The effect of chilling on mechanical properties of cast magnesium alloy (AZ92A) plates was investigated. The section size was 25mm and volumetric heat capacity of chill used was varied from 38cal/℃ to 645cal/℃.
Melting was conducted in an open-top electric furnace with steel crucible. In melting of AZ92A alloy the high purity magnesium ingot was charged first, aluminium was added shortly after meltdown, and then zinc was added at 680℃. The melt surface was covered with Magrex 33 flux to prevent burning of magnesium.
Degassing and grain refinement were obtained by carbon inoculation with Nucleant 200. Before pouring, the flux was skimmed from the melt and replaced with a light layer of sulfurboric acid mixture. The green sand mold was used for all plate castings and it was inhibited with 1.5 per cent sulfur and 1.5% boric acid mixture. Varying sizes of chills were placed at the plate extremity in the green sand for investigation of chill effect. Both unchilled and chilled AZ92A alloy plates were cast for comparison, and the melt was poured and cast in a series with different sizes of chills in the chilled casting practice.
The solution heat treatment was conducted under the $SO_2$ atmosphere, and subsequently, all the samples were blast aircooled and aged.
Tensile bars were cut and machined from various sections in the plates, and their mechanical properties were tested. A metallographic examination of the effect of chilling on microstructure was simultaneously carried out. The mechanical properties of sand cast AZ92A alloy plates decrease with increasing distance from chill end. Also, this investigation has shown that there is an effective threshold size of chills. Any larger chills that the threshold size do not seem to much improve the mechanical properties of cast AZ92A alloy.
Magnesium 합금 ( AZ92A ) 판상주조물의 기계적 성질에 미치는 냉금 ( chill ) 의 효과에 대하여 연구하였다. 본 연구에서 주조물의 두께는 25mm로 하였고 사용된 냉금의 열용량은 38 cal/℃에서 645cal/℃ 까지 변화시켰다.
용해작업은 철제도가니를 사용하여 전기로에서 행하여 졌으며, 고순도 magnesium ingot를 먼저 장입용해한 후 aluminium ingot를 첨가하고 680℃에서 Zinc를 첨가용해하였다. 용해시 magnesium의 산화연소를 방지하기 위해 Magrex 33 flux를 용탕표면에 도포하였다.
탈 gas 및 결정입자 미세화 처리는 Nucleant 200을 사용하여 C - 접종법 (carbon inoculation) 으로 행해졌다. 주입은 용탕표면의 flux를 걷어낸 다음 황 및 붕산분말을 용탕표면에 뿌려주면서 주입하였다. 주물사는 황 1.5%와 붕산 1.5%를 주물사중에 배합첨가하여 보호제의 역할을 하도록 하였고 냉금의 효과를 조사하기 위하여 여러가지 크기의 냉금을 주형에 설치하였다. 또한 비교를 위해서 주형에 냉금을 설치하지 않고 용탕을 주입주조하였다.
Magnesium합금의 용체화처리는 $SO_2$ 분위기에서 행하고 이어서 공냉시킨다음 석출경화열처리 하였다. 인장시편은 판상 주조물의 여러군데에서 절단가공하여 기계적 성질들을 측정하고 미세조직을 검사하였다. 사형주조된 AZ92A합금의 기계적 성질에 미치는 냉금의 크기에는 효과적인 냉금의 크기가 있으며 본 연구결과에 의하면 이 보다 큰 냉금을 사용하여도 AZ92A 합금의 기계적 성질은 크게 향상되지 않는 것이 판명되었다.
