阴极电流密度对6061铝合金在含Na_(2)WO_(4)电解液中微弧氧化膜结构和耐磨性能的影响

Effect of Cathodic Current Density on the Structure and Wear Resistance of PEO Film on 6061 Aluminum Alloy Formed in Electrolyte Containing Na_(2)WO_(4)

目的 研究恒流模式下阴极电流密度对6061铝合金在含Na2WO4的电解液中制备的微弧氧化膜厚度、形貌、相组成及耐磨性能的影响。方法 固定阳极电流密度为5.0 A/dm^(2),阴极电流密度分别为0、1.25、2.5、3.75、5.0 A/dm2,对6061铝合金进行微弧氧化40 min。用涡流测厚仪测量了氧化膜的厚度,用扫描电镜观察了微弧氧化膜的表面形貌和截面形貌,用能谱分析仪分析了氧化膜的表面成分,用X射线衍射分析仪分析了微弧氧化膜的相组成,用往复式摩擦磨损试验机测试了氧化膜的耐磨性能。结果 随着阴极电流密度的增加,氧化膜内的W含量逐渐减少,氧化膜颜色逐渐变浅,氧化膜厚度逐渐增加。微弧氧化膜的主要组成相为α-Al2O3和γ-Al2O3。当阴极电流密度从0 A/dm2增加到3.75 A/dm2时,氧化膜内孔洞的数量和尺寸逐渐减少,孔洞到氧化膜/基体界面的距离逐渐增加,氧化膜的耐磨性能逐渐提升。当阴极电流密度为3.75 A/dm2时,氧化膜的磨损率最低,仅为1.07×10-4mm3/(N·m)。但阴极电流密度增加到5.0 A/dm2时,氧化膜表层出现孔洞和剥落,耐磨性能下降。结论 阴极电流的加入有助于增加6061铝合金微弧氧化膜的厚度,提高氧化膜的致密性和耐磨性能,但过高的阴极电流会导致氧化膜表层出现孔洞,降低耐磨性能。

Plasma electrolytic oxidation(PEO)is one of the promising methods for creating wear resistance coatings on aluminum alloy.The qualities of PEO coatings are determined by many factors,such as substrate materials,power supply modes,electrolyte composition,oxidation time and additives.In the bipolar power supply mode,the cathodic current density plays an important role in the PEO process.Previous results showed that the increase of cathodic current density could reduce the strong plasma discharges which made the coatings more compact and have better wear resistance.The composition of electrolyte is another key factor affecting the structure and performance of PEO coating.The use of Na_(2)WO_(4) containing electrolytes has emerged in recent years due to the wear and corrosion resistant properties of the resulted coatings.However,there is no research on the effect of cathodic current density on the structure and wear resistance of PEO film prepared in Na_(2)WO_(4) containing electrolyte.In this work,PEO films were prepared on 6061 aluminum alloy under different cathodic current densities in electrolyte composed of Na_(2)SiO_(3),Na_(2)WO_(4) and KOH.The effect of cathodic current density on the thickness,composition,structure and wear resistance of the PEO films was studied.6061 aluminum alloy with a thickness of 6.0 mm was used as the matrix.The samples with the size of 20 mm×20 mm×6 mm were cut from the alloy and were grounded up to 1500 grit by waterproof abrasive paper.During the PEO process,the anodic current density was set as 5.0 A/dm2,and the cathodic current was set as 0 A/dm^(2),1.25 A/dm^(2),2.5 A/dm^(2),3.75 A/dm^(2),and 5.0 A/dm^(2) respectively.The power frequency was set as 500 Hz,the number ratio of positive and negative pulses was 1:1,and the duty cycle of cathodic current and anodic current was 40%.The electrolyte contained 8.0 g/L Na_(2)SiO_(3)·9H_(2)O,3.0 g/L Na_(2)WO_(4) and 4.0 g/L KOH.The duration of PEO process was 40 minutes.After the PEO process,the surface and cross-section morphology of the samples were analyzed by a scanning electron microscope(Gemini SEM 300).The surface composition of the oxide film was analyzed by EDAX.The phase composition of PEO film was analyzed by an X-ray diffraction analyzer(Bruker D8).The thickness of PEO film was measured by an eddy current thickness gauge(Beijing Times TT2501).The friction experiment of oxide film was carried out by a reciprocating friction tester(HSR-2M).After the friction test,the wear scar morphology was analyzed by a laser confocal microscope(Olympus OLS4000).With the increase of cathodic current density,the W content in the PEO film gradually decreased,the color of the PEO film gradually became lighter,and the thickness of the film increased.The PEO film was mainly composed ofα-Al_(2)O_(3) andγ-Al_(2)O_(3) phases.When the cathodic current increased from 0 A/dm^(2) to 3.75 A/dm^(2),the number and size of voids in the film gradually decreased and they were located far away from the oxide film/matrix interface and the wear resistance of the oxide film was gradually improved.The wear volume of the oxide film under the cathodic current of 3.75 A/dm^(2) was only 1.07×10^(‒4) mm^(3)/(N·m),which was the lowest among all the samples.However,when the cathodic current density increased to 5.0 A/dm^(2),voids and peeling appeared on the surface of the film,and the wear volume increased.The cathodic current is helpful to increase the thickness of 6061 PEO film and improve the compactness and wear resistance of the PEO film.However,too high cathodic current density will produce voids on the top of the oxide film,and reduce the wear resistance of the PEO film.