial spaces between the larger stainless-steel particles, improving packing and releasing more binder to lubricate particle flow.
🟢 Reduced interparticle friction: At constant solid loading, replacement of part of the stainless-steel powder with finer TiC particles reduced particle–particle friction and facilitated feedstock flow.
🟢 Intermediate-temperature behavior: At approximately 70 °C, TiC addition could slightly increase viscosity because the irregular/angular morphology of TiC became influential after interparticle friction in the base SS/binder system was reduced.
🟢 High-shear convergence: At shear rates above approximately 1000 s⁻¹ and temperatures above 70 °C, the influence of TiC addition on viscosity became relatively small for both 55 and 60% solid loadings.
🟢 Flow behavior index: The flow behavior index (n) ranged approximately from 0.15 to 0.75 depending on feedstock composition and temperature.
🟢 Enhanced shear sensitivity: At 70 °C, TiC addition reduced the flow behavior index, indicating greater shear-rate sensitivity and stronger pseudoplastic behavior of the SS–3TiC feedstock compared with the base stainless-steel feedstock.
🟢 Solid-loading influence: Increasing solid loading further decreased the flow behavior index of the composite feedstock, highlighting the role of shear in disrupting agglomerates within highly loaded systems.
🟢 Temperature sensitivity: Feedstock viscosity decreased with increasing temperature, while the extent of temperature sensitivity was characterized through the flow activation energy.
🟢 Activation-energy reduction: Incorporation of TiC reduced the flow activation energy, particularly at high shear rates, indicating reduced sensitivity of viscosity to temperature variations.
🟢 Quantitative activation-energy improvement: At 55% solid loading and a shear rate of 1181 s⁻¹, TiC addition reduced the flow activation energy from approximately 19 to 16 kcal mol⁻¹.
🟢 Bimodal packing effect: The improved packing of the SS–TiC bimodal powder mixture allowed more binder to function as a lubricant, contributing to lower viscosity and reduced temperature sensitivity.
🟢 Critical solid loading: A critical solid loading of approximately 64% provided satisfactory agreement with the experimental rheological data for several investigated feedstocks.
🟢 Relative-viscosity reduction: TiC addition decreased the relative viscosity of the feedstocks, further supporting the beneficial influence of fine ceramic particles on composite feedstock flow.
🟢 Mouldability enhancement: The general mouldability index increased following TiC incorporation, demonstrating that the composite SS–3TiC feedstock exhibited better overall mouldability than the base 316L stainless-steel feedstock.
🟢 Solid-loading optimization: Feedstocks containing 55% solids showed better mouldability than those containing 60% solids, despite the potential advantages of higher solid loading during subsequent debinding and sintering.
🟢 Temperature optimization: A processing temperature of 70 °C provided a favorable balance of viscosity, shear sensitivity, temperature sensitivity, and overall mouldability.
🟢 Optimal PIM condition: The best moulding condition identified for the 316L SS–3 wt.% TiC composite feedstock was 55% solid volume fraction at 70 °C.
🟢 Manufacturing implication: The study demonstrated that 316L stainless-steel matrix composites reinforced with TiC particles can be processed by PIM when suitable feedstock composition and processing conditions are selected.
Overall, the study demonstrates that adding 3 wt.% TiC to 316L stainless-steel PIM feedstock can improve its rheological performance by reducing viscosity under appropriate conditions, increasing pseudoplastic shear sensitivity, lowering flow activation energy, and enhancing the overall mouldability index. These improvements originate largely from the bimodal particle-size distribution, in which fine TiC particles occupy interstitial spaces between larger stainless-steel particles and facilitate binder-assisted flow. Among the investigated conditions, a 55% solid volume fraction processed at 70 °C provided the most favorable combination of rheological properties and mouldability for the SS–3TiC composite feedstock.
ARTICLE LINK
https://doi.org/10.1179/003258905X37747
DOI
10.1179/003258905X37747
HOW TO CITE
Khakbiz, M.; Simchi, A.; Bagheri, R. Investigation of rheological behaviour of 316L stainless steel–3 wt-%TiC powder injection moulding feedstock. Powder Metallurgy 2005, 48(2), 144–150. DOI: 10.1179/003258905X37747.
KEYWORDS
316L Stainless Steel • TiC Powders • Powder Injection Moulding • Rheological Behaviour • Pseudoplastic Flow • Mouldability • Flow Activation Energy • Composite Feedstock
Abstract
The rheological behaviour of powder injection moulding feedstock comprising of 316L stainless steel and 3 wt-%TiC powders was studied using a capillary rheometer. The flowability and the sensitivity of viscosity to shear rate and temperature of the feedstock were investigated and compared with those of the binder system and the 316L SS PIM feedstock. The general rheological indexes were examined through relevant equations and the influence of TiC addition on the mouldability of the 316L SS feedstock was determined. It was found that all the feedstocks are basically pseudoplastic but the values of flow behaviour index n are influenced by the TiC addition, the solid volume fraction and the temperature. In this context, the results show that TiC addition plays an important role in rheological parameters, i.e. TiC particles decrease the viscosity of PIM feedstock and this effect is more pronounced in the system comprising higher solid volume fraction. Furthermore, the flow activation energy decreases with the introduction of TiC particles, in particular at high shear rates. In other words, the addition of ceramic particles would ease the feedstock flow and suppress the sensitivity of the feedstock to strain rate and temperature variations, if a proper moulding temperature is applied. From the viewpoint of mouldability, the optimum PIM condition for the SS/TiC composite powder containing a wax based binder system was found at 55% solid volume fraction and 70uC. At this condition, the viscosity of feedstock is low enough to fulfill the requirements of a medium pressure, injection moulding process.