Fatigue Behavior Of Ti-5Al-5Mo Titanium Alloy Forgings

Ti-5Al-5Mo alloy has the characteristics of high specific strength and good corrosion resistance.It is often used to make aircraft parts and hull structures.Because the fatigue behavior of titanium alloy is affected by its structure,component shape and bearing mode,two kinds of structure forgings,basket and equiaxed forgings,are studied in this paper.The forgings are processed into three kinds of fatigue specimens with stress concentration factor?K_t?,and fatigue tests are carried out under two different stress ratios?R?.The effects of structure,stress concentration factor and stress ratio on fatigue behavior of titanium alloys are studied.The results are of great significance for rational arrangement of material use,improvement of fatigue life of components and reduction of fatigue damage.The S-N,P-S-N curves and Goodman-Smith diagrams of the two kinds of structures were drawn to observe the fatigue fracture morphology.The effects of K_t and R on the fracture morphology,fatigue strength?S?,fatigue life?N_f?and fatigue limit(?_-1)were analyzed.Local stress-strain and finite element software Fe-safe were used to predict the N_f of the two kinds of structures,and compared with the test life value.The main results are as follows:Under the same R and N_f,the maximum stress(?_max)decreases with the increase of K_t.At the same K_t,?_max decreases with the increase of R,while stress amplitude??_a?increases.The fatigue strength of equiaxed structure under different K_t and R is better than that of basket structure,but the S-N curve of equiaxed structure decreases rapidly,which shows that basket structure has excellent fatigue resistance.The P-S-N curve of equiaxed tissue is higher than that of basketball tissue under the same K_t and R,but the P-S-N curve of basketball tissue decreases slowly.Basketball tissue also shows better fatigue resistance under different survival rates.The Goodman-Smith diagram of equiaxed organization reliability under the same K_t and R is larger than that of basket organization.For the Goodman-Smith diagram of the same kind of organization,the range increases with the increase of R.The fatigue strength,fatigue limit and average stress sensitivity factor of Ti-5Al-5Mo alloy are affected by K_t and R.The fatigue life of equiaxed structure is gradually lower than that of basket structure with the increase of K_t.Combining Goodman's life curve with Peterson's and Neuber's formula,fatigue data of R=-1 and?_-1 of K_t=1 are used to predict three fatigue S-N curves of Ti-5Al-5Mo alloy K_t=1,3,5and R=0.5.Compared with experimental results,the overall prediction is more accurate,which proves that the S-N curves of different stress ratios have a clear functional relationship.The relationship between basket and equiaxed structure K_t and N_f is discussed by using generalized S-N surface.The results show that K_t has a significant effect on equiaxed structure S-N surface.The proportion of crack growth zone in the macro-fracture of the two alloys is an important index of the length of N_f.With the increase of K_t,the macro-fracture morphology becomes smoother,and the three typical regions of the fracture become more obvious with the decrease of R.From the crack growth zone of the micro-morphology,it can be seen that the fatigue glow of the basket structure is longer and deeper than that of the equiaxed structure,and the fatigue glow of the equiaxed structure is smoother and smaller.Using ANSYS finite element software,the local stress and strain of smooth and notched specimens under loading are simulated and calculated.Manson-Coffin,SWT model and finite element software Fe-safe are used to predict the fatigue life respectively,and compared with experimental data.The results show that the fatigue life predicted by Fe-safe software is in good agreement with the experimental values,and the correlation coefficients R are 0.96 and 0.97,respectively.