This paper deals with metal matrix composites (MMCs) of Al 7075 alloy containing different weight percentage (2.5, 5, 7.5, and 10) basalt short fiber reinforcement and unreinforced matrix alloy. The samples were produced by the permanent stir casting technique. The casting ingots were cut into blanks to be forged in single stage and double stage, using MN press and graphite-based lubricant. The microstructures and fatigue properties of the matrix alloy and MMC samples were investigated in the as cast state and in the single and double stage forging operations. The microstructure results showed that the forged sample had a uniform distribution of the basalt short fiber throughout the specimens. Evaluation of the fatigue properties showed that the forged samples had higher values than those of the as cast counterparts. After forging, the enhancement of the fatigue strength of the matrix alloy was so significant and high in the case of 2.5 and 5.0 wt. percentage basalt short fiber reinforced MMC, and there was no enhancement in 7.5 and 10 weight percentages short fiber reinforced MMCs. The fracture damage was mainly due to decohesion at the matrix-fiber interface.
Metal matrix composites (MMCs) with better mechanical strength are under development in order to introduce structural components for automotive and aerospace applications, by reducing drastically the weight and increasing the specific strength [
The fatigue resistance of short fiber reinforced MMCs depends on various factors such as the type and weight percentage of reinforcements, fiber aspect ratio, fiber and matrix interfacial bonding, the presence of inclusions or defects that arise from processing, and testing environment [
The matrix alloy used in the present investigation was Al alloy, which had Cu coated basalt short fiber of diameter 3–5
Chemical composition of Al 7075 alloy-weight percentage.
Element | Si | Fe | Cu | Mn | Mg | Cr | Zn | Ti | Al |
---|---|---|---|---|---|---|---|---|---|
Percentage | 0.4 | 0.5 | 1.6 | 0.3 | 2.5 | 0.15 | 5.5 | 0.2 | Bal |
The composites were prepared by adding 0, 2.5, 5, 7.5, and 10 weight. percentage of basalt short fiber using liquid metallurgy technique. The basalt short fibers were introduced into the molten metal pool through a vortex created in the melt by the use of an alumina-coated stainless steel stirrer. The coating of alumina on the stirrer is essential to prevent the migration of ferrous ions from the stirrer material into the molten metal. The stirrer was rotated at 550 rpm, and the depth of immersion of the stirrer was about two-thirds of the depth of the molten metal. The preheated (500°C) basalt short fibers were added into the vortex of the liquid melt which was degassed using pure nitrogen gas for about 3 to 4 min. The resulting mixture was tilt and poured into preheated permanent moulds (400°C).
Forging was carried out in an open die in both single and double stages, using a 20 MN press and a graphite-based lubricant. The temperature of the composites at the beginning of the process was about 500°C, while at the end of the forging it was 425°C. The forging parameters were temperature of the die 400°C; initial billet height 75 mm, final billet height 25 mm, and deformation ratio 3 : 1; average engineering strain rate 0.1 s-1 as per Hong et al. [
The microstructure of the Al/10% basalt short fiber MMCs (a) as cast and (b) in forged conditions.
After heat treatment, total-strain controlled fatigue tests were carried out with total-strain amplitudes ranging from 0.001 to 0.01 in servohydraulic universal testing machines. Strain was measured by clip-on extensometer attached directly to the gauge length at room temperature. The stress level corresponding to the LCF (compression and tension) was fixed nominal stress amplitude of 180 MPa with a constant load rate equal to 125 N/s. The fatigue tests have been carried out in load control (in a fully reversed push-pull mode) with hydraulic test machine, and a comparison of the stress versus cycles to failure behaviour of the unforged, single, and double forged specimens belongs to both Al7075 matrix alloy and Al/basalt composites as shown in Figures
Wöhler curves for Al and Al/basalt short fiber composites for (a) unforged, (b) first stage forging, and (c) second stage forging.
Wöhler curves for as cast, first stage forging, and second stage forging for Al, Al/2.5%, Al/5%, Al/7.5%, and Al/10% basalt short fiber MMCs.
Figure
Figure
However, the magnitude of enhancement fatigue resistance is higher at lower weight percentage of basalt short fiber (0–5 wt. percentage) but no significant change can be seen at a higher percentage (7.5 and 10 wt. percentage). Several studies have shown that the increasing weight percentage of reinforcement results in enhanced fatigue resistance [
The change in fatigue resistance as a function of the forging condition is given in Figure
The typical fractured surface of Al 7075 alloy and Al/5 weight percentage basalt short fiber composites (both as cast and two-stage forged condition) after fatigue test are shown in Figures
Fracture surface of (a) unforged and (b) forged condition Al7075 alloy specimen after fatigue. C-main crack, FC-fatigue crack, and V-void in matrix alloy.
Fracture surface of (a) unforged and (b) forged condition Al 7075/5 weight percentage of basalt short fiber MMCs after fatigue test. C-main crack, FC-fatigue crack, and V-void in matrix alloy.
Al 7075 alloy after fatigue failure contains two regions, “A” (ductile failure) and “B” (brittle failure). The region B is more dominante than the region “A” which due to foringing reduces the porosity and refines the grain boundaries in the matrix alloy. Similar to unforged specimen, the crack (C) and fatigue crack(FC) are continuous and formed in the severe damage region.
Figures
The fracture surfaces also displayed the presence of few cracks (Figure
In the present work, the effects of basalt short fiber content and the forging process on the fatigue properties of Al 7075/basalt short fiber MMCs are studied, and the conclusions are drawn from the experimental observations as follows. The microstructures of the as cast and forged Al/basalt short fiber MMCs exhibit uniform distribution of basalt short fiber, and porosity content in forged specimen is substantially reduced. The matrix alloy and MMCs behave more like a monolithic material, since the homogeneous spatial distribution of the basalt short fiber enables efficient load transfer from the matrix to the reinforcement fiber without producing stress concentration sufficient to initiate fatigue cracks. With increasing basalt fiber content, the fatigue value has been increasing up to 7.5 percent in the as cast, single, and double stage forged specimens, and it has been found that there is no significant change when more than 7.5 weight percentage of short basalt fiber is added. SEM analyses of the fracture surfaces showed the broken particles, surrounded by ductile region, and decohesion at the matrix and fiber interfaces. The tear ridges in the MMCs can be explained with the high local plastic constraints induced by fiber cluster.