Investigation Of Fatigue Reliability On Alloy ZL101A By Adding Titanium In Electrolysis

Fatigue/fracture is a broad failure mode for the metal parts under alternate loading and may result in tragedies for its sudden failure. The fatigue strength of materials is affected by many stochastic factors, so analyzing the fatigue reliability of the metal parts under alternate loading becomes a key subject in the area of engineering design and is becoming more and more important. Analyzing the fatigue reliability of the metal parts under alternate loading must accord the theory of reliability and use the method of probability and statistics.The alloying element of titanium in casting alloy ZL101A is usually added by the method of melting high-titanium aluminum alloy, but the 'alloy ZL101A by adding titanium in electrolysis' is prepared with 'electrolytic low-titanium aluminum base alloy' substituting for commercially pure aluminum and high-titanium aluminum alloy. Electrolytic low-titanium aluminum base alloy is a new type of material developed by the Key Laboratory of Material Physics of Zhengzhou University and electrolytic aluminum factory. Without changing the traditional technical conditions of electrolytic production, still using the alumina as the material, after directly adding a small quantity of TiO₂ into electrolytic tanks then electrolyzing can acquire the alloy of aluminum and titanium with the content of titanium lower than 0.3%.Alloy ZL101A is broadly used to produce the wheel hubs of car and motorcycle for its excellent casting, mechanical and processing properties. The running vehicles are under alternate loading. In order to ensure that the parts made from 'alloy ZL101A by adding titanium in electrolysis' can work safely, reliably and normally, it is necessary to investigate the fatigue reliability of 'alloy ZL101A by adding titanium in electrolysis' systematically and generally.In order to study the effects of element titanium and different titanium alloying methods on the fatigue reliability of alloy ZL101A systematically, three kinds of alloys were produced under the same technologies of melting, heat treatment and machining. One is the normal alloy ZL101 containing no titanium; the second is produced with the electrolytic low-titanium aluminum base alloy (referred as ZL101R); the third is produced with pure aluminum and Al-Ti master alloy (referred as ZL101R). The fatigue crack growth rate da/dN and thresholdΔK_th of the three kinds of alloys are tested respectively.It is convinced that every specimen of the same group should contribute equally to its mechanical property. A new data processing method for group testing data of different specimens is proposed. The testing data are simulated using Monte Carlo method. The thresholdΔK_th and parameters of the fatigue crack growth rate da/dN -ΔK curve ofthis group of alloy in Paris zone are obtained in the method of ultimately fitting the simulating data substituting for testing data.The process of fatigue/fracture generally can be divided into three successive phases: the initiation of the crack, the steady propagation of the crack and the fast growth of the crack leading up to fracture. ThresholdΔK_th reflects the ability of crack initiationresistance of the alloy in the first phase. The results of the tests show that the thresholds of the three kinds of the alloys don't differ from each other obviously, that is to say, the threshold is not sensitive to adding titanium and different titanium alloying methods.The most part of the fatigue life of alloy ZL101A is consumed by the second phases, namely subcritical crack growth. The fitting parameters of Paris zone reflect the fatigue crack growth resistance of the alloy and the fatigue life can be indirectly calculated according to the parameters. The da/dN -ΔK curves are presented in Paris zone with different confidence levels then the P - da/dN -ΔK curves are presented with different reliabilities.Alloy ZL101D have the lowest fatigue crack growth rate and the fatigue crack growth rate of ZL101R is lower than ZL101's in Paris zone. It's because that alloy ZL101A'sα(Al) phase are refined by titanium. The average diameter of eutectic silicon particles decreases and the roundness increases at the same time. And the refining effect of electrolytic titanium alloying method is better than that of melting method. That is why adding titanium can increase the fatigue crack growth resistance of the alloy.A reliability mold, using a fatigue crack growth rate as the failure index, is set up. The P-ΔK curves of ZL101, ZL101D and ZL101R are presented. The results show that with the increasing of the stress intensity factor rangeΔK, the fatigue crack stable propagation reliabilities of the three kinds of alloys all decrease, but ZL101D and ZL101R are slower than ZL101. This is in accordance with the results: adding titanium increases the fatigue crack growth resistance of alloy ZL101 A.