7075-T6航太鋁合金表面改質製程與破壞行為之研究

Surface Modification Processes and Fracture Behaviors of 7075-T6 Aerospace Al-Alloy

邱盛乾(Sheng-Chien Chiu)

指導教授 ： 許正勳

大同大學/工程學院/材料工程學系所/博士(2004年)

摘要

本研究主要是利用表面處理製程施於航空用高強度7075-T6鋁合金，探討其經過不同披覆硬質鍍膜後，其鍍膜附著性、耐腐蝕性、疲勞強度及沖蝕磨耗等特性，接著分析試驗後之破壞行為。
本實驗研究目的有三；首先，嘗試以析鍍三種不同製程之鍍膜於航太材料7075-T6鋁合金，耐腐蝕硬質陽極處理(Hard Anodizing, HA)氧化鍍膜，及無電鍍鎳磷（ENi）合金之耐磨耗與耐腐蝕鍍膜，另外設計以無電鍍鎳磷合金鍍膜為中介層，外層以物理氣相蒸鍍(PVD)披覆類鑽石膜(DLC)而成複合鍍層，尋求疲勞強度提昇的最佳製程條件。第二，析鍍高低兩種不同磷含量與三種不同厚度之無電鍍鎳磷合金鍍膜，探討高低兩種不同磷含量之附著性與表面型態，進而析鍍三種不同厚度之無電鍍鎳磷合金鍍膜，來分析鹽霧腐蝕(Salt spray corrosion)行為以及對疲勞強度的影響；第三，以固體顆粒為媒介物進行沖蝕實驗，探討7075-T6鋁合金與披覆三種不同厚度之無電鍍鎳磷合金鍍膜後，其破壞機制與沖蝕磨耗損失。
實驗結果顯示，ENi/PVD-DLC複合鍍膜（7μm/3μm）具有高的微硬度值(Hv)、較低的表面粗度(Ra)以及極佳的附著性，對7075-T6鋁合金表面改質後，可以提高疲勞強度約22.2％。經被覆ENi鍍膜（10μm）後之疲勞強度下降了3~5.5％。而硬質陽極處理的鍍膜（10μm）特性整體表現最差。綜合以上三種製程，HA與ENi鍍膜的製程簡單成本較低，但是HA鍍膜的表面粗糙度較高，不利於疲勞強度。而ENi鍍膜的表面粗糙度較低，鍍膜硬度有提高，但析鍍前的硝酸活化處理會造成鍍膜與基材之間的交界處表面粗化，對於疲勞強度無法有效提升。而ENi/PVD-DLC複合鍍層的表面粗糙度較低，鍍層硬度也提高，對於疲勞強度能有效提升，但是製程中的腔體(chamber)溫度高達150℃以上，會直接使材料強度及硬度下降。
選擇最經濟、具耐腐蝕與析鍍效率高的無電鍍鎳磷（ENi）合金鍍膜進行鹽霧腐蝕的研究，發現鍍膜的磷含量不同時，表面型態與附著性亦不相同。較高磷含量鍍膜的表面較平滑且附著性較稍低磷含量的鍍膜為佳。耐候性實驗中，發現較高磷含量鍍膜厚度達到25μm時，即可達到防止鹽霧腐蝕的效果，並能有效減少腐蝕後疲勞強度的下降，但是鍍膜越厚（大於25μm以上）時，其疲勞破壞起始於鍍膜本身，所以疲勞強度下降。
從Al2O3固體顆粒沖蝕磨耗的破壞行為中可以發現，7075-T6鋁合金試片在低角度沖蝕時之破壞機制為較寬且深的切削與剷犁，同時沖蝕磨耗率最大。中角度沖蝕時，因為其衝擊破壞的分力最大，則為陷入及剷犛的凹坑破壞機制，出現最大的沖蝕磨耗深度，並且呈現出高斯分佈輪廓。高角度沖蝕時，則為壓入凹坑及塑性變形破壞機制。另亦發現，次表面的應變硬化層會隨衝擊角度的增加而變大，且沖蝕的砂量增加時，沖蝕磨耗率會降低，這是材料加工硬化的影響。ENi鍍膜對於低角度的磨耗抵抗低於較高角度，又因鍍膜較薄及基材特性，整體而言，屬於延性破壞的型態。

關鍵字

7075-T6鋁合金、硬質陽極處理、無電鍍鎳磷、物理氣相蒸鍍、類鑽石膜、沖蝕磨耗、延性破壞.

並列摘要

In this investigation the adhesion strength, corrosion resistance, fatigue strength, and the erosion resistance are studied for various surface treatment applied on the high strength aerospace 7075-T6 Al-alloy. The morphology of the fracture surfaces, were also studied afterwards.
There are three objectives in this investigation: the first objective is to identify the optimum manufacturing processes of the three coating systems for possibly raising the fatigue strength of the coated system. The selected coatings for the experiments are the Hard Anodizing (HA) coating, commonly used in the aerospace industry for its corrosion resistance, the electroless nickel phosphorous (ENi) coating for its wear and corrosion resistance and the duplex coating system with the ENi as the under coat and the diamond like carbon top coating through physical vapor disposition (PVD) method. The second objective is to study the impact of the salt spray corrosion to the fatigue strength of the ENi coating in three thicknesses. The third objective is to investigate the fracture mechanism and wear loss of the 7075-T6 Al-alloy coated with three different thicknesses of ENi coating and sandblast them with solid particles.
The test results indicate that ENi/PVD-DLC duplex coating (7μm/3μm) has high micro-hardness (Hv), low surface roughness and high adhesion strength that improve the fatigue strength of the 7075-T6 Al-alloy by about 22.2%. The ENi coating, on the other hand, lowers the fatigue strength of the 7075-T6 Al-alloy by about 3 to 5.5%. The worst fatigue performance among the three coating systems is the HA coating. These test results help to conclude the importance of high hardness, good adhesion strength and low surface roughness of a coating system in reducing the chance of crack initiation at the treated surface thus improves the fatigue strength of the treated material.
The surface appearances and the adhesion strength of the most cost-effective and high deposition efficiency ENi coating are influenced by the amount of phosphor in the coating. The high phosphorous (Hi-P) ENi coating has lower surface roughness and higher adhesion strength when compared to the low phosphorous ENi coating. It is found that ENi coated to a 25μm thick is sufficient to prevent the corrosion from happening when exposing the coated 7075-T6 Al-alloy to a long term salt spray corrosion test. Also found in this investigation is that the ENi coating film when built up to 25μm thick can efficiently reduce the corrosion induced fatigue strength loss but doubling the coating thickness to 50μm thick it is surprising to find out that the fatigue strength is lower than the 25μm after exposing them to salt spray corrosion.
A wide and deep cutting with plowing like fracture mechanism was found on the 7075-T6 Al-alloy test specimen when exposed at lower angle Al2O3 solid particle erosion test. Crater and plowing like fracture mechanism was identified for the mid angle erosion test that has the highest erosion force for producing the Gaussian distribution profile. At the high angle erosion test, the fracture mechanism revealed an indented crater with plastic deformation fracture mechanism. Also found was the subsurface work hardened zone increases with the erosion amount of the solid particles in sandblasting test. The resistance to the wear is lower for ENi coating under low angle erosion than the high angle erosion. In addition, the coating film is thinner than others and the uniqueness of the substrates mechanical properties, overall, result in a ductile fracture mechanism.

並列關鍵字

7075-T6 Al-alloy ； Diamond like carbon ； Ductile fracture mechanism. ； Electroless nickel phosphorous ； Erosion ； Hard Anodizing ； Physical vapor disposition

參考文獻

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