2. Materials and Methods
1060 aluminum (purity 99.9 wt.%) and 6082 aluminum alloy (with Si 0.87 wt.% and Mg 0.65 wt.%) ingots were cut into 60 mm × 20 mm × 5 mm plates as the substrates. After sanding by sandpaper to remove the alumina film, these plates were ready for experiments.
The main equipment was a commercial GTAW welder (Model WP-300, Panasonic, Tangshan, China) and its torch was fixed on an automotive device moving at the given speed. The nitrogen gas and the argon gas were mixed by a mixer which can control the flow rates of each gas. Then the mixed gas was served as the ionizing gas and the shielding gas. The mixed gas flow rate was 10 L/min. The tungsten electrode of the welding torch was about 3 mm above the substrate where the mixed gas was ionized. The direct-current electrode negative (DCEN) polarity was applied, the travel speed was fixed to 2.5 mm/s, and the nitrided layers were prepared by single rows. In order to study the AlN strengthening layer structures on different substrates, nitrided layers were prepared on 1060 and 6082 substrates by a discharge current of 140 A with a nitrogen gas flow rate of 5 L/min. For studying the effects of nitrogen flow rates on nitrided layer structures and properties, nitrided layers were prepared by different nitrogen gas flow rates (1 L/min, 2.5 L/min, 5 L/min, and 7.5 L/min) on 6082 substrates with the discharge current 110 A. The argon gas flow rates are the differences between the mixed gas flow rates and the nitrogen gas flow rates.
After nitriding, the samples were cut along the cross-sections. To analyze the forming process, a sample (discharge current 110 A, N2 flow rate 2.5 L/min) was cut from the end of the nitrided layer against the travel direction. Then, after sanding, polishing, and etching (the etching solution was composed of 1 mL HF, 1 mL HCl, and 8 mL H2O), these specimens were analyzed by optical microscopy (OM; Model Axio Scope A1, Zeiss, Oberkochen, Germany) and scanning electron microscopy (SEM; Model JSM-5310, JEOL, Tokyo, Japan) with the energy-dispersive spectroscopy system (EDS; Model Link-Isis, Oxford Instruments, Oxford, Britain). Phase compositions of the nitrided layers were investigated by an X-ray diffractometer (XRD; Model D/Max 2500PC, Rigaku, Tokyo, Japan) with a Cu Kα radiation source. The XRD specimens were cut from the nitrided layer, and the surfaces of the nitrided layers needed to be ground to a flat plane before XRD tests, and the grinding depth was about 200 µm. Transmission electron microscopy (TEM; Model JEM-2100F, JEOL, Tokyo, Japan) was also used to show the microstructure of the nitrided layer.
The mechanical properties of the nitrided layers prepared by a discharge current of 110 A on 6082 substrates were measured. Vickers microhardness distributions of the cross-sections were measured by a microhardness tester (Model HXD-1000, Shangguang, Shanghai, China), loading 1 N for 10 s. Cylindrical wear specimens 6 mm in diameter and 5 mm in height were prepared from the nitrided layers. They were worn by an abrasive wear tester (Model ML-100, Jingcheng, Jinan, China) and the friction pair was #500 corundum sandpapers. The samples were worn for 20 m under 3 N loading, at the rotation speed of 60 rev/min and the radial feed rate of 4 mm/r without lubrication. The weights of the samples before and after the experiments were measured by an electronic balance having a readability of 10−4 g, and then the weight losses were calculated. Finally, the worn surfaces were analyzed by SEM for the wear mechanisms.