###### Abstract  
Due to its excellent strength, plasticity, and fracture toughness, titanium alloy has been widely used in the aerospace field. The specificity of its application environment places high demands on the surface quality of titanium alloy. In this paper, we study the effects of different lubrication methods on the microscopic topography, surface roughness, and microhardness of titanium alloy TC21 during the milling process. The lubrication methods include dry, high-pressure air cooling, and minimum quantity lubrication (MQL). Compared with dry milling and high-pressure air cooling conditions, the MQL environment can effectively suppress plastic deformation and surface defects of titanium alloy TC21. MQL is significant for improving the milling process of titanium alloy TC21. In addition, a surface roughness model considering milling vibration is developed. According to the results of orthogonal experiments, the prediction accuracy of the surface roughness model is acceptable, and the prediction errors are all below 20%.  
MQL; titanium alloy TC21; surface integrity; milling vibration +
Footnote †: journal: Applied  
Titanium alloys are characterized by high chemical activity, low thermal conductivity, and low modulus of elasticity. Compared with other conventional engineering materials, titanium alloys have poor machinability, which restricts the application and development in the aerospace field. The specificity of the application environment places higher demands on the machining quality, mechanical properties, and cost of titanium alloys [11; 12; 13]. In recent years, most scholars have conducted a series of studies about the milling of titanium alloy Ti-6 Al-4V. Due to the dimensional effect of cutting area, subsurface deformation and burrs are generated by chip squeezing and tearing failure during the conventional milling process. These defects can seriously affect the working performance and service life of critical components such as aeroengines. Thus, it is necessary to establish an advanced cutting theory and find a suitable machining method [14; 15]. In recent years, some novel machining methods such as electric discharge assisted milling (EDAM) and electric discharge assisted grinding (EDAG) have been gradually accepted and applied by modern industry. Due to the reduction in surface hardness and yield stress of the workpiece, EDAM can effectively suppress burr generation and improve the surface quality of Ti-6 Al-4V. The residual stress obtained by EDAM is much smaller than that of general milling [16; 17]. Many scholars have also developed the milling machining mechanism from milling temperature and milling force. Based on the established milling machining model, the theoretical results of these models are in better agreement with the experimental results [18; 19; 20].  
In summary, most scholars have conducted extensive studies on the milling properties of traditional materials such as titanium alloy Ti-6 Al-4V. Titanium alloy TC21 has been rarely reported as a novel metal material for aeronautical applications. In terms of milling theory, the modeling and prediction of milling force and temperature are the main studying directions. A surface roughness model considering milling vibration is developed in this study. Titanium alloy TC21 is milled using a coated tool under three machining methods (dry, high-pressure air cooling, and MQL). The coating and substrate materials are TiAlN and WC, respectively. The effects of lubrication methods, lubrication parameters, and machining parameters on the surface integrity of TC21 are studied. This work is significant to improve the surface quality and optimize the machining process of titanium alloy TC21 during milling.