마이크로-나노 혼합분말 피드스톡의 저온 분말사출성형에 관한 연구

Abstract

최근 부품산업은 전기, 전자산업의 발달에 따라 소형화, 복잡화되어가고 있으며, 마이크로 부품의 발전은 현대 산업의 다양화를 촉진하고 있다. 따라서 미소 부품 제조기술에 대한 중요성이 증가되고 있으며, 대량 생산에 적합한 전통의 부품제조기술을 마이크로 부품 제조에 적용하기 위한 연구가 진행되고 있다. 특히 분말사출성형(Powder Injection Molding)은 원재료 선택에 대한 제약이 없고, 복잡형상 부품의 대량 생산에 적합한 기술로서 주목받고 있지만, 마이크로 부품 제조를 위해서는 원료 분말, 바인더, 그리고 장비와 공정에 이르는 부분까지 새로운 기술적 접근을 필요로 한다. 본 연구에서는 실형상 마이크로 부품의 제조를 위한 기술로서 마이크로-나노분말을 사용한 피드스톡의 제조와 사출성형 및 탈지, 소결공정을 최적화하였다. 저온, 저압의 공정조건은 저점도, 저융점 특성의 왁스, 계면활성제의 2종 바인더를 적용하여 설계하고, 피드스톡 내 분말함량 개선과 미세구조 균일화는 마이크로-나노분말 혼합체를 사용하여 구현하였다. 마이크로 분말사출성형 공정을 위한 Fe 마이크로-나노분말 피드스톡은 혼합 시 토크와 미세구조의 변화거동을 통하여 최적화하였다. 혼합은 twin screw type mixer를 통해 수행하였으며, 분석은 피드스톡 내 분말의 함량이 증가함에 따라 관찰되는 토크와 온도 및 미세구조의 변화거동을 측정하는 방법으로 진행하였다. 특히, 전체 분말 내 나노분말의 첨가가 피드스톡의 혼합거동에 미치는 영향을 조사하였다. 토크와 온도변화는 피드스톡 내 분말의 누적함량에 비례하여 증가하였으며, 이는 나노분말의 함량이 증가하면서 두드러지게 나타났다. 그러나 토크의 최대값은 나노분말 함량 25% 조건이 아닌, 10% 조건에서 관찰되었다. 나노분말 응집체는 피드스톡의 혼합 시, roller bearing 효과를 통해 고상 바인더로서 작용하며, 피드스톡의 재배열과 높은 충진율을 야기한다. 이러한 효과는 마이크로 분말과 나노분말의 혼합비율이 75 : 25인 조건에서 가장 크게 나타나며, 이때, 혼합토크는 오히려 낮은 상태로 안정화되고, 분말의 함량은 최대 71 vol.%까지 증가되는 것으로 확인되었다. Fe 마이크로-나노분말 피드스톡은 66 vol.%의 분말함량과 34 vol.%의 파라핀 왁스, 스테아린 산 바인더로 구성하였다. 특히 25%의 나노분말 첨가 시, 피드스톡은 70oC, 4 MPa 이하의 조건에서 사출성형공정이 가능하였다. 이때 나노크기의 기공채널은 액상 바인더와 작용하여 모세관 현상을 나타내었으며, 이는 피드스톡의 혼합 및 사출성형 공정에서, 분말과 바인더의 분리, 액상 바인더의 추출 현상을 억제하였다. 또한 나노분말은 마이크로 분말의 사이 공간에 자리한 채 시편의 치밀도를 증가시키고, 탈지공정 직 후, 나노분말의 초기 소결효과는 탈지시편의 성형강도를 효과적으로 개선하였다. 사출성형한 소형 더블기어는 소결공정을 통해 완전 치밀화되었으며, 등방 수축거동을 나타내었다. 이때, 나노분말의 첨가로 인해 시편 내 계면의 수는 증가하고, 소결과정에서 발생하는 잔류기공은 주로 결정립 계면에 존재하는 것으로 확인되었다. 잔류기공은 결정립 계면의 이동을 방해하였으며, 이로 인하여 소결 시 시편의 결정립 성장은 억제되었다. 또한 다수의 결정립 계면은 소결 시 원자의 확산경로로서 작용하여, 마이크로-나노분말 소결체는 마이크로 분말 소결체와 비교하여 우수한 치밀화 거동을 나타내었다. 마이크로-나노분말 피드스톡을 이용하여, 저온, 분말사출성형 공정으로 제조한 SUS 316L 실형상 부품은 상용 부품과 비교하여 더욱 작은 결정립 크기와 우수한 치수정밀도 및 치밀도를 나타내었다. 특히 작게 유지된 결정립 크기와 높은 치밀화도는 마이크로-나노분말 부품의 기계적 특성을 상용 부품과 비교하여 약 10~30% 개선시키는 것으로 확인되었다. 결과적으로, 마이크로-나노분말 피드스톡을 적용한 저온 분말사출성형기술에 대한 연구는 마이크로 부품 제조기술에 대한 새로운 접근 방법을 제공한다.| Recently, machinery component industry is developing the same way as electronic system and its size is reducing from centimeter to micrometer. The successful fabrication and operation of micro-components provide opportunity to produce miniaturised machines and mechanical systems. Thus, a reliable and highly accurate shaping process is imperative in manufacturing micro-components. In particular, micro powder injection molding(m-PIM) is a potential method for mass producing near net shape micro-components. However, in order to replicate micro feature, powders smaller than that used in conventional PIM are employed in m-PIM, which imposes additional and more stringent requirements in the processing conditions. In this study, in order to successfully manufacture the net shaped micro components, PIM processes such as feedstock design, injection molding, debinding, and sintering using micro-nano mixed powders were considered. A binder system composed of paraffin wax and stearic acid was used for specific process conditions with low temperature and pressure. Also, application of micro-nano powders led to improved powder loading rate and homogenization of microstructure in feedstock. The optimization of Fe micro-nano powder feedstock for micro-powder injection molding process has been performed to study the mixing behavior and micro-structural development during the mixing. The mixing experiment using a screw type blender system was conducted to measure the variations of torque and temperature during mixing of Fe powder-binder feedstock with progressive powder loading for various nano-powder compositions up to 25%. It was found that the torque and the temperature required to the mixing of feedstock increased proportionally with increasing cumulative powder loading. Such an increment was larger in the feedstock containing higher content of nano-powders at the same powder loading condition. However, the maximum value was obtained at the nano-powder composition of 10% instead of its composition of 25%. It was due to the 'roller bearing effect' of agglomerate type nano-powder acting as lubricant during mixing, consequently leading to the rearrangement of micro-nano powders in the feedstock. It is concluded that the improvement of packing density by rearrangement of nano-powders into interstices of micro-powders is responsible for the maximum powder loading of about 71vol.% in the nano-powder composition of 25%. The optimal feedstock consisted of 66vol.% powder and 34vol.% wax binder in which the powder composition was 75vol.% Fe micro powder (~4mm) and 25vol.% nano powder (~100nm). Injection molding was conducted under the conditions of 70oC and 4MPa. The nano-sized capillary pore channels prevented some drawbacks such as powder-binder disintegration, binder extraction and collapse of powders during mixing and molding. The debound part maintained uniform and sound surface structure due to strong networks between the micro powders by enhanced sintering effect of nano powders during the debinding process. The PIMed gear underwent isotropic shrinkage in reaching full densification during sintering. In particular, it was investigated that addition of the nano powder increased the number of grain-boundaries of which migration was interrupted by most of remained pores on the grain-boundaries. Therefore, micro powder grains rarely grow because of the inhibited boundary migration. It was also found that micro-nano powder sintered part had a better densification behavior compared to the sintered micro powder part because of the grain boundaries acting as a fast diffusion path. It is concluded that the low temperature and low pressure PIM process using micro-nano powders and low viscosity binder, enables us to fabricate the micro parts with high precision and fine microstructure.; Recently, machinery component industry is developing the same way as electronic system and its size is reducing from centimeter to micrometer. The successful fabrication and operation of micro-components provide opportunity to produce miniaturised machines and mechanical systems. Thus, a reliable and highly accurate shaping process is imperative in manufacturing micro-components. In particular, micro powder injection molding(m-PIM) is a potential method for mass producing near net shape micro-components. However, in order to replicate micro feature, powders smaller than that used in conventional PIM are employed in m-PIM, which imposes additional and more stringent requirements in the processing conditions. In this study, in order to successfully manufacture the net shaped micro components, PIM processes such as feedstock design, injection molding, debinding, and sintering using micro-nano mixed powders were considered. A binder system composed of paraffin wax and stearic acid was used for specific process conditions with low temperature and pressure. Also, application of micro-nano powders led to improved powder loading rate and homogenization of microstructure in feedstock. The optimization of Fe micro-nano powder feedstock for micro-powder injection molding process has been performed to study the mixing behavior and micro-structural development during the mixing. The mixing experiment using a screw type blender system was conducted to measure the variations of torque and temperature during mixing of Fe powder-binder feedstock with progressive powder loading for various nano-powder compositions up to 25%. It was found that the torque and the temperature required to the mixing of feedstock increased proportionally with increasing cumulative powder loading. Such an increment was larger in the feedstock containing higher content of nano-powders at the same powder loading condition. However, the maximum value was obtained at the nano-powder composition of 10% instead of its composition of 25%. It was due to the 'roller bearing effect' of agglomerate type nano-powder acting as lubricant during mixing, consequently leading to the rearrangement of micro-nano powders in the feedstock. It is concluded that the improvement of packing density by rearrangement of nano-powders into interstices of micro-powders is responsible for the maximum powder loading of about 71vol.% in the nano-powder composition of 25%. The optimal feedstock consisted of 66vol.% powder and 34vol.% wax binder in which the powder composition was 75vol.% Fe micro powder (~4mm) and 25vol.% nano powder (~100nm). Injection molding was conducted under the conditions of 70oC and 4MPa. The nano-sized capillary pore channels prevented some drawbacks such as powder-binder disintegration, binder extraction and collapse of powders during mixing and molding. The debound part maintained uniform and sound surface structure due to strong networks between the micro powders by enhanced sintering effect of nano powders during the debinding process. The PIMed gear underwent isotropic shrinkage in reaching full densification during sintering. In particular, it was investigated that addition of the nano powder increased the number of grain-boundaries of which migration was interrupted by most of remained pores on the grain-boundaries. Therefore, micro powder grains rarely grow because of the inhibited boundary migration. It was also found that micro-nano powder sintered part had a better densification behavior compared to the sintered micro powder part because of the grain boundaries acting as a fast diffusion path. It is concluded that the low temperature and low pressure PIM process using micro-nano powders and low viscosity binder, enables us to fabricate the micro parts with high precision and fine microstructure.