Technical Paper: PIM International, Vol.5 No.2 June 2011, pages 54-59, 3855 words

Authors: Paul Ewart[1,2], Seokyoung Ahn[1] and Deliang Zhang[2]

[1] Mechanical Engineering Department, University of Texas-Pan American, Edinburg, Texas, USA
[2] Waikato Centre for Advanced Materials, University of Waikato, New Zealand

  

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Abstract

Although metal injection moulding (MIM) is increasingly being used to produce metal parts with complex geometries, less than1% of the market utilises titanium powders [1]. As a low energy method for producing parts of complex geometry, MIM is a suitable process for reducing the cost of producing titanium and titanium alloys. It is accepted that the final quality of MIM parts is a function of the moulding, debinding and sintering processes. If the green part has defects they will exacerbate during subsequent debinding and sintering due to in-homogeneity of binder components and non-uniformity in particle distribution. Although this is understood there is little available data quantifiably supporting this. In our study, a co-rotating twin-screw extruder is used for batch mixing titanium fine powder with a polyethylene based binder to form a feedstock enabling investigation of limits to homogeneity and to determine how this affects particle adhesion and the strength of green parts.

Introduction

Titanium is found in at least 35 mineral forms, is the ninth most abundant element in the earth’s crust and fourth most abundant metallic element [2]. As an advanced material it has good strength (typically over 650 MPa) and ductility (typical elongation to fracture > 20 %) with a moderate density (4.5 g/cm3) providing good specific properties over other materials and it is corrosion resistant (chemical industry) and highly bio-compatible (medical and dental application). When alloyed (i.e. Ti 6Al 4V) even more desirable properties are found: high strength (>1000 MPa), and high resistance to creep at temperatures up to 650 ºC and high corrosion resistance. The negative aspects that keep titanium from materials selection are generally associated with costs and, for many titanium alloys, poor machinability [3]. Metal injection moulding (MIM) offers a low energy process, compared with casting, press forming, forging and machining. It offers low waste through low production numbers of high complexity at near net shape with little machining requirements and best suited for small to micro-scale parts.

Although MIM is increasingly being used to produce metal parts with and in many cases the only process able to produce complex geometries, titanium powder usage is less than 1% of the MIM market [1]. Researchers and industrialists are working together to increase the awareness of the benefits of using Ti MIM [4]. This is evidenced by the number of presentations at the recent MIM2011 conference [5-7] and the twelve full sessions offered at PowderMet2011 conference where MIM and Ti development and applications is extremely active [8]..........

Further sections of this paper include:

• Experimental Procedure
• Results and Discussion
• Conclusion
• Acknowledgments
• References
  

 

Figures and Tables:

Fig. 1 Pre-mixed powder and binder was repeatedly extruded in the twin screw extruder and feedstock samples taken at 1, 2, 3, 4, 5, 7 and 10 passes
Fig. 2 SEM micrographs showing feedstock for a) one, b) five and c) ten extrusion passes
Fig. 3 Flow rates of extruded feedstock determined using the extrusion plastometer
Fig. 4 TGA plots for extruded feedstock 0, 1, 2, 3, 4, 5, 7 and 10 passes
Fig. 5 DSC thermal plots for feedstock mixes 1, 2, 3, 4, 5, 7, and 10 passes
Fig. 6 a) Green part plates moulded using equivalent injection moulding temperature, pressure and duration. Plates cut into bar and token specimens for analysis and debinding, b) solvent debound (SD) and c) thermal debound (TD)
Fig. 7 Surface of as moulded specimen a) single pass with signs of binder agglomeration, b) four pass uniform appearance and c) binder separation area of ten pass specimen
Fig. 8 Surface of solvent debound specimens a) single pass, b) four pass and c) ten pass
Fig. 9 Surface of thermal debound specimens a) single pass, b) four pass and c) ten pass
Fig. 10 Plot of percentage full density for both sets of AM, SD and TD specimens
Fig. 11 Batched sample EM0 showing unmixed binder location
Fig. 12 Thermal expansion of extruded AM specimens from TMA analysis
Fig. 13 Comparative DMA analysis of AM and SD specimens
Fig. 14 The effect of mixing on powder particles post binder burnout, a) as received powder, b) single pass, c) seven passes and d) ten passes

Table 1 Density values of as-moulded (AM), solvent debound (SD) and thermal debound (TD) samples respectively
Table 2 Flexural modulus values taken from DMA plot at 40 °C, distinct difference between AM and SD specimens

 

 

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