Effect of Vanadium Addition to Aluminum Grain Refined by Ti or Ti + B on Its Microstructure, Mechanical

Behavior, Fatigue Strength and Life.

ADNAN i. O. ZALD, PROFESSOR OF MECHANICAL & INDUSTRIAL

ENGINEERING, APPLIED SCIENCE UNIV.AMMAN 11931 Jordan

1). A luminum and its alloys are widely used materials in automobile, aircraft and space craft industries due to their attractive properties, e.g. high strength- to-weight ratio, corrosion resistance, ability to be ionized beside their other useful properties.

2). Against these attractive properties they have the disadvantage of solidifying in columnar structure with large grain size which has adverse effect on their mechanical strength and surface quality.

3). Therefore it became a necessity to grain refine their structure by either Ti or Ti+B to avoid these deficiencies and master alloys of Al-Ti and Al-Ti-B are manufactured for this purpose and are now commercially available.

4). The available literature reveals that most of the published work on grain refiners is directed towards the metallurgical aspects, and very little work is published on the effect of these refiners on their mechanical behavior, fatigue life and strength, wear resistance and corrosion resistance.

3). Therefore it became a necessity to grain refine their structure by either Ti or Ti+B to avoid these deficiencies and master alloys of Al-Ti and Al-Ti-B are manufactured for this purpose and are now commercially available.

4). The available literature reveals that most of the published work on grain refiners is directed towards the metallurgical aspects, and very little work is published on the effect of these refiners on their mechanical behavior, fatigue life and strength, wear resistance and corrosion resistance.

5). IMPROVING FATIGUE LIFE AND STRENGTH

The following methods are suggested to increase fatigue life and strength:i). grain refinement: by using rare earth elements which modifies microstructure by decreasing the grain size .This results in increasing the grain boundaries; hence hindering the propagation of the fatigue cracks which are initiated at the surface from propagating inside the main matrix.

6). ii) Shot peening [: this is a surface treatment process by which the surface of the component is subjected to multiple impact by hard particles producing a compressed layer on the surface; hence it improves its fatigue life and strength .

7). Experimental Procedure: it started by preparing the master alloys, From which the microalloys shown in the table were prepared.

Sample Alloys Ti% B% V% Balance

1 Al 0 0 0 Al

2 Al-Ti 0.15 0 0 Al

3 Al-Ti-B 0.046 0.0092 0 Al

4 Al-V 0 0 0.1 Al

5 Al-Ti-V 0.15 0 0.1 Al

6 Al-Ti-B-V 0.046 0.0092 0.1 Al

TABLE 1. CHEMICAL COMPOSITION OF THE DIFFERENT AL MICRO-ALLOYS

7). After then three sets of specimens were machined from Al of the dimensions shown in Fig.1 and a second set for the compression test to determine the mechanical behavior and the third set for measuring hardness and grain size.

FIG.1. A STANDARD FATIGUE SPECIMEN

8).Results

0

20

40

60

80

100

120

140

160

180

Pure AL AL-Ti AL-Ti-B AL-Ti-V AL-Ti-B-V

Microalloys

Gra

in S

ize

(mic

ron

)

FIG.2. EFFECT OF REFINERS ON AL GRAIN SIZE

FIG.3. EFFECT OFGRAIN REFINERS ON AL HV.

24

24.5

25

25.5

26

26.5

27

Pure AL AL-Ti AL-Ti-B AL-Ti-V AL-Ti-B-V

Microalloy

VH

N

FIG.4.EFFECT OFGRAIN REFINERS ON AL FATIGUE LIFE.

9). Discussion of Result

The total period of fatigue life (total life may be considered to consist of three phases):

i). Initial fatigue damage that produces crack initiation.

ii). Propagation of crack or cracks that results in partialseparation of cross section of the member, until the remaining un-cracked cross section is unable to support the applied load.

iii). Final failure of the member.  

10). Mode of Deformation

Fatigue is a progressive structural damage which occurswhen the part is subjected to cyclic loading. The stress value at which fatigue failure occurs is much less than the UTS or even may be well below the yield strength of the material.Fatigue failure is caused by initiation of cracks at thesurface of the part which extend to cause complete failureof the part.

11). The corresponding number of load cycles or the time during which the number is subjected to these loads before fracture occurs is referred to as fatigue life and the cyclic stress which can be applied infinite number of cycles without causing failure is referred to as endurance limit.

The endurance limit is only well defined in ferrous materials. In non-ferrous materials the failure stress which stands for the endurance limit is determined at a specified number of cycles on the S-N curve of the material.

11). The mode of fatigue failure in the different micro-alloys is almost identical to a great extent, which indicates that the specimens suffer a certain amount of plastic deformation prior to fracture, and the amount of the plastic deformation varies with the added grain refiner. 12). The SEM photoscans of Figs.9, 10, 11, 12and 13 show the fractured surface (At 60X) of pure Al, Al-Ti, Al-Ti-B, Al-Ti-V and Al-Ti-B-V respectively. On the whole, the fatigue failure in aluminum is not sudden as with other materials, e.g. steel, Since the total failure is proceeded with a state of plastic deformation.Examination of the fractured surface reveals that it consists of two regions regarding the crack propagation, which isdenoted as (Region I and Region II). Region I contains the fatigue crack initiation as well as initial crack propagation.

13). It shows that the crack propagates almost is transgranular type. As the crack travels through the grains rather than at the grain boundaries. In addition, this region consists of a ductile region at the outer part. The area of the ductile fracture is reduced as the percentage of the hard particles increases in aluminum matrix this was observed in case of addition of vanadium to aluminum grain refined by titanium where a combination of TiAl3 and Val3 particles exist in aluminum matrix. Both of them are hard particles.

Region II is characteristic of higher rates of crack propagation observed as larger crack size, being much more irregular than Region I.

Furthermore some transgranular cracking is observed.

Fig.12: Photoscan showing the fracture surface of AL-Ti-Vmicroalloy (At 60X)

Fig.13: Photoscan showing the fracture surface of AL-Ti-B-V micro-alloy (At 60X)

14). The mode of fatigue failure in the different micro-alloys is almost identical to a great extent, which indicates that the specimens suffer a certain amount of plastic deformation prior to fracture, and the amount of the plastic deformation varies with the added grain refiner. The SEM photoscans of Figs.9, 10, 11, 12and 13 show the fractured surface (At 60X) of pure Al, Al-Ti, Al-Ti-B, Al-Ti-V and Al-Ti-B-V respectively. On the whole, the fatigue failure in aluminum is not sudden as with other materials, e.g. steel, Since the total failure is proceeded with a state of plastic deformation.

15). Examination of the fractured surface reveals that it consists of two regions regarding the crack propagation, which is denoted as (Region I and Region II). Region I contains the fatigue crack initiation as well as initial crack propagation,. It shows that the crack propagates almost is a transgranular type. As the crack travels through the grains rather than at the grain boundaries. In addition, this region consists of a ductile region at the outer part. The area of the ductile fracture is reduced as the percentage of the hard particles increases in aluminum matrix this was observed in case of addition of vanadium to aluminum grain refined by titanium where a combination of TiAl3 and Val3 particles exist in aluminum matrix. Both of them are hard particles.

16). Region II is characteristic of higher rates of crack propagation observed as larger crack size, being much more irregular than Region I. Furthermore some transgranular cracking is observed in this Region, II. The crack propagation in this region is primarily intergranular, and the number of the trans-granular cracked grains of the fractured surface decrease as the crack length increases, resulting in more surface irregularity away from the initiation site.

17). It is worth noting, that the fracture surface of the Al-Ti-B-V specimen reflected larger plastic deformation region due to existence of the borides TiB2 and VB2 which are soft particles. Also there is evidence of fatigue striations on the fractured surface of Al-Ti-V described as region D in Fig.12. This may be attributed to the existence of the hard particles TiAl3 and VAl3.

Conclusions

i). Addition of vanadium to commercially pure aluminum grain refined by either titanium or titanium + boron, enhances the grain refining efficiency, and the addition of vanadium to aluminum grain refined by titanium alone gives better result than the addition of vanadium to aluminum grain refined by titanium + boron. ii.). Addition of vanadium to commercially pure aluminum grain refined by titanium resulted in improvement of its hardness by 4.7%. 

iii). . Addition of vanadium to commercially pure aluminum grain refined by titanium + boron reduced its hardness by 1.2%. 1v). . Addition of any of the grain refiners used in this work namely Ti, Ti-B, Ti-V and Ti-B-V to aluminum, resulted in modifying the mode of failure and crack initiation and propagation, being a mixture of trans-granular and inter-granular of different percentages, being more of the latter in case of the intermetallic compounds, i.e. the hard particles) of TiAl3 and VAl3.

Thank you for Attention