- Email: [email protected]
Manufacture studies and impact behaviour of light metal matrix composites reinforced by steel wires
archives of civil and mechanical engineering 12 (2012) 265–272
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/acme
Original Research Article
Manufacture studies and impact behaviour of light metal matrix composites reinforced by steel wires W. Hufenbach, H. Ullrich, M. Gude, A. Czulak, P. Malczykn, V. Geske Technische Universita¨t Dresden, Institute of Lightweight Engineering and Polymer Technology (ILK), Holbeinstr. 3, 01307 Dresden, Germany
art i cle i nfo
ab st rac t
Article history:
Magnesium alloys play an important role in the development of light metal matrix
Received 1 June 2012
composites. Magnesium based metal matrix composites reinforced by particles and fibres
Accepted 4 June 2012
(especially carbon fibres) are successfully applied in various fields of automotive and
Available online 26 June 2012
aircraft industry. Equally high potential in large-batch production regarding to relatively
Keywords:
low price and high strength is expected from MMC with wires made of iron-based alloys.
Magnesium alloy
However, their application is hampered by the absence of intermetallic phase between iron
Steel
and magnesium and low solubility of iron in magnesium.
GPI process
This paper makes a contribution to the investigation of the effect of steel wires surface
Composite
preparation and of optimised production methods to improve the quality and type of
Impact resistance
adhesion with selected industrial magnesium alloys. The Fe/Mg-MMC specimens with steel wires reinforcement were manufactured by the help of advanced gas pressure infiltration method (GPI) in a graphite mould at Institute of Lightweight Engineering and Polymer Technology (ILK) of TU Dresden. For the examination of the obtained composites the computer tomography (CT), SEM microscopy with EDX, strength tests and fracture surface inspection has been used. & 2012 Politechnika Wrocławska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved.
1.
Introduction
Increasing technical requirements for automotive and aerospace products require continuous improvements in lightweight design to achieve high-tech products and associated efficient manufacture processes. Due to the high production cost for continuous fibre reinforced metal matrix composites intense research to develop new low-cost solutions has been done. Magnesium and its alloys offer a high lightweight potential because of high specific strength and stiffness, low density, welding ability and high thermal and electric conductivities.
Industrial companies whose products are subjected to high loads pay great attention to the development of new materials and new manufacturing technologies [3,5,9,11]. The selection of an appropriate reinforcement with regard to applications, operating conditions and simplicity of production is a key aspect in the initial stage of the product design. Compared to carbon fibres steel reinforcements provide higher rigidity in the die casting mould, higher temperature stability in contact with other metals and possibility to thermal treatment. Despite the fact that the higher density of steel may decrease light weight potential, a high impact resistance is expected by steel–magnesium composites.
n
Corresponding author. Tel.: þ49 691855091. E-mail address: [email protected] (P. Malczyk).
1644-9665/$ - see front matter & 2012 Politechnika Wrocławska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved. http://dx.doi.org/10.1016/j.acme.2012.06.005
266
archives of civil and mechanical engineering 12 (2012) 265–272
Since the main obstacles are a lack of intermetallic phase and poor wettability between magnesium and steel the efforts to receive favourable adhesion have been intensified. The main aim of this work is to achieve high strength composites characterised by improved impact resistance [9–11].
2.
Selection of the matrix and reinforcement
Mg–Al–Zn is the most common group of Mg-based alloys used in casting. Maximum solubility of aluminium in magnesium is 12.7% at 437 1C and it decreases to 2% in ambient temperature. The effect of addition of aluminium to magnesium alloys is noticed in castability improvement although it also causes a tendency to microporosity and corrosion. Moreover, aluminium in magnesium alloys contributes to solid solution hardening and to the formation of eutectic precipitations. The addition of zinc to Mg–Al alloys causes strengthening. However, the amount of zinc is limited due to an increase in the susceptibility to hot cracking during solidification. The corrosion resistance of Mg–Al alloys is improved by replacing the zinc with manganese, which also increases hardness without decreasing strain at fracture. This fact explains why the Mg–Al–Mn alloys are successfully used in casting as well as in forging. Considering the influence of Al, Zn and Mn the two industrial available alloys AZ91 and AM50 were selected. The iron based alloys are commonly used materials, for which manufacturing, shaping and microstructure development have been very well understood. Depending on the composition and type of treatment steel can provide a wide range of properties, which combination is difficult to obtain by other materials. The main influence on the impact resistance behaviour depends on the quality and type of bonding between magnesium matrix and steel wires. Two methods of surface preparation of steel reinforcements have been selected: the acid etching and the zinc plating. To improve
the adhesion by an increase of matrix-reinforcement contact surface, etching of the stainless steel mesh in highly concentrated sulphuric acid has been done. The ferritic steel with high content of Fe has been coated by zinc in order to investigate the possibility of the formation of a favourable intermetallic surface and to determine the influence of Zncover on the corrosion phenomena between iron and magnesium. The chemical compositions of the reinforcements and the matrix alloys are shown in Table 1 [5–11].
3.
Principals of manufacturing
The prototypes of steel/Mg-MMC specimens have been manufactured at the ILK by the help of an advanced gas pressure infiltration method. Before the infiltration the reinforcement was subjected to a surface treatment. Preparation of stainless steel wires in high condensed sulphuric acid has been carried out with different durations. Two times of bathing—10 and 20 min—have been used. The reinforcement was rinsed in acetone to clean the insert from residues and impurities due to the acid treatment. The ferritic steel was industrially covered by zinc and only cleaned in acetone before manufacturing process. Due to low wettability of steel by magnesium, which causes poor adhesion of the reinforcement and the matrix and therefore insufficient mechanical bonding, extremely high pressures of infiltration are indispensable. The gas pressure infiltration method, which allows programming the appropriate temperature, vacuum and pressure, is particularly favourable to produce composites that require a highly sophisticated and accurate production process [1–6]. A laboratory autoclave (Fig. 1c) with a maximum pressure of 120 bar, maximum temperature up to 1300 1C is equipped with two independent thermocouples to measure the temperature of melted material and the temperature of infiltrated product.
Table 1 – Chemical composition of used materials.
Ferritic steel Austenitic steel AM50 magnesium AZ91 magnesium
C (%)
Si (%)
Mn (%)
Cr (%)
Ni (%)
Mo (%)
Cu (%)
Al (%)
Zn (%)
0.05 0.03 – –
0.30 0.75 0.05 0.05
0.3 2.0 0.8 0.15
0.2 16 – –
0.25 10 0.001 0.001
0.05 2 – –
0.3 – 0.025 0.008
– – 5 9.5
– – 0.2 0.9
Fig. 1 – (a) Inner graphite mould with attached steel mesh; (b) external graphite mould and (c) autoclave.
archives of civil and mechanical engineering 12 (2012) 265–272
The infiltration of steel/Mg-MMC was realized with precision graphite moulds which allow to manufacture thin-walled structures. The mould consists of a precision inner-mould, where the infiltration process of attached reinforcement occurs (Fig. 1a), and an external mould (Fig. 1b) which allows maintaining vacuum and pressure in the infiltration area. The GPI process with steel mesh reinforcement can be divided into four steps. The first step starts with the temperature rising and cyclic vacuum ventilation to clean the autoclave chamber form impurities. The second step consists the increasing of temperature under vacuum conditions to melt the matrix. In the third step the melted alloy flows between the walls of the inner mould where the reinforcement has been attached. The ceasing of the vacuum maintaining creates the pressure difference between the inner mould—where vacuum is still retained—and the rest of the chamber. The inert gas pressurisation in the last step of the process contributes to the matrix porosity reduction and to the improvement in the filling of inaccessible places of reinforcement by melted magnesium. A defined cooling rate can be obtained by chamber ventilation and by adjusting the heating devices [3–5].
4.
267
Fig. 2 – Surface between uncoated ferritic steel and AM50 magnesium alloy with cracks at the contact area.
Material analysis
To investigate the microscopic structure the SEM TM3000 equipped with EDX scanning module has been used. The aim of this observation was to analyse the quality and type of bonding by the presence of specific phase precipitations in the area of contact surface between matrix and reinforcement. The Fe–Mg phase diagram shows low solubility and no intermetallic precipitations between the components. With regards to this information and to galvanic corrosion phenomenon the direct contact of magnesium and high pure iron is undesirable. The Zn-coating has been performed to reduce the Fe–Mg contact. The stainless steel consist of about 16–18% Cr, 8–11% Ni, 2% Mo and notable additions of Mn and Si as well as a carbon content of as low as 0.05%. Therefore the content of iron is decreased to 67%. The volume fraction of steel wires was different for the composites with ferritic steel reinforcement and stainless steel reinforcement. The ferritic steel reinforced composites were characterised by 8% of steel in the volume of specimen and a density of 2.15 g/cm3. Stainless steel reinforced composites have about 20% of steel and a density of 3.05 g/cm3. Considering the relative long time of magnesium being above the liquid temperature and the fast cooling rate the residual stresses generated by a mismatch of the thermal expansion coefficients between the magnesium alloy and steel are expected. Thus an interface with insufficient bonding strength is expected to be characterised by cracks on the contact surface between both materials. Fig. 2 shows no intermetallic bonding between uncoated ferritic steel and AM50 magnesium alloy. The absence of an interface is demonstrated by oxidation in cracks between the reinforcement and matrix which leads to a significant decrease of mechanical properties. The contact surface between infiltrated uncoated steel mesh with AZ91 magnesium alloy shows no discontinuities as well as no intermetallic phases (Fig. 3). The Zn coated steel wires infiltrated by magnesium AM50 show a multi-layer interface with zinc, manganese and
Fig. 3 – Surface between uncoated ferritic steel and AZ91 alloy with no intermetallic precipitations.
aluminium components (Fig. 4). The arrangement of phases is as following: steel reinforcement—zinc coating–manganese with aluminium precipitations—magnesium alloy matrix (Fig. 5). The thickness of the multi-layer interface is in a range of 10–15 mm. Microsections of specimens with AZ91 magnesium alloy show aluminium and Al–Mg precipitations in the surrounding of the steel. The sulphuric acid treatment caused better distribution of precipitates and higher magnesium diffusion into the steel wire. No cracks were visible in the contact area between reinforcement and matrix. Figs. 6 and 7 illustrate the difference between both interfaces in diffusion rate of magnesium into the steel wire and in the size and distribution of Al-based precipitations at the boundary of the wire. The microsection of AM50 reinforced by stainless steel shows an Al–Mn intermetallic interface (Figs. 8 and 9) which have a positive effect on the adhesion. This interface has a thickness of 2–5 mm and is similar to the multi-layer interface from the zinc coated specimens.
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archives of civil and mechanical engineering 12 (2012) 265–272
Fig. 4 – Multi-layer intermetallic interface between Zncoated ferritic steel and AM50 magnesium alloy.
Fig. 7 – Surface between acid treated stainless steel and AZ91 magnesium alloy shows higher distributions of Al–Mg precipitations and diffusion rate of Mg into reinforcement.
Fig. 5 – EDX analysis of Zn-coated ferritic steel and AM50 magnesium alloy intermetallic interface arrangement. Fig. 8 – Intermetallic interface between acid prepared stainless steel and AM50 magnesium alloy.
Fig. 6 – Surface between untreated stainless steel and AZ91 magnesium alloy shows poor distribution of Al–Mg precipitations and low diffusion rate of Mg into reinforcement.
Fig. 9 – EDX analysis of stainless steel and AM50 magnesium alloy intermetallic interfaces.
archives of civil and mechanical engineering 12 (2012) 265–272
269
Due to the extreme brittleness the Al–Mg phases have an influence on the decrease of bonding strength between matrix and reinforcement. Therefore, the magnesium alloys with aluminium content as high as AZ91, will bring insufficient improvement of impact resistance of the stainless steel reinforced composites produced by GPI method. There is a reasonable presumption that the composites manufactured using AM50 magnesium alloy will be characterised by better quality of adhesion between matrix and reinforcement due to the presence of Al–Mn interface. Further investigations to determine the exact composition and formation mechanism of the obtained Al–Mn precipitations are necessary.
5.
Impact resistance test
Fig. 11 – (a) Representative photograph of planar impact test specimen and (b) drawing of specimen with dimensions.
The impact resistance tests have been performed by the help of the charpy test machine at the ILK (Fig. 10). The specimens have a dimension of 2.5 10 80 mm and have been prepared from planar steel/Mg-MMC plates (Fig. 11). Two groups of manufactured composites have been investigated. The group of ferritic steel reinforced composites with lower density has been selected to analyse the influence of zinc coating on the adhesion between reinforcement and matrix by creating the multi-layer intermetallic interface. The results of impact test are shown in Fig. 12. The composites reinforced with stainless steel have been selected to analyse the influence of surface etching on the mechanical properties of Fe–Mg connection. Rough surface made by bathing in acid and higher content of alloying Fig. 12 – Impact test results for ferritic steel reinforced composites.
Fig. 13 – Impact test results for stainless steel reinforced composites.
Fig. 10 – Charpy test machine.
elements contribute better mechanical adhesion and increase the diffusion rate of magnesium into steel wires. The results are shown in Fig. 13. Considering the difference in densities between stainless steel reinforced and ferritic steel reinforced specimens the impact energy absorption related to density of manufactured composite has been shown in Table 2. Based on the quotient of impact energy absorption and density the most favourable material combination has been selected as an acid etched stainless steel mesh infiltrated by AM50 magnesium alloy.
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archives of civil and mechanical engineering 12 (2012) 265–272
Table 2 – The impact energy absorption and density of manufactured specimens.
Energy absorption (%) Density (g/cm3)
AZ91
AM0
a-steel / AZ91
a-steel / AM50
a-steel(Zn) / AZ91
a-steel(Zn) / AZ91
g-steel / AZ91
g-steel (acid prepared) / AZ91
g-steel /AM50
g-steel (acid prepared) / AM50
3.48
3.8
4.18
6.1
4.67
9.6
14.7
19.2
16.7
21.1
1.82
1.78
2.15
2.15
2.15
2.27
Fig. 14 – Specimen infiltrated by AZ91 alloy with no plastic deformation.
6.
3.23
3.05
3.07
2.99
Fig. 15 – Plastic deformation of specimen infiltrated by AM50 alloy.
Fracture analysis
The fracture surface of specimens after the impact resistance test has been investigated by the help of TM3000 SEM microscope. The AZ91 alloy as a matrix shows no plastic deformation during the test (Fig. 14), which is a main reason for the lower impact resistance. The brittleness is caused by a high content of Al, which results in hard Al–Mg eutectic. Specimens with AM50 alloy show significant plastic deformation (Fig. 15). Figs. 14 and 15 show that the stainless steel wire is subjected to plastic deformation during the test. The ferritic steel reinforcement shows a brittle fracture preceded by plastic deformation (Fig. 16). The proper inference about the fracture surface requires an understanding of the fact that the more ductile reinforcement is infiltrated by the brittle alloy. This aspect indicates that the created interface and the quality of bonding between both materials are extremely important. The specimens with lower impact resistance are characterised by no intermetallic interface and fracturing along brittle phases of matrix (Fig. 17). The composites with high impact resistance show an interface containing less brittle intermetallic precipitations. Cracks occurred along the magnesium brittle phases. This fracture surface can be recognised by the plastic deformation of matrix (Fig. 18) and significant remains of the intermetallic interface on the reinforcement (Fig. 19). The impact resistance highly depends on the quality and type of adhesion between reinforcement and matrix. The reinforcement characterised by higher stiffness with sufficient good bonded matrix maintains the high impact force during the
Fig. 16 – Fracture of composite reinforced by ferritic steel without interfacial surface and insignificant plastic deformation.
deformation. After the fracture of matrix the impact force is received only by the reinforcement that in consequence leads to damage of specimen (Fig. 20).
7.
Computer tomography analysis
The matrix-wire interfaces of stainless steel reinforced samples were investigated by high-resolution CT on a GE phoenix x-ray v9tome9x L 450. Small rods of 5 mm in length, 2 mm in
archives of civil and mechanical engineering 12 (2012) 265–272
271
Fig. 20 – Representative impact test graph of AM50 reinforced by stainless steel.
Fig. 17 – Fracture of stainless steel reinforced composite infiltrated by AZ91 alloy.
Fig. 21 – Roughness of the surface of reinforcement after infiltration. Fig. 18 – Fracture of AM50 reinforced by Zn-coated ferritic steel.
Fig. 19 – Fracture of AM50 reinforced by stainless steel.
width and 80 mm in height were prepared by cutting. A 300 kV microfocus tube was equipped with a tungsten target and a 0.2 mm copper foil was used as a filter for the xray beam. With an acceleration voltage of 100 kV and a beam current of 100 mA resolutions of up to 4 mm were achieved. The volumes were reconstructed with the Datos 2 Rec software by GE and visualised with VGStudio 2.0 by Volume Graphics. Since there is a large difference of densities and atomic numbers when comparing the magnesium-rich matrix with the austenitic steel picturing the matrix without artifacts can only be achieved by using a higher beam power while at the same time effectively lowering the resolution. Hence, the visualisation threshold was chosen in such a way that only the reinforcement is shown in the image and the matrix including the beam-hardening artefacts is not shown in the following pictures. In Fig. 21 there is a typical illustration of the stainless steel wire after the infiltration. The infiltration and the resulting
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archives of civil and mechanical engineering 12 (2012) 265–272
processes in the interface lead to an increase in surface roughness of the stainless steel wire. This is believed to be caused by the reinforcement etching in acid, diffusion of matrix elements into the reinforcement, and intermetallic precipitations on the contact surface between both phases.
8.
Conclusions
The impact resistance of magnesium can be successfully improved with insignificant decrease of light weight potential by the steel mesh reinforcement. It is really important to know how indispensable the selection of appropriate materials is to receive favourable bonding and to avoid disadvantageous interfaces. Considering the results of this paper following conclusions can be raised:
The contact surface area between steel and magnesium
alloy are disadvantageous and causes the corrosion phenomenon. For good adhesion special preparation of reinforcement before infiltration is needed. With longer time of acid etching the increase of contact surface and higher magnesium diffusion rate into the steel wires can be observed. The Al–Mg precipitations are disadvantageous as an interface between steel reinforcement and magnesium alloy and because of their brittleness should be avoided. The addition of manganese into the magnesium alloy is advantageous because of creating less brittle Al–Mn precipitations. The Al–Mn precipitations at the surrounding of the wires strongly create adhesive interface. The stainless steel due to the less iron content does not need Zn-coating to create favourable interface. The increasing of contact surface by etching in acid is necessary to receive better mechanical infiltration and causes higher diffusion rate of magnesium into steel wires. The intermetallic interface improves strength and energy absorption. The specimens with AM50 matrix reinforced by stainless steel prepared with 20 minutes of acid bathing have received the best results and improvement of impact resistance of more than 400% compared to the impact resistance of unreinforced specimens.
Intermetallic coating and acid etching of steel reinforcement is advantageous and brings improvements in the connection between steel reinforcement and magnesium matrix. The Zn coating of reinforcement is more attractive
for favourable Al–Mn precipitations in contrast to the clean Fe-surface of wires which provide favourable conditions for separation of Al–Mg phase. This aspect is extremely important for further steps of manufacturing where high impact resistance of composites is the main point of interest. The advanced methods of connecting metals by creating favourable interface which are unavailable in traditional production methods have been still insufficient developed and further continuation of research in this field are necessary.
r e f e r e n c e s
[1] W. Hufenbach, M. Gude, A. Czulak, J. S´leziona, A. DolataGrosz, M. Dyzia, Development of textile-reinforced carbon fibre aluminium composites manufactured with gas pressure infiltration methods, Journal of Achievements in Materials and Manufacturing Engineering 35 (2) (2009) 177–183. [2] W. Hufenbach, M. Gude, A. Czulak, F. Engelmann, Study of manufacture of Cf/Al-Mmc with aid of the gas pressure infiltration method, Kompozyty (Composites) 10 (3) (2010) 213–217. [3] W. Hufenbach, M. Andrich, A. Langkamp, A. Czulak, Precision moulds of graphite for fabrication of carbon fibre reinforced magnesium, Journal of Achievements in Materials and Manufacturing Engineering 12 (2003) 381–384. [4] W. Hufenbach, M. Gude, A. Czulak, F. Engelmann, Influence of different aluminium alloys on the material properties of Cf/Al-Mmc manufactured by Gpi method, Kompozyty (Composites) 10 (2) (2010) 143–148. [5] W. Hufenbach, M. Gude, A. Czulak, A. Gruhl, P. Malczyk, F. Engelmann, Carbon fibre light metal composites manufactured with gas pressure infiltration methods, Production Engineering Innovations & Technologies of the Future;Wroclaw (2011) 91–105. [6] W. Hufenbach, M. Gude, A. Czulak, F. Engelmann, K.J. Kurzydlowski, J. Sleziona, Characterisation of CF/Al-MMC Manufactured by Means of Gas Pressure Infiltration; Fifth International Light Metal Technology Conference 2011; 19–22 July 2011; Lu¨neburg, Germany, 116–120. [7] Jianqiu Jian Chen, Enhou Wang, Junhua Han, Dong, Wei Ke, States and transport of hydrogen in the corrosion process of an AZ91 magnesium alloy in aqueous solution, Corrosion Science 50 (2008) 1292–1305. [8] Meirong Jian Chen, Jianqiu Ai, En-Hou Wang, Wei Han Ke, Stress corrosion cracking behaviors of AZ91 magnesium alloy in deicer solutions using constant load, Material Science and Engineering A 515 (2009) 79–84. [9] Liu. Liming, Qi. Xiaodong, Wu. Zhonghui, Microstructural characteristics of lap joint between magnesium alloy and mild steel with and without the addition of Sn element, Materials Letters 64 (2010) 89–92. [10] Y.Z. Lu¨, Q.D. Wang, W.J. Deng, X.Q. Zeng, Y.P. Zhu, Fracture behavior of AZ91 magnesium alloy, Materials Letters 44 (2000) 265–268. [11] Gang Xiaodong Qi, Song, Interfacial structure of the joints between magnesium alloy and mild steel with nickel as interlayer by hybrid laser-TIG welding, Material and Design 31 (2010) 605–609.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/acme
Original Research Article
Manufacture studies and impact behaviour of light metal matrix composites reinforced by steel wires W. Hufenbach, H. Ullrich, M. Gude, A. Czulak, P. Malczykn, V. Geske Technische Universita¨t Dresden, Institute of Lightweight Engineering and Polymer Technology (ILK), Holbeinstr. 3, 01307 Dresden, Germany
art i cle i nfo
ab st rac t
Article history:
Magnesium alloys play an important role in the development of light metal matrix
Received 1 June 2012
composites. Magnesium based metal matrix composites reinforced by particles and fibres
Accepted 4 June 2012
(especially carbon fibres) are successfully applied in various fields of automotive and
Available online 26 June 2012
aircraft industry. Equally high potential in large-batch production regarding to relatively
Keywords:
low price and high strength is expected from MMC with wires made of iron-based alloys.
Magnesium alloy
However, their application is hampered by the absence of intermetallic phase between iron
Steel
and magnesium and low solubility of iron in magnesium.
GPI process
This paper makes a contribution to the investigation of the effect of steel wires surface
Composite
preparation and of optimised production methods to improve the quality and type of
Impact resistance
adhesion with selected industrial magnesium alloys. The Fe/Mg-MMC specimens with steel wires reinforcement were manufactured by the help of advanced gas pressure infiltration method (GPI) in a graphite mould at Institute of Lightweight Engineering and Polymer Technology (ILK) of TU Dresden. For the examination of the obtained composites the computer tomography (CT), SEM microscopy with EDX, strength tests and fracture surface inspection has been used. & 2012 Politechnika Wrocławska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved.
1.
Introduction
Increasing technical requirements for automotive and aerospace products require continuous improvements in lightweight design to achieve high-tech products and associated efficient manufacture processes. Due to the high production cost for continuous fibre reinforced metal matrix composites intense research to develop new low-cost solutions has been done. Magnesium and its alloys offer a high lightweight potential because of high specific strength and stiffness, low density, welding ability and high thermal and electric conductivities.
Industrial companies whose products are subjected to high loads pay great attention to the development of new materials and new manufacturing technologies [3,5,9,11]. The selection of an appropriate reinforcement with regard to applications, operating conditions and simplicity of production is a key aspect in the initial stage of the product design. Compared to carbon fibres steel reinforcements provide higher rigidity in the die casting mould, higher temperature stability in contact with other metals and possibility to thermal treatment. Despite the fact that the higher density of steel may decrease light weight potential, a high impact resistance is expected by steel–magnesium composites.
n
Corresponding author. Tel.: þ49 691855091. E-mail address: [email protected] (P. Malczyk).
1644-9665/$ - see front matter & 2012 Politechnika Wrocławska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved. http://dx.doi.org/10.1016/j.acme.2012.06.005
266
archives of civil and mechanical engineering 12 (2012) 265–272
Since the main obstacles are a lack of intermetallic phase and poor wettability between magnesium and steel the efforts to receive favourable adhesion have been intensified. The main aim of this work is to achieve high strength composites characterised by improved impact resistance [9–11].
2.
Selection of the matrix and reinforcement
Mg–Al–Zn is the most common group of Mg-based alloys used in casting. Maximum solubility of aluminium in magnesium is 12.7% at 437 1C and it decreases to 2% in ambient temperature. The effect of addition of aluminium to magnesium alloys is noticed in castability improvement although it also causes a tendency to microporosity and corrosion. Moreover, aluminium in magnesium alloys contributes to solid solution hardening and to the formation of eutectic precipitations. The addition of zinc to Mg–Al alloys causes strengthening. However, the amount of zinc is limited due to an increase in the susceptibility to hot cracking during solidification. The corrosion resistance of Mg–Al alloys is improved by replacing the zinc with manganese, which also increases hardness without decreasing strain at fracture. This fact explains why the Mg–Al–Mn alloys are successfully used in casting as well as in forging. Considering the influence of Al, Zn and Mn the two industrial available alloys AZ91 and AM50 were selected. The iron based alloys are commonly used materials, for which manufacturing, shaping and microstructure development have been very well understood. Depending on the composition and type of treatment steel can provide a wide range of properties, which combination is difficult to obtain by other materials. The main influence on the impact resistance behaviour depends on the quality and type of bonding between magnesium matrix and steel wires. Two methods of surface preparation of steel reinforcements have been selected: the acid etching and the zinc plating. To improve
the adhesion by an increase of matrix-reinforcement contact surface, etching of the stainless steel mesh in highly concentrated sulphuric acid has been done. The ferritic steel with high content of Fe has been coated by zinc in order to investigate the possibility of the formation of a favourable intermetallic surface and to determine the influence of Zncover on the corrosion phenomena between iron and magnesium. The chemical compositions of the reinforcements and the matrix alloys are shown in Table 1 [5–11].
3.
Principals of manufacturing
The prototypes of steel/Mg-MMC specimens have been manufactured at the ILK by the help of an advanced gas pressure infiltration method. Before the infiltration the reinforcement was subjected to a surface treatment. Preparation of stainless steel wires in high condensed sulphuric acid has been carried out with different durations. Two times of bathing—10 and 20 min—have been used. The reinforcement was rinsed in acetone to clean the insert from residues and impurities due to the acid treatment. The ferritic steel was industrially covered by zinc and only cleaned in acetone before manufacturing process. Due to low wettability of steel by magnesium, which causes poor adhesion of the reinforcement and the matrix and therefore insufficient mechanical bonding, extremely high pressures of infiltration are indispensable. The gas pressure infiltration method, which allows programming the appropriate temperature, vacuum and pressure, is particularly favourable to produce composites that require a highly sophisticated and accurate production process [1–6]. A laboratory autoclave (Fig. 1c) with a maximum pressure of 120 bar, maximum temperature up to 1300 1C is equipped with two independent thermocouples to measure the temperature of melted material and the temperature of infiltrated product.
Table 1 – Chemical composition of used materials.
Ferritic steel Austenitic steel AM50 magnesium AZ91 magnesium
C (%)
Si (%)
Mn (%)
Cr (%)
Ni (%)
Mo (%)
Cu (%)
Al (%)
Zn (%)
0.05 0.03 – –
0.30 0.75 0.05 0.05
0.3 2.0 0.8 0.15
0.2 16 – –
0.25 10 0.001 0.001
0.05 2 – –
0.3 – 0.025 0.008
– – 5 9.5
– – 0.2 0.9
Fig. 1 – (a) Inner graphite mould with attached steel mesh; (b) external graphite mould and (c) autoclave.
archives of civil and mechanical engineering 12 (2012) 265–272
The infiltration of steel/Mg-MMC was realized with precision graphite moulds which allow to manufacture thin-walled structures. The mould consists of a precision inner-mould, where the infiltration process of attached reinforcement occurs (Fig. 1a), and an external mould (Fig. 1b) which allows maintaining vacuum and pressure in the infiltration area. The GPI process with steel mesh reinforcement can be divided into four steps. The first step starts with the temperature rising and cyclic vacuum ventilation to clean the autoclave chamber form impurities. The second step consists the increasing of temperature under vacuum conditions to melt the matrix. In the third step the melted alloy flows between the walls of the inner mould where the reinforcement has been attached. The ceasing of the vacuum maintaining creates the pressure difference between the inner mould—where vacuum is still retained—and the rest of the chamber. The inert gas pressurisation in the last step of the process contributes to the matrix porosity reduction and to the improvement in the filling of inaccessible places of reinforcement by melted magnesium. A defined cooling rate can be obtained by chamber ventilation and by adjusting the heating devices [3–5].
4.
267
Fig. 2 – Surface between uncoated ferritic steel and AM50 magnesium alloy with cracks at the contact area.
Material analysis
To investigate the microscopic structure the SEM TM3000 equipped with EDX scanning module has been used. The aim of this observation was to analyse the quality and type of bonding by the presence of specific phase precipitations in the area of contact surface between matrix and reinforcement. The Fe–Mg phase diagram shows low solubility and no intermetallic precipitations between the components. With regards to this information and to galvanic corrosion phenomenon the direct contact of magnesium and high pure iron is undesirable. The Zn-coating has been performed to reduce the Fe–Mg contact. The stainless steel consist of about 16–18% Cr, 8–11% Ni, 2% Mo and notable additions of Mn and Si as well as a carbon content of as low as 0.05%. Therefore the content of iron is decreased to 67%. The volume fraction of steel wires was different for the composites with ferritic steel reinforcement and stainless steel reinforcement. The ferritic steel reinforced composites were characterised by 8% of steel in the volume of specimen and a density of 2.15 g/cm3. Stainless steel reinforced composites have about 20% of steel and a density of 3.05 g/cm3. Considering the relative long time of magnesium being above the liquid temperature and the fast cooling rate the residual stresses generated by a mismatch of the thermal expansion coefficients between the magnesium alloy and steel are expected. Thus an interface with insufficient bonding strength is expected to be characterised by cracks on the contact surface between both materials. Fig. 2 shows no intermetallic bonding between uncoated ferritic steel and AM50 magnesium alloy. The absence of an interface is demonstrated by oxidation in cracks between the reinforcement and matrix which leads to a significant decrease of mechanical properties. The contact surface between infiltrated uncoated steel mesh with AZ91 magnesium alloy shows no discontinuities as well as no intermetallic phases (Fig. 3). The Zn coated steel wires infiltrated by magnesium AM50 show a multi-layer interface with zinc, manganese and
Fig. 3 – Surface between uncoated ferritic steel and AZ91 alloy with no intermetallic precipitations.
aluminium components (Fig. 4). The arrangement of phases is as following: steel reinforcement—zinc coating–manganese with aluminium precipitations—magnesium alloy matrix (Fig. 5). The thickness of the multi-layer interface is in a range of 10–15 mm. Microsections of specimens with AZ91 magnesium alloy show aluminium and Al–Mg precipitations in the surrounding of the steel. The sulphuric acid treatment caused better distribution of precipitates and higher magnesium diffusion into the steel wire. No cracks were visible in the contact area between reinforcement and matrix. Figs. 6 and 7 illustrate the difference between both interfaces in diffusion rate of magnesium into the steel wire and in the size and distribution of Al-based precipitations at the boundary of the wire. The microsection of AM50 reinforced by stainless steel shows an Al–Mn intermetallic interface (Figs. 8 and 9) which have a positive effect on the adhesion. This interface has a thickness of 2–5 mm and is similar to the multi-layer interface from the zinc coated specimens.
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Fig. 4 – Multi-layer intermetallic interface between Zncoated ferritic steel and AM50 magnesium alloy.
Fig. 7 – Surface between acid treated stainless steel and AZ91 magnesium alloy shows higher distributions of Al–Mg precipitations and diffusion rate of Mg into reinforcement.
Fig. 5 – EDX analysis of Zn-coated ferritic steel and AM50 magnesium alloy intermetallic interface arrangement. Fig. 8 – Intermetallic interface between acid prepared stainless steel and AM50 magnesium alloy.
Fig. 6 – Surface between untreated stainless steel and AZ91 magnesium alloy shows poor distribution of Al–Mg precipitations and low diffusion rate of Mg into reinforcement.
Fig. 9 – EDX analysis of stainless steel and AM50 magnesium alloy intermetallic interfaces.
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Due to the extreme brittleness the Al–Mg phases have an influence on the decrease of bonding strength between matrix and reinforcement. Therefore, the magnesium alloys with aluminium content as high as AZ91, will bring insufficient improvement of impact resistance of the stainless steel reinforced composites produced by GPI method. There is a reasonable presumption that the composites manufactured using AM50 magnesium alloy will be characterised by better quality of adhesion between matrix and reinforcement due to the presence of Al–Mn interface. Further investigations to determine the exact composition and formation mechanism of the obtained Al–Mn precipitations are necessary.
5.
Impact resistance test
Fig. 11 – (a) Representative photograph of planar impact test specimen and (b) drawing of specimen with dimensions.
The impact resistance tests have been performed by the help of the charpy test machine at the ILK (Fig. 10). The specimens have a dimension of 2.5 10 80 mm and have been prepared from planar steel/Mg-MMC plates (Fig. 11). Two groups of manufactured composites have been investigated. The group of ferritic steel reinforced composites with lower density has been selected to analyse the influence of zinc coating on the adhesion between reinforcement and matrix by creating the multi-layer intermetallic interface. The results of impact test are shown in Fig. 12. The composites reinforced with stainless steel have been selected to analyse the influence of surface etching on the mechanical properties of Fe–Mg connection. Rough surface made by bathing in acid and higher content of alloying Fig. 12 – Impact test results for ferritic steel reinforced composites.
Fig. 13 – Impact test results for stainless steel reinforced composites.
Fig. 10 – Charpy test machine.
elements contribute better mechanical adhesion and increase the diffusion rate of magnesium into steel wires. The results are shown in Fig. 13. Considering the difference in densities between stainless steel reinforced and ferritic steel reinforced specimens the impact energy absorption related to density of manufactured composite has been shown in Table 2. Based on the quotient of impact energy absorption and density the most favourable material combination has been selected as an acid etched stainless steel mesh infiltrated by AM50 magnesium alloy.
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Table 2 – The impact energy absorption and density of manufactured specimens.
Energy absorption (%) Density (g/cm3)
AZ91
AM0
a-steel / AZ91
a-steel / AM50
a-steel(Zn) / AZ91
a-steel(Zn) / AZ91
g-steel / AZ91
g-steel (acid prepared) / AZ91
g-steel /AM50
g-steel (acid prepared) / AM50
3.48
3.8
4.18
6.1
4.67
9.6
14.7
19.2
16.7
21.1
1.82
1.78
2.15
2.15
2.15
2.27
Fig. 14 – Specimen infiltrated by AZ91 alloy with no plastic deformation.
6.
3.23
3.05
3.07
2.99
Fig. 15 – Plastic deformation of specimen infiltrated by AM50 alloy.
Fracture analysis
The fracture surface of specimens after the impact resistance test has been investigated by the help of TM3000 SEM microscope. The AZ91 alloy as a matrix shows no plastic deformation during the test (Fig. 14), which is a main reason for the lower impact resistance. The brittleness is caused by a high content of Al, which results in hard Al–Mg eutectic. Specimens with AM50 alloy show significant plastic deformation (Fig. 15). Figs. 14 and 15 show that the stainless steel wire is subjected to plastic deformation during the test. The ferritic steel reinforcement shows a brittle fracture preceded by plastic deformation (Fig. 16). The proper inference about the fracture surface requires an understanding of the fact that the more ductile reinforcement is infiltrated by the brittle alloy. This aspect indicates that the created interface and the quality of bonding between both materials are extremely important. The specimens with lower impact resistance are characterised by no intermetallic interface and fracturing along brittle phases of matrix (Fig. 17). The composites with high impact resistance show an interface containing less brittle intermetallic precipitations. Cracks occurred along the magnesium brittle phases. This fracture surface can be recognised by the plastic deformation of matrix (Fig. 18) and significant remains of the intermetallic interface on the reinforcement (Fig. 19). The impact resistance highly depends on the quality and type of adhesion between reinforcement and matrix. The reinforcement characterised by higher stiffness with sufficient good bonded matrix maintains the high impact force during the
Fig. 16 – Fracture of composite reinforced by ferritic steel without interfacial surface and insignificant plastic deformation.
deformation. After the fracture of matrix the impact force is received only by the reinforcement that in consequence leads to damage of specimen (Fig. 20).
7.
Computer tomography analysis
The matrix-wire interfaces of stainless steel reinforced samples were investigated by high-resolution CT on a GE phoenix x-ray v9tome9x L 450. Small rods of 5 mm in length, 2 mm in
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Fig. 20 – Representative impact test graph of AM50 reinforced by stainless steel.
Fig. 17 – Fracture of stainless steel reinforced composite infiltrated by AZ91 alloy.
Fig. 21 – Roughness of the surface of reinforcement after infiltration. Fig. 18 – Fracture of AM50 reinforced by Zn-coated ferritic steel.
Fig. 19 – Fracture of AM50 reinforced by stainless steel.
width and 80 mm in height were prepared by cutting. A 300 kV microfocus tube was equipped with a tungsten target and a 0.2 mm copper foil was used as a filter for the xray beam. With an acceleration voltage of 100 kV and a beam current of 100 mA resolutions of up to 4 mm were achieved. The volumes were reconstructed with the Datos 2 Rec software by GE and visualised with VGStudio 2.0 by Volume Graphics. Since there is a large difference of densities and atomic numbers when comparing the magnesium-rich matrix with the austenitic steel picturing the matrix without artifacts can only be achieved by using a higher beam power while at the same time effectively lowering the resolution. Hence, the visualisation threshold was chosen in such a way that only the reinforcement is shown in the image and the matrix including the beam-hardening artefacts is not shown in the following pictures. In Fig. 21 there is a typical illustration of the stainless steel wire after the infiltration. The infiltration and the resulting
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processes in the interface lead to an increase in surface roughness of the stainless steel wire. This is believed to be caused by the reinforcement etching in acid, diffusion of matrix elements into the reinforcement, and intermetallic precipitations on the contact surface between both phases.
8.
Conclusions
The impact resistance of magnesium can be successfully improved with insignificant decrease of light weight potential by the steel mesh reinforcement. It is really important to know how indispensable the selection of appropriate materials is to receive favourable bonding and to avoid disadvantageous interfaces. Considering the results of this paper following conclusions can be raised:
The contact surface area between steel and magnesium
alloy are disadvantageous and causes the corrosion phenomenon. For good adhesion special preparation of reinforcement before infiltration is needed. With longer time of acid etching the increase of contact surface and higher magnesium diffusion rate into the steel wires can be observed. The Al–Mg precipitations are disadvantageous as an interface between steel reinforcement and magnesium alloy and because of their brittleness should be avoided. The addition of manganese into the magnesium alloy is advantageous because of creating less brittle Al–Mn precipitations. The Al–Mn precipitations at the surrounding of the wires strongly create adhesive interface. The stainless steel due to the less iron content does not need Zn-coating to create favourable interface. The increasing of contact surface by etching in acid is necessary to receive better mechanical infiltration and causes higher diffusion rate of magnesium into steel wires. The intermetallic interface improves strength and energy absorption. The specimens with AM50 matrix reinforced by stainless steel prepared with 20 minutes of acid bathing have received the best results and improvement of impact resistance of more than 400% compared to the impact resistance of unreinforced specimens.
Intermetallic coating and acid etching of steel reinforcement is advantageous and brings improvements in the connection between steel reinforcement and magnesium matrix. The Zn coating of reinforcement is more attractive
for favourable Al–Mn precipitations in contrast to the clean Fe-surface of wires which provide favourable conditions for separation of Al–Mg phase. This aspect is extremely important for further steps of manufacturing where high impact resistance of composites is the main point of interest. The advanced methods of connecting metals by creating favourable interface which are unavailable in traditional production methods have been still insufficient developed and further continuation of research in this field are necessary.
r e f e r e n c e s
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