low cycle fatigue properties laminate aa2519-ti6al4v

Procedia Engineering 114 ( 2015 ) 26 – 33

1877-7058 © 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of INEGI - Institute of Science and Innovation in Mechanical and Industrial Engineeringdoi: 10.1016/j.proeng.2015.08.022

ScienceDirectAvailable online at www.sciencedirect.com

21st International Conference on Structural Integrity

Low cycle fatigue properties laminate AA2519-TI6AL4V

Ireneusz Szachogluchowicza, Lucjan Sniezeka, Volodymyr Hutsaylyuka aFaculty of Mechanical Engineering, Military University of Technology, 2 Gen. S Kaliskiego str., Warsaw, 00-908, Poland

Abstract

This paper presents test results of AA2519/TI6AL4V composite laminate structure and mechanical properties. The composite laminate was merged by explosive welding using a transition AA1050 alloy layer. Weld endurance properties were determined through fatigue testing performed in constant total deformation amplitude (εac) conditions. The paper also describes the influence of test conditions on hysteresis loop shape, process of cyclic material strengthening and weakening, and fatigue life. Low cycle test results were approximated using a straight line with the following formula: σa = K'(εap)n which enables the K' fatigue strength coefficient, and the n' cyclic strengthening index to be determined. Test results also include fatigue fracture surface analysis. Distinctive fatigue fissure characteristics were investigated through studying fatigue fracture surface microstructure using a scanning electron microscope.

© 2015 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of INEGI—Institute of Science and Innovation in Mechanical and Industrial Engineering.

Keywords: explosive welding, composite laminates, Al/Ti composites;

1. Introduction

Modern construction and technology solutions require the use of materials which would provide more advantageous mechanical properties in comparison to conventional ones. They also require innovative and more effective bonding techniques. One of the techniques utilized to bond materials, which complements traditional methods utilized for special constructions, is explosive welding. It enables materials of various mechanical properties to be bonded, e.g. non-weldable metals and light alloys. Explosive welding enables composite laminates to be produced, which are efficiently utilized by aviation, space, and power industries. The prospects of using such composite laminates as construction materials seem extensive, based on the analyzed literature, especially in cases when using other types of bonds is onerous, impossible or commercially unviable.

© 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of INEGI - Institute of Science and Innovation in Mechanical and Industrial Engineering

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27 Ireneusz Szachogluchowicz et al. / Procedia Engineering 114 ( 2015 ) 26 – 33

Materials utilized for mechanical constructions are usually under cyclic stress. Such conditions may lead to various damages, both to the laminate base material, and to the material bond zone [1]. One common example of such damage is layer separation — delamination. Such fractures appearing within a composite laminate may locally significantly reduce the stiffness and bending resistance. Delamination appearing centrally within the wall, i.e. separating the laminate into two sublaminates, may reduce the bending resistance of the entire section as much as twice [16].

Literature analysis shows a significant number of publications devoted to Al-Ti laminates. Most of these papers cover mechanical properties of explosive weld materials, material structure, and bonding zones.

A lot of consideration is given to the appearance of Al3Ti intermetallic diffusion [2,3,4] and its influence on the essential characteristics of produced laminates. They cause local material strengthening, which significantly increases laminate resistance to foreign body dynamic impacts [12-15]. Al3Ti molecule forming introduces a risk of material structure discontinuity and strain concentration caused by the diffusion creation process. They may perform the role of indentations, which increase the probability of fissures and stratification [5-9]. Numerous published papers are dedicated to Al-Ti composite laminate heat treatments aimed to relax the strains introduced through explosive welding. This process, as described in paper [10], brings expected results, but also causes changes in the microstructure of the aluminium alloy. Paper [11] is dedicated to Al-Ti laminate heat treatment considering the composite oxidation process, and its influence on physical characteristics of the material.

An analysis of research performed up to date only enables one to evaluate the application of Al-Ti laminates produced through explosive welding in statically loaded constructions. Very few studies consider the issues related to laminate damage due to cyclic stress. In case of layered materials bonded through explosive welding this issue is clearly one of significant importance, which enables the laminate to be characterized as a construction material, with prospects of utilizing it for mechanical construction.

The purpose of this paper is to determine the fatigue properties through fatigue testing performed in constant total deformation amplitude (εac) conditions. The paper describes the influence of test conditions on hysteresis loop shape, process of cyclic material strengthening and weakening, and fatigue life. The process of fatigue failure is described based on the results of electro-optical examination of fatigue fracture surfaces, indicating distinctive characteristics.

Nomenclature

Nf Number of cycles N Cycle number

ac Total deformation amplitude ap Ductile deformation amplitude ae Elastic deformation amplitude a Strain amplitude ’f Fatigue resistance coefficient ’f Fatigue ductility coefficient

E Young module b Fatigue resistance index K' Fatigue resistance coefficient n' Cyclic strengthening index Rm Maximum rupture resistance Rp0,2 Ductility limit A Elongation at fracture


2. Material and test procedure

Research was based on composite laminate produced through explosive welding from the following base materials: AA2519 aluminium alloy (AlCuMgMn+ZrSc) and Ti6Al4V titan alloy. The laminate is composed of base materials of 5mm thickness (each). An additional 0.8 mm transition AA1050 alloy layer was required for the explosive welding process.

It was assumed, that such material would have unique properties, combining advantageous properties of both titanium and aluminium alloys (high durability, high ductility, low specific weight), as well as alloys based on Ti-Al intermetallic phases (high hardness and rigidity) [13]. The chemical composition and endurance properties of base materials are presented in Tables 1, 2, and 3.

Table 1. Chemical composition of the Ti6Al4V alloy

%

O V Al Fe H C N Ti

<0.20 3.5 5.5 <0.30 <0.0015 <0.08 <0.05 rest

Table 2. Chemical composition of the AA2519 alloy

%

Si Fe Cu Mg Zn Ti V Zr Sc 0.06 0.08 5.77 0.18 0.01 0.04 0.12 0.2 0.36

Table 3. Mechanical properties of basic materials

Rm,[MPa] R0,2[MPa] A, [%]

AA2519 alloy 335 312 6.5

Ti6Al4V alloy 1020 950 14

In the process of selecting proper explosive welding parameters it was found that the AA1050 alloy must be

used as a transition/bonding layer (Fig. 1a). The weld shows a distinctive, corrugated surface of the AA2519-AA1050 bond, and a flat surface of the Ti6Al4V-AA1050 bond (Fig. 1b). ,

a)

b)

Fig. 1. (a) cross-sectional area of the laminate AA2519-AA1050-Ti6Al4V; (b) AA2519-AA1050-Ti6Al4V bond surface

produced through explosive welding

Determining material behaviour in variable stress conditions for low cycle numbers is especially important, because it provides a chance to examine cyclic deformation mechanisms, which lead to deformation being localized, fractures appearing, and in consequence to the destruction of a construction element. Low cycle test was performed in constant total deformation amplitude conditions (εac = const.), utilizing flat samples as depicted at Fig. 2. Fatigue testing was performed using an INSTRON 8802 fatigue testing system. The total deformation amplitude (εac) was modified in a sinusoidal manner throughout the test, using a cycle asymmetry coefficient R = 0. Tests were

0,2 mm 1 mm


conducted utilizing a frequency (f) of 0.8 Hz. Four deformation levels were examined: εac= 0.3%; 0.4%, 0.5% and 0.6%. Deformations were registered using a extensometer and a 25 mm measurement base. Basic laminate fatigue characteristics were developed based on distinctive loop parameters obtained approximately half-way to the sample being damaged (N/Nf = 0.5), and the criteria assumed for sample destruction was a fissure appearing in one of the sublaminate material layers.

Fig. 2. Sample dimensions for low cycle tests

3. Discussion and results

Horizontal deformations were defined following analysis of the static tension graph [14]. Endurance testing under conditions of axial tension was conducted in accordance with the PN-EN ISO10002-5:2004 standard. The properties defined on the basis of the monotonic tension graph are quoted in Fig. 3.

Fig. 3. Monotonic tension graph for the flat AA2519-Ti6Al4V composite laminate sample

Preparation of the results of the low cycle tests began with analysis of the changes in the dependent variable value (stress amplitude σa) as a function of the number of relapses. The results of these measurements, for the adopted levels of deformation, are presented in Fig. 4.

a)

b)

Fig. 4. (a) changes of the stress amplitude with the number of cycles stabilized hysteresis loops; b) change in the stress

amplitude σa as a function of the number of cycles Nf


Based on the above graph, significant changes in stress amplitude can be claimed depending on the current number of cycles of strain changes for the adopted amplitudes of total strain εac=0.3%, 0.4% as well as 0.5% and 0.6%. The tested composite shows a tendency to cyclic stability. The waveforms σa=f(Nf) have a characteristic inflection as marked in Fig. 4b by an arrow, illustrating the effect of damage to one of the layers. In each of the tested samples, the AA2519 alloy layer was the first that was damaged. In order to develop the results of the tests of hysteresis loop parameters (εac, εap and σa), durability was introduced corresponding to 0.5Nf (where Nf is the number of cycles until the failure/destruction of the sample). This approach for determining the period of stabilization (in the case of cyclic volatile materials) can be found quite frequently in the literature.

The data recorded during the LCF tests were used to determine the low cycle properties of the Figure 5b tested materials. The coefficient of fatigue strength K' and the index of cyclic strengthening n' were defined on the basis of the parameters obtained on the basis of the stabilized hysteresis loops. It was assumed that the curves described by the equation would be presented in a straight form described by the formula:

σa=k'(εapl)n' (1)

The results were included in a logarithmic scale arrangement σa - logεap. presented in figure 5a;

a)

b)

Fig. 5.(a) logarithmic scale arrangement σa - εap; (b) plastic strain amplitude εapl versus of the number of cycles Nf behaviour

The assigned values of the coefficient and index allow a curve description of cyclic strain using the constitutive Ramberg-Osgood equation (2).

'1

'n

KE (2)

The data obtained was also used to perform the Manson-Coffin Basquin equation (3) based on fatigue analysis.

cffb

f

fpe NNE

2'2

'222

(3)

The values obtained made it possible to assign a material strain-life curve (Fig.6)


Fig. 6. Strain-life curves of the laminate AA2519-Ti6Al4V with the transition layer AA1050

4. Discussion and results

The characteristics of fatigue failure were investigated by examination of the microstructure of fracture surfaces using a JEOL JSM 6610 scanning electron microscope. Details of the structure of a fracture surface on a composite laminate sample tested at ac =0.3% by Nf=2728 cycles are shown in the photographs (Figures 7-12). The photographs come from a narrow band of material comprising a transition layer made of AA1050 alloy and the neighbouring zones of AA2519 and Ti6Al4V alloys (Fig. 7). The fracture surface in this area is characterized by a microstructure having predominantly the properties of ductile breakage interwoven with images of friable-like fissure. Initiation of fatigue fissures occurred on the border of the bonding, in the transition layer zone. What was noted here was a diverse nature of failure in each of the composite’s component layers. The fissure developed in three different planes, creating faults on the borders of AA1050-AA2519 and AA1050-Ti6Al4V (Fig. 7).

Fig. 7. The fracture surface of the laminate AA2519-Ti6Al4V with the transition layer AA1050

The process of material failure in the plane perpendicular to the direction of the load was accompanied by lamellar fissures, extending most often on the border of AA1050-Ti6Al4V (Fig. 8). No delaminations were observed on the border of AA1050-AA2519 (Fig. 9).

Fig. 8. Fracture surface of the connection border zone of Ti6Al4V/AA1050 alloy

Fig. 9. Fracture surface of the connection border zone of AA2519/AA1050 alloy

100 m

AA 1050

Fig. 13

Fig. 10

Fig. 9

Fig. 12

Fig. 11

AA

251

9 T

i6A

l4V

AA 2519

AA 1050

50 m 20 m

AA 1050


The fracture surface within the transition layer of AA1050 alloy is characterized by the features of ductile fissure with a considerable number of bands of fatigue striations (Fig. 10). The varied inter-striation distances constitute ‘microscope’ evidence of acceleration or delay in the failure of this layer of the material. The surfaces of the striations are locally deformed and against their background can be seen traces of ductile micro-strains. This image is completed by numerous shears and faults being evidence of a fissure jumping between the development planes of many micro-fissures, propagating in adjacent layers the AA1050 alloy structure, highly deformed during explosive welding.

Fig. 10. Fracture surface of the transition layer of alloy AA1050

The fracture surface within the base materials AA2519 and Ti6Al4V are characterized by arrangements of indentations and protrusions typical of ductile breakage, forming a so-called honeycomb structure (Figures 11-12). The indentations are formed in the area of micro-voids that form during the imposition of the load and grow larger right up to bonding as a result of ductile deformations and decohesion. The shape and dimensions of the observed indentations depends on grain size and the ductile properties of the material. In the tested material were distinguished uniform indentations corresponding to uniaxial, even tension. In Ti6Al4V alloy, ductile shears forming faults in the form of ‘terraces’ were also observed, connecting areas of stabilized development of fissures.

Fig. 11. Fracture surface of the connection border zone of AA2519 alloy

Fig. 12. Fracture surface of the connection border zone of Ti6Al4V alloy

The diffusive nature of the process by which the Al-Ti connector is formed means that the mechanical strength properties of the plating may be shaped by appropriate selection of the parameters of the heat treatment carried out following the process of explosive welding. For this reason, during analysis of the surface of fatigue fractures in composite laminates, the application of heat treatment processes should be included. A comprehensive publication on this subject is currently under development.

50 m

20 m

20 m

50 m


4. Conclusions

Based on these results it can be concluded that composite laminates are promising construction materials which can be subjected to modifications designed to improve its mechanical properties. On the surfaces of the samples of AA1050 and AA2519 aluminium alloy a significant heterogeneity of the deformation fields was observed. The diffusive nature of the process by which the Al-Ti connector is formed means that the mechanical strength properties of the plating may be shaped by appropriate selection of the parameters of the heat treatment carried out following the process of explosive welding. Test results indicate cyclical stability of the produced laminate. The strengthening index assigned during the low cycle tests amounts to n' and the coefficient k'. Analysis of the microstructure of the surfaces of fatigue fractures indicates a complex failure process. The sources of the fatigue fissures were located within the bonding zone of the base materials on the border of the transition layer of AA1050 and Ti6Al4V titanium alloy. The results set the direction for further research to determine the effect of annealing parameters, under high vacuum conditions or in the atmosphere of a protective gas, on the microstructure, phase transitions and structure stability and the fatigue properties of Al-Ti type composites obtained by means of the explosive welding process.

Acknowledgments This work was supported by The National Centre for Research and Development of Poland under the grant No. PBS/A5/35/2013

References

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low cycle fatigue properties laminate aa2519-ti6al4v

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