藉由等通道轉角擠型熱機處理探討Mg-5wt.%Sn合金之顯微結構對乾磨耗性質之影響

Abstract

本篇論文主要利用光學顯微鏡(OM)、掃描式電子顯微鏡(SEM)以及X-Ray繞射儀(XRD)來分析Mg-5wt.%Sn合金的鑄造狀態與經過一、二、四道次之等通道轉角擠型後之顯微結構，並施以300℃之1小時、2小時、3.5小時退火處理，此外對鑄造狀態合金施以520℃14小時之固溶處理與300℃18小時、36小時、54小時之時效處理，隨後以維氏微硬度計探討以上組別之硬度值，並探討其對乾磨耗性質的影響。Mg-5wt.%Sn合金鑄件於鑄造狀態下，在晶界處為連續層狀Mg2Sn析出，晶粒內部有明顯黑色偏析區域，在經過四道次等通道轉角擠型後，晶粒內部偏析區因熱擴散完全消除，晶粒尺寸由鑄態117.6μm降至24.4μm，晶界處之連續層狀Mg2Sn受到純剪力作用而被破斷，平均分散於晶粒之中，且有奈米級Mg2Sn在擠型過程中動態析出，平均顆粒大小為5~10nm，其體積分率達3.12%，試片硬度達到52.5Hv。鑄造狀態經 520℃14小時固溶處理後經300℃54小時時效處理，析出物體積分率為1.340%，硬度達到61.2Hv。 在pin-on-disc磨耗測試下，晶粒尺寸 88.0μm(一道次)下的磨耗阻 抗為佳，達73.77m/mm3，此為塊材強化機制與氧化傾向競爭所致，隨 著晶粒細化強化與析出硬化強化，整體硬度上升，但隨著擠型道次上升， 內應力與差排密度上升，合金氧化傾向上升，形成硬脆MgO隨著磨耗過程剝落，而變成三體刮除磨耗下的界面間磨料，進而降低磨耗阻抗。 在晶粒大小分別為88.0μm、49.5μm、24.4μm擠型後經1小時退火處理後，磨耗阻抗皆大幅上升，退火處理釋放內應力並形成再結晶新生小晶粒，磨耗表現以晶粒大小24.4μm之試錠退火後為佳，達118.60 m/mm3，為退火前的 1.8倍，退火時間增長後磨耗表現隨著晶粒成長而下降。時效處理18小時後磨耗阻抗最佳，達107.76m/mm3，之後隨著時效時間而下降，因為隨著時效析出，半整合性邊界造成失配，而在基地內形成應力場，進而增加氧化傾向，其結果為析出強化與氧化傾向競爭所致。 磨耗表面隨著晶粒細化與析出強化機制硬度上升，磨耗機制由黏著磨耗與刮除磨耗轉變成疲勞磨耗與氧化磨耗為主，磨屑隨著擠型道次上升而Mg2Sn含量上升，並含有大量的MgO成分。動摩擦係數分析中，析出物體積分率上升與晶粒細化下擁有最低的平均磨擦係數值，α-Mg基地相與對磨盤有黏著傾向，隨著Mg2Sn析出與均勻化，進而降低基地與對磨盤的接觸面積，基地相強度上升亦能降低凸點與對磨盤的接觸面積，此外，堅硬的Mg2Sn與MgO磨屑有固態潤滑效果，從而降低摩擦係數值。平均表面粗糙度的結果與磨耗阻抗結果相依，高磨耗阻抗下會擁有較低的表面粗糙度。

The Mg-5wt.%Sn alloy in as-cast condition, were solution heat-treated (SHT) in 520℃for 14 hours and aged in 300℃ for 18 hours、36 hours and 54 hours. Another group, Mg-5wt.%Sn alloy, were processed by Equal Channel Angular Extrusion (ECAE) through 4 passes respectively, then annealed in 300℃ for 1 hours、2 hours and 3.5 hours. The microstructure of all groups was investigated by using optical microscopy (OM), scanning electron microscopy (SEM), and X-Ray Diffraction system (XRD). Besides, the hardness were investigated by Vickers Hardness Tester. Pin-on-disc wear test was conducted on all groups to investigate dry sliding wear behaviors. In the as-cast condition, continuous eutectic α-Mg + Mg2Sn were precipitated in grain boundary, and segregation was placed in matrix. After four passes of ECAE, the segregation was eliminated and the grain size was refined from 117.6μm (As-cast) to 24.4μm. The eutectic α-Mg + Mg2Sn were broken and distributed to both grain and grain boundary. Nano Mg2Sn particles were dynamic precipitated, average size were 5nm to 10nm, and the volume fraction was 3.12%. Hardness reached to 52.2Hv. Another group of as-cast sample was solution treated at 520℃for 14 hours, then aged at 300℃ for 54 hours, the volume fraction reached to 1.340% and the hardness reached to 61.2Hv. Grain size condition 88.0μm (1 pass) had the highest wear resistance reached to 73.77m/mm3, that was an outcome caused by grain size refining and precipitate hardening against oxidation. High dislocation density and stress field causing high internal energy occurred during multi-passes ECAE, which increased the oxidation rate of processed alloy. Debris composed of hard and brittle MgO was scratched down and became abrasive behavior of three-body abrasive wear leading to lower wear resistance. Samples annealing for 1 hour under grain size conditions 88.0μm、49.5μm、24.4μm led to increase the wear resistance due to residual stresses releasing and recrystallization, but decreased the wear resistance with more than 2 hours, due to the grain growth. Samples aging treatment for 18 hours had the highest wear resistance reached to 107.76 m/mm3, but then decrease with aging time. It was attributed to the stress field caused by the misfit of precipitate and matrix leading to increase oxidation rate. Wear mechanisms were mainly adhesive and abrasive wear under grain size condition 117.6μm, but turned to delamination and oxidation under grain size condition 24.4μm. Mg2Sn contents and grain size were in inverse proportion in XRD results of debris, and both had large amounts of MgO. Condition with finest grain and the more volume fraction of Mg2Sn had the lowest coefficient of friction (COF). Greater α-Mg asperity contact was responsible for higher COF, grain refining and precipitate hardening decreased the contact area. More than that, Mg2Sn and MgO caused solid lubricant effect. Average surface roughness was in keeping with wear resistance, higher wear resistance surface tended to have lower surface roughness.