Powder flowability is critical in additive manufacturing processes, especially for laser powder bed fusion. Many powder features, such as powder size distribution, particle shape, surface roughness, and chemical composition, simultaneously affect the flow properties of a powder; however, the individual effect of each factor on powder flowability has not been comprehensively evaluated. In this study, the impact of particle shape (sphericity) on the rheological properties of Ti-6Al-4V powder is quantified using an FT4 powder rheometer. Dynamic image analysis is conducted on plasma-atomized (PA) and gas-atomized (GA) powders to evaluate their particle sphericity. PA and GA powders exhibit negligible differences in compressibility and permeability tests, but GA powder shows more cohesive behavior, especially in a dynamic state, because lower particle sphericity facilitates interaction between particles during the powder flow. These results provide guidelines for the manufacturing of advanced metal powders with excellent powder flowability for laser powder bed fusion.

This study was performed to fabricate the porous titanium foam by space holder method using NaCl powder, and to evaluate the effect of NaCl volume fractions (33.3~66.6 vol.%) on the porosities, compressive strength, Young's modulus and permeability. For controlling pore size, CP titanium and NaCl particles were sieved to different size range of 70~150 and 300~425 respectively. NaCl of green Ti compact was removed in water followed by sintered at for 2 hours. Total porosities of titanium foam were in the range of 38-70%. Pore shape was a regular hexahedron similar that of NaCl shape. Porous Ti body showed that Young's modulus and compressive strength were in the range of 0.6-6 GPa and 8-127 MPa respectively. It showed that pore size and mechanical properties of Ti foams was controllable by NaCl size and volume fractions.

Nondestructive instrumented indentation test is the method to evaluate the mechanical properties by analyzing load - displacement curve when forming indentation on the surface of the specimen within hundreds of micro-indentation depth. Resistance spot welded samples are known to difficult to measure the local mechanical properties due to the combination of microstructural changes with heat input. Particularly, more difficulties arise to evaluate local mechanical properties of resistance spot welds because of having narrow HAZ, as well as dramatic changed in microstructure and hardness properties across the welds. In this study, evaluation of the local mechanical properties of resistance spot welds was carried out using the characterization of Instrumented Indentation testing. Resistance spot welding were performed for 590MPa DP (Dual Phase) steels and 780MPa TRIP (Transformation Induced Plasticity) steels following ISO 18278-2 condition. Mechanical properties of base metal using tensile test and Instrumented Indentation test showed similar results. Also it is possible to measure local mechanical properties of the center of fusion zone, edge of fusion zone, HAZ and base metal regions by using instrumented indentation test. Therefore, measurement of local mechanical properties using instrumented indentation test is efficient, reliable and relatively simple technique to evaluate the tensile strength, yield strength and hardening exponent.

The evolution of sinterability, microstructure and mechanical properties for the spark plasma sintered(SPS) Ti from commercial pure titanium(CP-Ti) was studied. The densification of titanium with 200 mesh and 400 mesh pass powder was achieved by SPS at under 10 MPa pressure and the flowing +Ar mixed gas atmosphere. The microstructure of Ti sintered up to consisted of equiaxed grains. In contrast, the growth of large elongated grains was shown in sintered bodies at with the 400 mesh pass powder and the lamella grains microstructure had been developed by increasing sintering temperature. The Vickers hardness of 240~270 HV and biaxial strength of 320~340 MPa were found for the specimen prepared at .

테헤란밸리의 쇠퇴 이후 대다수의 벤처기업들이 새롭게 자리 잡은 대표적인 곳은 구로디지털단지 지역과 성남 지역이다. 구로의 기업 군집화는 정부의 의도된 계획과 개입을 통해 성장한 반면, 성남 지역의 기업 군집화는 자생적이고 자기조직화의 과정을 통해 성장하였으며 이는 복잡성 이론의 관점에서 설명될 수 있다. 본 논문은 조직생태학적 관점에서 급진적이고 지속적인 혁신을 통해 성장한 대표적 벤처 생태계인 성남지역을 하나의 기업생태계 단위로 바라보고, 성남 지역 기업생태계에 일어난 조직변화의 양상을 복잡성 이론의 관점에서 분석하였다.성남 지역기업생태계는 1990년대까지 전통적 제조업 기반이었으나, 이후 오랜 시간동안 지속적으로 여러 가지 작은 변화들이 누적되어 첨단기업 중심의 기업생태계로 급진적인 진화를 거듭하고 있다. 이러한 지속적이면서도 급진적인 변화 과정은 기존의 조직변화 이론으로는 설명되기 어려우므로, 본 연구에서는 복잡성 이론의 관점에서 평형으로부터 먼 상태, 초기조건의 민감성, 상호작용 네트워크와 긍정 피드백 고리를 통한 변화의 증폭, 공진화라는 4단계에 따라 분석을 시도하였다.또한, 지역의 시스템 요소들 간의 복잡한 상호작용을 통해 지역기업생태계의 자생적 변화가 이루어지는 과정을 복잡성 이론의 관점에서 연구한 본 논문은 우리나라 지역혁신체제의 지속적 성장과 효율적 발전에 대한 정책적 시사점을 제시한다.