简介概要

Injection molding of micro-porous titanium alloy with space holder technique

来源期刊：Rare Metals2011年第6期

论文作者：Gülsah Engin Bülent Aydemir H.zkan Gülsoy

文章页码：565 - 571

摘要：The powder space holder（PSH） and powder injection molding（PIM） methods have an industrial competitive advantage because they are capable of the net-shape production of micro-sized porous parts.In this study,micro-porous Ti6Al4V alloy（Ti64） parts were produced by the PSH-PIM process.Ti64 alloy powder and spherical polymethylmethacrylate（PMMA） particles were used as a space holder material.After molding,binder debinding was performed by thermal method under inert gas.Debinded samples were sintered at 1250oC for 60 min in a vacuum（10-4 Pa） .Metallographic studies were conducted to determine densification and the corresponding microstructural changes.The surface of sintered samples was examined by SEM.The compressive stress and elastic modulus of the micro-porous Ti64 samples were de-termined.The effects of fraction of PMMA on the properties of sintered micro-porous Ti64 alloy samples were investigated.It was shown that the fraction of PMMA could be controlled to affect the properties of the Ti alloy.

Rare Metals 2011,30(06),565-571

Injection molding of micro-porous titanium alloy with space holder technique

Gülsah Engin Bülent Aydemir H.zkan Gülsoy

Marmara University,Institute Graduate Studies Pure and Applied Sciences

TUBITAK,National Metrology Institute

Marmara University,Technical Education Faculty,Materials Department

Tubitak-MRC,Materials Technologies Research Institute

作者简介：H.zkan Gülsoy,E-mail:ogulsoy@marmara.edu.tr;

收稿日期：11 November 201

基金：supported by the Scientific Research Project Program of Marmara University (No.FEN-C-YLP-280110-0004);Marmara University for their financial support and the provision of laboratory facilities;

Injection molding of micro-porous titanium alloy with space holder technique

Abstract：

The powder space holder(PSH) and powder injection molding(PIM) methods have an industrial competitive advantage because they are capable of the net-shape production of micro-sized porous parts.In this study,micro-porous Ti6Al4V alloy(Ti64) parts were produced by the PSH-PIM process.Ti64 alloy powder and spherical polymethylmethacrylate(PMMA) particles were used as a space holder material.After molding,binder debinding was performed by thermal method under inert gas.Debinded samples were sintered at 1250oC for 60 min in a vacuum(10-4 Pa) .Metallographic studies were conducted to determine densification and the corresponding microstructural changes.The surface of sintered samples was examined by SEM.The compressive stress and elastic modulus of the micro-porous Ti64 samples were de-termined.The effects of fraction of PMMA on the properties of sintered micro-porous Ti64 alloy samples were investigated.It was shown that the fraction of PMMA could be controlled to affect the properties of the Ti alloy.

Keyword：

powder injection molding; titanium powder; porous metal; sintering;

Received： 11 November 201

1. Introduction

Powder injection molding is a high volume manufacturing process that combines plastic injection molding and conventional powder metallurgy(PM)technologies.This technique combines the advantages of plastic injection molding with the material versatility of the traditional powder metallurgy,producing highly complex parts with a small size,tight tolerance,and low production cost.The process overcomes the shape limitation of traditional powder compaction,cost of machining,productivity limits of isostatic pressing and slip casting,and defect and tolerance limitations of conventional casting.The mechanical properties of a well-processed powder injection molded material are indistinguishable from cast and wrought materials.The PIM process is composed of four sequential steps:mixing of the powder and organic binder,injection molding,debinding(binder removal),and sintering[1].

Porous metal materials have been widely studied and used,but they are newly classified materials with low densities,large specific surfaces and novel properties useful in a variety of applications because of their easily tailored properties[2-3].An open cell structure is useful for applications such as heat exchangers and heat sinks for thermal management,medical implants,filters,and electrodes in biological and chemical reactions.Conventional press-sinter manufacturing methods can be used with a small subset of methods to create porous materials with a limited range of cell size and porosity[4-5].In practical ways,it is very difficult for metals to produce controlled pore sizes in the tens of micrometers with both open and closed cell structures and specified porosity.Currently,few methods can produce net-shaped metal components with high production efficiency.Furthermore,it is not easy to control the cell size and its distribution in practice and much more so to produce such micro-porous metal and ceramics components with complicated shapes and high dimensional accuracy[4-6].

In a recent study,the thermal decomposition temperature of PMMA and the best sintering temperature of mixtures of aluminum powder and PMMA powder have been optimized experimentally[4-7].Nishiyabu et al.[4-5]investigated the manufacturing parameters(size and fraction of PMMA particles and bending properties of the micro-porous graded structure)of commercial micro-porous metal components by PIM.Their study showed that PMMA particles can be decomposed during debinding and micro-porous metal components can be easily manufactured by PIM.Gain et al.[6]investigated the thermal decomposition temperature of PMMA and the best sintering temperature for a mixture of aluminum and PMMA powders.The effects of the porosity and pore size on the Young’s Modulus and strength were also discussed in their study.Gulsoy and German[7]investigated the effect of PMMA particle size and fraction on porosity and comprehensive properties of micro-porous 316L stainless steel produced by PIM.Manonukul et al.[8]investigated the effects of replacing metal powder with a powder space holder on stainless steel foam parts produced by metal injection molding with the space holder technique.Their results showed that closed-cell stainless steel foams can be produced by the PIM method with a high volume fraction of50%PMMA.K?lh et al.investigated several fabrication parameters of injection-molded Ni Ti shape memory alloys for implant applications[9].

In the present study,the micro-porous Ti64 alloy was produced by PIM with the space holder technique.The effects of fraction of PMMA particles on density,compressive stress,and Young’s Modulus of the micro-porous sintered Ti64 samples were investigated.Thermogravimetric analysis(TGA)was measured as a function of temperature for a debinding schedule.The morphology of the powders and the surface of sintered samples were analyzed using a scanning electron microscope(SEM).The compressive stress and Young’s modulus of the micro-porous Ti64 samples were determined.

2. Experimental

A schematic illustration of the manufacturing process of micro-porous Ti64 parts is shown in Fig.1.In a conventional PIM process,the feedstock materials consists of the mixture of metal powder and binders,followed by debinding and sintering processes for PIM parts.In addition to metal powder and organic binders,a coarse spherical particle made of polymer is used in the formation of a controlled porous structure in the PIM parts.The material combination of a space-holding particle and metal powder in addition to sintering conditions determines principally the porous structure.

Fig.1.The production steps of micro-porous Ti64 by PIM with the space holder technique.

In this research,gas atomized Ti64 powders(Ti-5.9Al-3.9V-0.19Fe-0.12O-0.01C-0.01N-0.004H)provided by SOLEA Corporation(France)was used.It has a particle size distribution of D₁₀=10.32μm,D₅₀=24.61μm,D₉₀=45.61μm.Spherical particles with 10μm and 41μm in mean diameter(Sunjin Chemical Co.,Ltd.)made of PMMA were used as space holder material.Particle size distributions of Ti64 and PMMA powder were determined on a Malvern Mastersizer equipment and shown in Fig.2.The morphology of the powder was observed using scanning electron microscopy,as shown in Fig.3.The gas-atomized Ti64 powder and PMMA powder are spherical in shape.

A multiple-component binder system consisting of paraffin wax(PW),polypropylene(PP),carnauba wax(CW),and stearic acid(SA)was used.Table 1 indicates the characteristic properties of the binders and PMMA particles.The properties of the feedstock and sample codes are given in Table 2.The amount of PMMA particles was adjusted to give 50,60and 70 vol.%in the starting mixture and each mixture was blended in a Turbula mixer for 2 h.Feedstock was prepared at 175°C for 30 min with the binder being melted first and then the powder and space holder being added incrementally The feedstock was injected at a 10 MPa pressure using a specially made injection-molding machine to obtain a cylindrical green body with a height of 20 mm and a diameter of15 mm.The melting temperature was 170°C,the mold temperature was kept at 35?C and cycle time was 20 s.

TGA is measured as a function of temperature while the sample is subjected to a controlled temperature program.This can be achieved as a function of the increasing temperature or isothermally as a function of time with high purity argon(Ar)atmosphere.The TGA measurements were done on a SII 6300 TGA-DTA(SII Nanaotechnology Inc.,Tokyo,Japan)with a heating rate of 10°C/min from room temperature up to 600°C for the binders and feedstock.

Fig.2.Cumulative particle size distributions for Ti64 and PMMA powders.

Table 1.Characteristics of binder components used in the binder system and space holder material 下载原图

Table 1.Characteristics of binder components used in the binder system and space holder material

Table 2.The properties of feedstock and sample codes in the experiments 下载原图

Table 2.The properties of feedstock and sample codes in the experiments

The samples molded using the multiple-component binder and space holder materials were all thermally debound and sintered under high purity Ar gas and high vacuum.Green parts were thermally debound step by step at1-2°C/min to 600°C for 4 h and pre-sintered at 5°C/min to900°C for 1 h in pure Ar gas.The sintering cycle applied to the samples was as follows:samples were heated to 1250°C at a rate of 10°C/min.and held for 60 min.under high vacuum(10^-4Pa).No distortion or other visible reduction in part quality or surface finish was observed.

The densities of the sintered samples were measured by means of the Archimedes water-immersion method.The microstructures of sintered steel samples were examined by optical as well as scanning electron microscopes(SEM-Jeol-JSM 6335F-Japan).The compression tests of the specimens were conducted on a Zwick Z250 materials testing machine at a crosshead speed of 1 mm/min up to a deformation of 50%.The carbon and oxygen values of the sintered samples were measured by a Carbon/Sulfur Combustion Analyzer(Horiba/Emia,Japan)and SEM-EDS.At least three specimens were tested under the same conditions to guarantee the reliability of the results.

3. Results and discussion

The TGA of feedstock helps to design the thermal debinding cycles.Fig.4 shows the weight loss-temperature plot with the 10°C/min heating rate for the aluminum feedstock.Below 200°C,no materials decomposed,but at 200°C the wax started to decompose and it created many paths for degassing around the Ti64 particles.Then at 300°C,all binders decomposed with the increased temperature.Finally,at above 475°C,all of the binder constituents decomposed.This is a basic debinding mechanism in the binder system.

Fig.3.Scanning electron micrographs of the powders:(a)Ti64;(b)10μm PMMA;(c)41μm PMMA.

Fig.4.TGA curves of binders and feedstock.

The effect of fraction of PMMA particles on the sintered density of micro-porous Ti64 is shown in Fig.5.As the PMMA particle content increases,the sintered density decreases in all the samples.The density of Ti64 samples was4.30 g/cm³ without PMMA addition sintered at 1250°C for60 min.The sintered density reduced to 3.09 g/cm³,2.42g/cm³ and 1.91 g/cm³ in 50%,60%and 70%PMMA(41μm)added samples,respectively.Similarly,sintered density reduced to 3.90 g/cm³,3.10 g/cm³ and 2.70 g/cm³ in 50%,60%and 70%PMMA(10μm)added samples,respectively.With the increasing fraction and average size of PMMA,the sintered density decreases for all types of samples[8-9].

Fig.5.Sintered densities as a function of the fractions and average sizes of PMMA.

Fig.6 shows SEM images of sintered micro-porous Ti64sample.It can be seen that the samples have a homogeneous structure with open pores,which represent PMMA particle characteristics,and uniformity of mixture.It is also possible to obtain a purposely tailored distribution of the pore size in the foam by using PMMA particles with different particle size ranges[5,9].For all samples,neck formation,which is the starting point for sintering,can be clearly seen in microstructures at 1250°C for 60 min.When the structures of A,B,C and the basic type of samples are compared,the A,B and C type samples include higher pore size than the basic type of samples and can be readily seen in Fig.6.Likewise,D,E,F and the basic type of samples can be compared.With the increasing fraction and average size of PMMA,the porosity and pore size were increased for all types of samples.

Fig.7 shows the microstructures of the basic and micro-porous samples.The microstructures of the basic material have significantly lower porosity in the grain boundary and inside the grain and high sintered density.PMMA(70%)particles added into the samples presented lower density and higher porosity than that of the basic material.It can be seen that the increase in the average size of PMMA particles increased pore sizes in the microstructures.For the basic samples,in the matrixα,the small second-phase particlesareβin all samples.The Ti64 was found to have a coarse acicular microstructure,revealingαgrains with intergranularβ-phase.The percentage ofα-phase in the alloys depends on the sintering conditions[10-11].

Fig.7.Optical microscopy images of microstructures of sin-tered samples at 1250°C for 60 min:(a)C type sample;(b)F type sample;(c)base material.

Fig.8 shows the compressive stress-strain curves of basic and micro-porous Ti64 samples fabricated by PIM with the space holder technique with different fractions and average sizes of PMMA.The corresponding strengths for PMMA(41μm)added samples at 50%and 70%are 1169 and 653MPa;for PMMA(10μm),the strengths of added samples a50%and 70%are 1242 and 711 MPa,respectively.Lower compression stresses and a decrease in the Young’s modulus are obtained with the increment in porosity[9].Table 3 shows the sintered density,compressive stress,Young’s modulus and amounts of oxygen and carbon in micro-porous and full dense Ti64 samples fabricated by the PIM with the space holder technique.According to Table 3,the fraction and size of PMMA particles decreased the sintered density and Young’s modulus.The Young’s modulus increased with a decrease in porosity.The samples with 41μm and 10μm PMMA particles at 50%gave a Young’s modulus of 2.14 and 2.19 GPa respectively.When the porosity increased,41μm and 10μm PMMA at 70%gave a Young’s modulus of 0.89 and 1.72GPa,respectively.Control of carbon and oxygen content was the most important issue in producing the Ti64 alloy by PIM It determined not only the mechanical properties,but also the dimensional stability and corrosion properties.Sintered samples(1250°C for 1 h)contained a larger amount of oxygen and carbon than the starting powders.Residual carbon and oxygen amounts could be found in the debinding stages.The lower levels of oxygen and carbon in the PIM materials are the likely cause of both higher strength and ductility[10-11].

Fig.8.Compressive stress-strain curves of micro-porous Ti64samples with 50 vol.%and 70vol.%PMMA space holders.

Table 3.The properties of micro-porous Ti64 samples 下载原图

Table 3.The properties of micro-porous Ti64 samples

4. Conclusions

In conclusion,our experimental results show that micro-porous Ti64 parts were produced by applying a polymer powder space holder method to the metal-injection molding process.The feedstock produced using the PMMA particles and metal powders had good moldability and no separation of the binder was observed during molding.No distortion or other visible reduction in part quality or surface finish was observed when the samples were sintered at 1250°C for 60min.By comparing the physical and mechanical properties of the samples with the homogeneous porous structure,sintered density decrement,porosity increment and the Young’s modulus decrement with a fraction of PMMA incremen was observed.