Technical Paper: PIM International, Vol.4 No.2 June 2010, pages 60-65, 3713 words

Authors: Gemma Herranz[1], Gloria Patricia Rodríguez[1], Rubén Alonso[1], Grzegorz Matula[2]

[1] Escuela Técnica Superior de Ingenieros Industriales. Universidad de Castilla-La Mancha, Metallic Materials Group, Avda. Camilo José Cela s/n, 13071 Ciudad Real
[2] Institute of Engineering Materials Biomaterials, Silesian University of Technology ul. Konarskiego 18a, 44-100 Gliwice, Poland.

Abstract

Metal matrix composites (MMCs) based on M2 HSS (High Speed Steel) are processed using a metal injection moulding route. Different types of reinforcement are added to the mixture and the effect in the sintering behaviour has been analysed.

An optimised feedstock of M2 with carbides based on polyethylene and paraffin wax has been designed. The mixing procedure and the moulding conditions have been optimised to obtain parts without defects. A debinding schedule of thermal treatment has been established to partially eliminate the binder in order to promote the presence of some residual carbon into the parts.

Finally, the sintering was studied under N2-H2 atmosphere. Density and microhardness measurements and SEM microstructures were used to determine the optimum sintering temperature and the sintering window of each system.

This research has demonstrated that the addition of carbides produces an important reduction in the sintering temperature. Grain growth and coarsening of the grains are inhibited by the effect of the addition of the reinforcements.

Although the addition of carbides reveals some general secondary effects in both cases, the addition of different reinforcement shows particular effects depending on which reinforcement is used. The mixture of carbides (WC+TiC+TaC+NbC) produces an important enlargement of the sintering window and the addition of VC produces an important reduction in the optimum sintering temperature.

Introduction

Metal injection moulding (MIM) is a cost-effective production method for small, complex and high performance components. This process allows the production of more uniform microstructures and improvement of mechanical properties. Moreover, it is possible to achieve higher densities, more intricate shapes and better surface finish [1]. Feedstock formulations are the area of greatest patent coverage in PIM. This factor serves to increase the cost of the final product.

Basic exploration of innovative formulations increases knowledge and competitiveness, especially in high volume components. As can be seen in Table 1 [2] the part costs are dominated by feedstock price. This is the main justification for our research. The vendor A, who produces his own feedstock, is able to reduce his costs by 12% in the production of each part. For this reason, we focused our research on the development of innovative mixtures.

The production of HSS by MIM is considered to be better than other manufacturing techniques due to the inherent capacity of this technique to produce near net shape components, avoiding costly machining and obtaining more uniform shrinkage during sintering [3]. The principal difficulty presented by these steels is the complex densification process by SLPS (supersolidus liquid phase sintering) [4]. During the sintering process the high speed steel undergoes an unusual melting process; the liquid forms on the particle contact and grain boundaries and particle rearrangement causes rapid densification............

Further sections of this paper include:

- Experimental Procedure

- Results and Discussion

- Mixture of carbides

- Vanadium carbide reinforcement

- Conclusions

- Acknowledgements

Figures and Tables:

Fig. 1 Effect of residual carbon in the sinterability of M2 HSS processing by MIM [10]

Fig. 2 M2 HSS powder as observed by SEM

Fig. 3 Mixture of carbides as observed by SEM

Fig. 4  Vanadium carbide as observed by SEM

Fig. 5 Thermal debinding cycle applied

Fig. 6 SEM micrograph of M2 sintered under N2-H2, 1280ºC

Fig. 7 Comparison of the densification curves M2 and M2+mixture of carbides

Fig. 8 Microstructures of M2 reinforced with a mixture of carbides sintered at (A) 1240ºC and (B) 1260ºC

Fig. 9 Microstructure of M2 reinforced with (A) 1%wt of VC at 1260ºC and (B) 3%wt of VC at 1250ºC

Fig. 10 Comparison of the densification curves of M2, M2+1%VC and M2+3%VC

Fig. 11 Microstructure detail of M2 feedstock reinforced with 3%wt of VC at 1250ºC

Fig. 12 Microstructure of M2 feedstock reinforced with 6%wt of VC at 1170ºC

Fig. 13 Comparison of the densification curves of M2, M2+6%VC and M2+10%VC

Fig. 14 Microstructure of M2 reinforced with 10%wt of VC sintered at 1125ºC


Table 1 Contrast of component costs between two vendors on the same part, where vendor A uses self-mixing and vendor B relies on purchased premixed feedstock (values are US$ per part) [2]

Table 2 Chemical composition (%wt) as received M2 powder together AISI specification

Table 3 Feedstock formulations