Effect of cutting feed rate on machining performance and surface integrity in cutting process of Ti-6Al-4V alloy

Abstract

Titanium alloy is widely used in the aerospace industry due to many excellent properties such as high strength, low density, and good corrosion resistance. However, it is considered to be difficult to machine owing to their poor machinability, which is characterized by their low thermal conductivity and low chemical stability. To study the influence of cutting feed rate conditions on surface integrity, in this work, the experimental setup for the cutting test was developed with the online monitoring system using the dynamometer and infrared temperature tester. Variable cutting feed rates were applied in the cutting of Ti-6Al-4V alloy. The cutting force and cutting temperature during machining process were analyzed. Additionally, the surface roughness, hardness, residual stress, and microstructure of the machined surface of Ti-6Al-4V alloy were investigated. The cutting force, cutting temperature, and residual stress of workpiece were simulated based on the cutting experiments. The average error of cutting force and temperature between simulation and experiment was less than 10%, which showed a good agreement. The surface roughness exhibited an increasing trend as the feed rate increased. But there is a rapid increase of the surface roughness occurred when the feed rate exceeded 0.1 mm/rev. The compressive residual stress of the machined surface was characterized as increased at first and then decreased, reaching the maximum compressive residual stress value of 358.6 MPa at the feed rate of 0.10 mm/rev. The phase transition analysis was carried out by MATLAB after cutting Ti-6Al-4V. It was found that the α phase was decreasing and the β phase was increasing compared with the Ti-6Al-4V matrix. The transition from α phase to β phase during cutting Ti-6Al-4V could be facilitated by the high stress and rapid heating. The hardness evolution of the machined subsurface of Ti-6Al-4V was also characterized as increased at first and then decreased, reaching a maximum hardness value of 354.64 HV at the feed rate of 0.10 mm/rev. The work hardening-affected layer was about 400 μm. In summary, the high cutting force and high cutting temperature would induce the tool wear and deterioration of surface integrity of the machined workpiece when the feed rate exceeds 0.1 mm/rev. This suggests the existence of an optimal feed rate close to 0.1 mm/rev for the Ti-6Al-4V machining. It could provide a solid foundation for the further high-performance manufacturing of the key components with Ti-6Al-4V alloy.

Appendix. MATLAB program

Image1 = imread (['Graphics file location']);

imshow(image1)

[x, y] = ginput (2);

x = round(x);

y = round(y);

x1 = min(x);

x2 = max(x);

y1 = min(y);

y2 = max(y);

image2 = image1(y1: y2 x1: x2);

imshow(image2)

[xx, yy] = size(image2);

figure (2)

mmax = max(max(image2));

mmin = min(min(image2));

image4 = image2-188;

imshow(image4)

figure (3)

pcolor(image4)

colormap('jet')

shading flat

set(gca,'YDir','reverse')

for i = 1:xx

for j = 1:yy

if image4(i,j) > 0

image4(i,j) = 1;

else

image4(i,j) = 0;

end

end

end

sumwiter = sum(sum(image4));

ratio = sumwiter/(xx*yy);

disp('The β-phase ratio is: ')

ratio