N-Nitrosodimethylamine (NDMA) is a potent carcinogen associated with chloramination of wastewater and wastewater-impacted drinking waters. Substantial effort has been expended to identify the precursors and mechanisms leading to NDMA formation. One of the major discoveries has been that molecules in the N,N-dimethyl-α-arylamine class, including the common pharmaceutical ranitidine, form NDMA in high yield during chloramination. Simultaneously, it was hypothesized that these precursors react with monochloramine, the dominant species in most chloramine mixtures, to form NDMA. This monochloramine hypothesis contradicts past mechanistic work with simple secondary amines, as well as practical experience showing that minimization of dichloramine reduces the level of NDMA formation during wastewater reuse and drinking water treatment. In this work, we address the contradiction between practical experience and model precursor studies by showing that N,N-dimethyl-α-arylamines form NDMA chiefly via reactions with dichloramine, rather than monochloramine. We experimentally demonstrate substantially higher NDMA yields from dichloramination than from monochloramination of four N,N-dimethyl-α-arylamine compounds, including ranitidine, and computationally rationalize declining NDMA yields at large dichloramine doses, which may explain past results reporting low NDMA yields from dichloramination of ranitidine. These results provide support for NDMA control strategies currently under evaluation at wastewater reuse facilities.

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In the selective laser melting process, metal powder melted by the laser heat source generates large instantaneous energy, resulting in transient high temperature and complex stress distribution. Different temperature gradients and anisotropy finally determine the microstructure after melting and affect the build quality and mechanical properties as a result. It is important to monitor and investigate the temperature and stress distribution evolution. Due to the difficulties in online monitoring, finite element methods (FEM) are used to simulate and predict the building process in real time. In this paper, a thermo-mechanical coupled FEM model is developed to predict the thermal behaviors of the melt pool by using Gaussian moving heat source. The model could simulate the shapes of the melt pool, distributions of temperature and stress under different process parameters through FEM. The influences of scanning speed, laser power, and spot diameter on the distribution of the melt pool temperature and stress are investigated in the SLM process of Al6063, which is widely applied in aerospace, transportation, construction and other fields due to its good corrosion resistance, sufficient strength and excellent process performance. Based on transient analysis, the relationships are identified among these process parameters and the melt pool morphology, distribution of temperature and thermal stress. It is shown that the maximum temperature at the center point of the scanning tracks will gradually increase with the increment of laser power under the effect of thermal accumulation and heat conduction, as the preceded scanning will preheat the subsequent scanning tracks. It is recommended that the parameters with optimized laser power (P = 175–200 W), scanning speed (v = 200–300 mm/s) and spot diameter (D = 0.1–0.15 mm) of aluminum alloy powder can produce a high building quality of the SLM parts under the pre-set conditions.