選擇性雷射熔融（SLM）積層製造技術是透過雷射光束聚焦燒熔基板上金屬粉末，藉此將粉末逐層燒熔推疊直至工件完成。本研究目的為探討不同積層方向及後處理之回火效應對積層工件之顯微組織及各項機械性質之影響，選用之材料為AISI 420麻田散鐵系不銹鋼，包含研究拉伸性質、破裂韌性、疲勞裂縫成長等各項機械性質，並探討積層工件熱處理前後，殘留應力分布、顯微組織及異向性機械性質之間的關係。 在第一部分中，以SLM技術製作兩種不同積層方向之AISI 420拉伸試驗試片並進行拉伸試驗，分別是積層方向平行於試片施力方向的平行（PRL）試片以及垂直於試片施力方向的垂直（RERP）試片。接著試片區分三種熱處理狀態，分別是未經熱處理之初始狀態、200 °C及400 °C兩種回火溫度之後處理。兩組不同積層方向之殘留應力及硬度分佈有明顯的差異。PRL試片在各種狀態之降伏強度、抗拉強度與延伸率皆大於PERP試片。在初始狀態下拉伸破斷面呈現典型的脆性斷裂模式，另外回火狀態破斷面除了脆性劈裂的主要特徵，依回火條件不同呈現部分延性破裂的破斷面組合。SLM製作的兩種試片均具有沿積層方向生長的細長形柱狀結構，回火熱處理對於原先形成的細長形柱狀晶界並未有明顯改變，但對於晶界內的麻田散鐵影響較大，也因此在熱處理後異向拉伸性質仍然沒有消失，而且降伏強度的異向性隨回火溫度提高而提升，但延伸率卻有著相反的趨勢。 在第二部分中，所進行破裂韌性試驗之不同積層方向試片製作條件，與第一部分研究設定相同。PRL試片之破裂韌性在初始狀態及回火條件下皆大於PERP試片。在400 °C回火條件下兩種積層工件之破裂韌性值均優於初始狀態及200 °C回火條件，主要歸因於粗大化回火麻田散鐵體之熱處理效應。由於積層方向之影響，細長形柱狀結構仍然是影響異向破裂性質的主因，因此，回火效應雖明顯提升破裂韌性，但幾乎沒有對破裂韌性的異向性造成影響。 在第三部分中，針對不同積層方向製作的試片進行疲勞裂縫成長（FCG）試驗，試片包含未經熱處理之初始狀態及400 °C回火熱處理兩種狀態。實驗結果顯示，與PERP試片相比，PRL試片在初始狀態及回火狀態下均呈現較佳的 FCG 阻抗，有較低的疲勞裂縫成長速率。此外，回火效應能明顯減少疲勞裂縫成長的異向性行為，不過也會導致疲勞裂縫成長應力強度因子門檻範圍值降低。關於破裂韌性及疲勞裂縫成長所觀察到的異向性行為，主要歸因於裂縫成長路徑與細長形柱狀結構之間的相互作用所致。 ;The aim of this study was to investigate the relationship between build orientation, heat treatment, microstructure, and mechanical properties of AISI 420 martensitic stainless steel fabricated by selective laser melting (SLM). In Part I, tensile specimens were directly built by SLM in the directions perpendicular and parallel to the loading direction of the tensile test. Heat treatments at 200 °C and 400 °C were conducted to study the tempering effect. The difference in the residual stress and hardness distribution between the two build directions was noticeable. The parallel group exhibited better strength and ductility than the perpendicular group in both as-built and tempered states. Typical brittle fracture pattern was visible in the as-built state while a combination of primarily brittle and secondarily ductile fracture features was seen in the tempered conditions. Both as-built and tempered SLM builds exhibited elongated cellular structures growing along the build direction. The directional microstructure was responsible for the anisotropic tensile properties. The anisotropy of tensile strength of SLM AISI 420 build could be reduced to a certain extent by the given tempering treatments, while the elongation exhibited an opposite trend. In Part II, the anisotropic fracture toughness of SLM AISI 420 built in two orientations was investigated. Post-process heat treatments were performed at 200 °C and 400 °C to study the tempering effect. The parallel group exhibited better fracture toughness than the perpendicular group in both as-built and tempered states. KIC value of the 400 °C tempered state was superior to that of other states due to a tempered martensite phase. Elongated cellular structure was responsible for the anisotropic behavior because of a build direction effect, while the anisotropy was barely changed by the tempering treatments. In Part III, the anisotropic fatigue crack growth (FCG) behavior of SLM AISI 420 builds in two different build directions was investigated, in consideration of the effect of tempering treatment at 400 °C. The parallel group, in both as-built and tempered states, had a lower fatigue crack growth rate (FCGR) than the perpendicular group. The given heat treatment reduced the anisotropy of FCG behavior to a less extent, as the difference in FCGR between the two orientations was decreased in the tempered state, compared to the as-built state. The anisotropic FCG behavior was attributed to the interaction between the crack path and the elongated cellular structure. Moreover, it was found that tempered martensite had a greater influence on the perpendicular group than on the parallel group.
