Novosibirsk State Technical University (NSTU) is working to determine rational methods for finishing plastic aluminum alloys without removing the material. The advantages of such methods compared to traditional cutting processes are resource savings and surface hardening.
Aluminum alloys are used literally everywhere today — from food packaging to aircraft structures, they are indispensable due to their lightness and high specific strength. But for plastic, relatively soft materials like aluminum, there is little data on how to properly perform ultrasonic treatment in order to avoid negative effects, most of the research is devoted to steels and titanium, says Alina Ivanova, an assistant at the Department of Mechanical Engineering Technology at NSTU-NETI.
Working with aluminum and its alloys has a number of nuances due to the physical and chemical properties of the metal. This is, for example, the appearance of flow waves — surface defects in the form of wavy or ring-shaped patterns formed due to plastic deformation of the material; sticking — when chips adhere to the cutting edge of the tool, which worsens the surface quality and increases tool wear.
As part of the research, the scientists used two alloy processing methods — diamond smoothing (AB) and ultrasonic surface plastic deformation (USPD). They relate to finishing processing operations, that is, they are used at the last stage of manufacturing a part, when the geometry has already been set and it is necessary to improve the surface properties.
"The essence of the first method is as follows: a special tool with a working part made of a hard material (synthetic diamond or hard alloy) is pressed against a rotating part with a certain static force and moves along its surface. Local plastic deformation occurs, micro-dimensions are smoothed out, the surface layer is strengthened, compressive residual stresses are formed in it, increasing fatigue strength and preventing the formation of cracks. All this happens without removing the material and without using an abrasive tool. During ultrasonic surface plastic deformation, ultrasonic vibrations of the order of 20 kHz are additionally applied to the instrument. This turns the continuous contact into a series of micro-impacts that intensify the deformation and facilitate the flow of metal. For soft aluminum alloys, we used a tangential oscillation input scheme, when the tool does not oscillate perpendicular to the surface, but tangentially, which avoids undesirable effects," said Alina Ivanova.
The scientists established rational AB and USPD modes for ductile aluminum alloys and compared the two methods on the same samples under the same conditions. The studies were carried out on cylindrical samples made of two aluminum alloys (AMg6 and D16T), which differ in physical properties. AMg6 is a ductile, corrosion—resistant alloy with low hardness, used in the aerospace industry for fuel tanks and welded structures. D16T is duralumin, which is much harder and more durable, and is widely used in aviation as a structural material.
Under rational USPD conditions, the surface roughness decreased almost fivefold (from Ra 0.679 microns to Ra 0.158 microns), while the microhardness increased by 30-40%. In addition, the surface acquired a characteristic cellular structure that improves wettability and lubrication retention, which directly affects wear resistance under friction conditions. Diamond smoothing gave a higher microhardness for AMg6, but USPD consistently provided a smoother surface and a more favorable morphology. The scientists also investigated technological heredity — how the initial condition of the surface affects the result after processing and how many tool passes are needed.
"The practical value of the results obtained is that now industries have specific data on what load and feed to work with a particular alloy, which method to choose for a specific task: if maximum surface hardness is needed, diamond smoothing is preferable, if a combination of properties is important, that is, low roughness, wear resistance, fatigue strength, ultrasonic surface plastic deformation works better. This will help to reduce the rejection rate and reduce the cost of selecting modes by trial and error. As for the field of application, these are the aviation and aerospace industries, shipbuilding, chemical engineering, that is, those areas where aluminum parts with increased requirements for reliability and durability are used," adds Alina Ivanova.
Now scientists are preparing a new series of experiments using a roller tool. Unlike a diamond deformer, the roller has a much larger contact area with the surface, which means that significantly higher loads are required to create the necessary pressure in the processing area. To make this possible, the missing tooling was designed and manufactured. This is the next step in researching processing modes and improving the finishing technology of aluminum alloys.