Surface Chemical Inhomogeneity of High-Pressure ...

Surface Chemical Inhomogeneity of High-Pressure Die Cast Al-Si Alloys and its Effects on Corrosion Behaviors and Adhesive Bonding Capabilities J. Shi*, H. Pries, K. Dilger Institut of Joining and Welding, Technische Universität Braunschweig, Braunschweig, Germany

M. Klein, F. Walther Department of Materials Test Engineering, Technische Universität Dortmund, Dortmund, Germany |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||| Copyright 2014 World Foundry Organization ABSTRACT High-pressure die casting of Al-Si alloys is an attractive manufacturing process for light-weight structural components in the automobile industry. The die casting processes usually result in the contamination of casting surfaces due to die lubricants and in the segregation of alloying elements during solidification. In this paper surface chemical compositions of high-pressure die cast Al-Si alloys were determined quantitatively using surface sensitive analysis techniques, with emphasis on the distribution of residual die lubricants and alloying elements. Inhomogeneity of surface chemical compositions and its effects on corrosion behaviors and bonding capabilities were also examined. Better corrosion resistance and adhesive bonding capability were found in the surface regions near the melt-ingate than in the regions away from the melt-ingate. The different corrosion behaviors and bonding properties observed at different locations on the surface of die cast plates can be well explained in terms of the location-dependent surface chemistry which is thought to result from different temperature gradients in the die cavity and cooling rates during solidification processes. Keywords: High-pressure die casting, AlSi9Cu3Fe, AlSi10MnMg, Al-Si alloy, Element distribution, Chemical inhomogeneity, Segregation, Corrosion, Eectrochemical measurement, Potentiodynamic polarization, Adhesive bonding, Adhesive failure. INTRODUCTION Aluminum alloys are widely used in the construction, automotive and aerospace industries due to their good mechanical properties, corrosion resistance, and

especially a high strength to weight ratio. Al-Si alloys, together with other additives as modifiers for mechanical properties, have been routinely formed into automobile components at low cost by high-pressure die casting processes (HPDC), such as transmission housings, cylinder heads, inlet manifolds, engine sumps and decorative trim.1 Various lubricants in HPDC processes are used to maintain HPDC operations and the production rate through lubricating moving components, preventing solder formation and removing heat from the mold for thermal balance. The lubricants used in HPDC processes, particularly mold release agents, often lead to the contaminations on casting surfaces as residual lubricants and their decomposed products. The amount of residual lubricants on the surface of die castings is largely dependent on the type and the dilution ratio of die lubricants sprayed during HPDC processes. Another typical characteristic of HPDC plates is the inhomogeneous distribution of alloying elements in the cross-section of the castings2. A surface skin layer with a microstructure distinct from the bulk is often observed.2-4 Because adhesive bonding properties are sensitive to surface states, particularly surface chemistry, a quantitative determination of surface chemical compositions of casting surfaces is necessary for the evaluation of the bonding capability of HPDC Al-Si alloys and for the development of suitable surface pretreatment processes. Therefore, a research project between the industry and the research institute was established in order to develop a manufacturing process chain to ensure the initial bonding strength and the longterm stability of adhesive joints involving HPDC alloys. As part of the project, this paper will report on the surface chemical compositions and their effects on corrosion behaviors and adhesive bonding capabilities of HPDC AlSi castings.

EXPERIMENTAL METHODS HIGH-PRESSURE DIE CASTING OF AI-SI ALLOY PLATES Two Al-Si alloys, A226 (AlSi9Cu3Fe) and Trimal-05 (EN AC-AlSi10MnMg) were melted in a furnace at about 700 oC (1292 F) and then cast into plates (260 × 150 × 4.0 mm) using a HPDC machine (Buehler Evolution 530B ) at the Institute of Joining and Welding, Technische Universität Braunschweig. Besides the main element of Al in both alloys, the A226 contains 8.0-11.0 % Si, 2.04.0 % Cu, 1.33 % Fe, 0.05-0.55 % Mg, 0.55 % Mn, 0.15 % Cr, 0.55 % Ni, 1.2 % Zn, 0.35 % Pb, 0.25 % Sn and 0.25 % Ti whereas the concentrations of alloying elements in EN AC-AlSi10MnMg are 9.5-11.0 % Si, 0.25 % Fe, 0.4-0.7 % Mn, 0.1-0.4 % Mg, 0.07 % Zn, 0.03-0.12 % Ti, 0.05 % Cu with unknown impurities of about 0.2 %. The casting parameters were selected based on previous casting experiments. During HPDC polysiloxane-based mold lubricants (ChemTrend, Germany) with dilution ratios from 1:50 to 1:125 respectively, were sprayed onto the mold surfaces in each casting cycle. SURFACE COMPOSITIONAL CHARACTERIZATION Surface chemical compositions were analyzed on selected areas as shown in Fig. 1 using an electron probe microanalyzer (JEOL, JXA-8800L). This technique can not only provide quantitative compositional information at a spot of micron sizes but also the elemental distribution in a selected area using a spatially resolved mode. The residual lubricants on the HPDC surface can be reflected

by the carbon-containing species which is expressed as an equivalent carbon-film thickness in the measured area of 1×1 mm, consisting of 104 spot analyses. The distribution of alloying elements Si, Cu or Mn was also measured in a similar way in the selected regions. CORROSION CHARACTERIZATION Immersion test An accelerated corrosion test was performed by immersing an HPDC plate into 500 ml 10 mM CuSO4 solutions at room temperature for 18 h. After rinsing and drying, the precipitation of Cu on the surface of HPDC plates was compared. Some selected areas were also investigated using Scanning Electron Microscopy (SEM, JEOL 6400). Electrochemical measurement Potentiodynamic polarization measurements were performed in a 0.1 M NaCl solution at 25 oC (77 F) using a potentiostat (Gamry PCI 4300). The specimens used as working electrodes were connected to a copper wire and the whole specimen, apart from its front-side, was embedded in epoxy resin. For the electrochemical measurements, a standard three-electrode system was used with an Ag/AgCl electrode with a Luggin capillary as reference electrode and a graphite electrode as counter electrode. The experimental setup can be found elsewhere.5 Before each polarization measurement, the electrolyte was purged for 30 min with argon and afterwards the open circuit potential was measured for 30 min. The potentiodynamic polarization measurements were conducted at a scanning rate of 0.2 mV/s. ADHESIVE BONDING EVALUATION

Fig. 1. Schematic of various sampling locations for surface chemical analysis of an HPDC plate (260×150×4.0), S1, S2 and S3 are surface regions away from the melt-ingate, S4, S5 and S6 represent the surface regions near the melt-ingate.

The adhesive bonding property of HPDC Al alloy plates was evaluated by floating roller peel test according to the German standard DIN EN 285106. The adhesive joints were fabricated by gluing three grit-blasted Al sheets (300 × 25 × 0.3 mm) on the surface of a casting plate with an epoxy adhesive (Dow Corning, Germany) followed by curing at 180 oC (356 F) for 30 min. Three specimens were cut from each HPDC plate. The specimen located near the melt-ingate regions (the bonding area covering S4, S5 and S6 in Fig. 1) and at the regions away from the melt-ingate (the bonding area covering S1, S2 and S3 in Fig. 1) is designated as AN and AF, respectively. The one located between the specimens AN and AF is denoted as

MIT. The flexible Al-sheet of each specimen was peeled off from the HPDC panel using a Zwick/Roell machine with a peeling speed of 100 mm/min. After the peel test the failure mode was visually checked. RESULTS AND DISCUSSION LOCATION DEPENDENCE OF SURFACE CHEMICAL COMPOSITIONS OF HPDC PLATES Figure 2 shows the concentration distributions of Si, Cu at the two typical locations, S5 (near the melt-ingate) and S2 (away from the melt-ingate) on the surface of an AlSi9Cu3Fe plate produced using a polysiloxane-based mold lubricant with a dilution ratio of 1:125. An average atomic concentration of Si and Cu in the measured area of S5 is found to be 11.3 at.% and 0.6 at.% respectively, showing a slight enrichment of the Si element in the region near the melt-ingate. On the contrary, the averaged concentration of Si and Cu in the region S2 is 6.3 at. % and 7.0 at%, respectively, suggesting a strong segregation of Cu in the region away from the melt-ingate. The concentration of Cu in the region S2 is more than 10 times higher than that in the region S5. The inhomogeneous distribution of alloying elements across the casting thickness was reported for AZ91D cast plates.2 These phenomena were attributed to the mold filling and subsequent solidification processes.2 The observed segregation of Si and Cu in the HPDC AlSi9Cu3Fe plates can also be explained by different temperature gradients in the mold cavity and the cooling conditions during solidification. The enrichment of Cu in the regions away from the melt-ingate would result in the high fraction of Al-Cu intermetallic phases which have a higher electrochemical potential than the Al-matrix phase.7,8 The difference in the concentration of Cu-containing phases on the casting surface will lead to different corrosion behaviors as observed in the corrosion tests. Residual mold lubricants on casting surfaces can be characterized by the concentration distribution of Ccontaining species. The distribution of C-species within a measured area can be represented by an equivalent C-film with different thickness.9-11 The equivalent thickness of Ccontaining species on the surface of HPDC AlSi10MnMg plates, fabricated using a polysiloxane-based mold lubricant with dilution ratios of 1:50 and 1:125, was measured as a function of locations, Fig. 3. It is often found that in the region S5 the thickness of an equivalent C-film is much thinner than in other regions, indicating a relatively clean surface with respect to the contamination by residual lubricants. It is also found that the residual

mold lubricants strongly depend on the dilution ratio used. A higher dilution ratio usually leads to, on the one hand, less residual lubricant on the casting surface, on the other hand, a better homogeneity across the casting surface, e.g. the averaged C-film thickness on the surface of an AlSi10MnMg plate is approximately 10 ± 3 nm in the measured regions S1, S2, S3 and S5, which is nearly half of that on the plate cast with a dilution ratio of 1:50 (18 ± 6 nm). The different coverage of residual lubricants in different regions will affect adhesive bonding properties as the residual lubricants are detrimental to adhesive bonding capabilities.

Fig. 2. Distributions of alloying elements, Si and Cu in the different regions of the surface of an HPDC AlSi9Cu3Fe plate cast using a polysiloxane-based die lubricant with a dilution ratio of 1:125. S2 represents the regions away from the melt-ingate and S5 is located in the region near the melt-ingate. Pictures in the left column are SEM micrographs of the regions S2 and S5, in the middle and on the right are corresponding elemental distributions of Si and Cu in the regions S2 and S5, respectively. For details see the text.

Fig. 3. Variations of the equivalent carbon-film thickness on the surface of HPDC AlSi10MnMg plates cast using a polysiloxane-based lubricant with dilution ratios of 1:50 and 1:125, reflecting the inhomogeneous coverage of residual lubricants on the HPDC casting

surfaces. S1, S2, S3 and S5 are different surface regions as defined in Fig. 1.

CORROSION BEHAVIORS Cu-precipitation on the surface of HPDC AlSi9Cu3Fe plates in CuSO4 solutions The surface morphology of an HPDC AlSi9Cu3Fe plate, produced using a polysiloxane-based lubricant with a dilution ratio of 1:100, after an immersion test in a 10 mM CuSO4 solution, is shown in Fig. 4. The brown areas in the photograph (Fig. 4a) are indicative of Cu precipitation from the CuSO4 solution and are confirmed by SEM investigation (Fig. 4c). The light-colored areas in Fig. 4a exhibit dissolution morphologies under SEM (Fig. 4b). These changes in surface morphology can be explained by the corrosion processes during immersion test. The alloying elements in HPDC plates can be dissolved in the matrix Al phase or form intermetallic phases in the micro-

Fig. 4. a) Photograph of an HPDC AlSi9Cu3Fe plate cast using a polysiloxane-based mold lubricant and a dilution ratio of 1:100 after immersion for 18 h in a 10 mM Cu2SO4 solution, showing more precipitation in the areas away from the melt-ingate than in those near the melt-ingate, b) SEM micrograph of anodic dissolution of the Al matrix phase and c) SEM micrograph of the precipated Cu on cathodic sites

structure. Copper-containing intermetallic phases, such as Al2Cu, Al-Cu-Fe etc. can serve as a cathode in a local galvanic cell upon exposure to an electrolyte, because they are more noble than the Al matrix phase.7,8 When immersed in a CuSO4 solution, the anodic dissolution of the Al-matrix phase processes (Al = Al3+ + 3e). Electrons generated in this anodic reaction flow to the cathodic sites where the precipitation of Cu from the CuSO4 solution (Cu2+ + 2e = Cu) occurs. The inhomogeneous distribution of Cu-containing phases on the casting surface leads to the distribution of microcathodes in an electrolyte and forms different numbers of local electrochemical cells. As shown in Fig. 4 many more Cu-precipitation areas can be

observed in the regions away from the melt-ingate than in the regions near the melt-ingate. This observation is consistent with the segregation of the Cu element in the regions away-from the melt-ingate, indicating many Cucontaining intermetallic phases which act as Cuprecipitation sites in the CuSO4 electrolyte. Electrochemical measurements of HPDC AlSi9Cu3Fe plates The difference in corrosion behaviors observed during immersion tests can be further studied by electrochemical measurements. Figure 5 shows typical potentiodynamic polarization curves in 0.1 M NaCl solutions at 25 oC (77 F) for different surface regions, S2 and S5 of AlSi9Cu3Fe plates produced using a polysiloxane-based lubricant with a dilution ratio of 1:100. From this diagram the corrosion potentials, Ecorr and the corrosion current densities, icorr were obtained. In most cases a higher corrosion current density was observed in the regions away from the meltingate (S2) than in the regions near the melt-ingate (S5). The corresponding corrosion rates (mass loss per square centimeter per year), rcorr were determined according to Faraday’s law and are given in Table 1. The high corrosion current density and high corrosion rate for the regions away from the melt-ingate are caused by the high content of Cu-containing intermetallic phases as shown by the enrichment of Cu element in these regions. The corrosion potential show a range of 0.47 - 0.67 V (vs. Ag/AgCl) for S5 and 0.59 - 0.83 V (vs. Ag/AgCl) for S2, suggesting that the Cu-containing intermetallic phases in the region, S2 or S5 are also different from plate to plate, e.g., with or without Fe, or other alloying elements.8 H

Fig. 5. Potentiodynamic polarization curves of the regions S2 and S5 of an HPDC AlSi9Cu3Fe plate cast using a polysiloxane-based mold lubricant and a

o

dilution ratio of 1:100 in a 0.1 M NaCl solution at 25 C (77 F). For the designation of S2 and S5, see Fig.1. Table 1. Corrosion parameters and corrosion rates of the regions S2 and S5 of different HPDC AlSi9Cu3(Fe) plates cast using a polysiloxane-based mold lubricant and a dilution ratio of 1:100 in a 0.1 M NaCl solution at o 25 C (77 F). For the designation of S2 and S5, see Fig.1. Ecorr. mV vs. Ag/AgCl

icorr. µA/cm2

rcorr. mg/a.cm2

S5

-474

0.45

1.3

S2

-588

3.11

9.1

S5

-687

0.13

0.4

S2

-828

2.04

6.0

S5

-685

3.45

10.1

S2

-714

2.50

7.3

S5

-674

2.29

6.7

S2

-757

3.16

9.3

Sampling location Plate 1 Plate 2 Plate 3 Plate 4

force versus peel distance. For the specimen MIT, adhesive failure on the surface of the HPDC substrate occurred also at the very beginning of the peeling test (Fig. 6b). It is noticed that adhesive failure at the surface of HPDC plates often occurred in the regions away from the melt-ingate which is characterized by higher concentration of residual lubricants, Fig. 3. The difference in the initial bonding capability of HPDC Al-Si plates can be ascribed to the inhomogeneous coverage of residual lubricants on their surface, which affects the initial bonding properties of HPDC Al-Si alloy plates.

ADHESIVE BONDING PROPERTIES After bonding Al sheets using an epoxy adhesive in three regions, i.e. AF, MIT and AN, three peel test specimens with an adhesive layer thickness of about 0.5 mm were cut from a HPDC Al-Si alloy plate. The specimens were examined using the floating roller peel test procedure.6 In this test, the Al-sheet is peeled off at a constant angle from the HPDC panel and the peel force of the adhesive joint is recorded versus peel distance. As an example, Fig. 6a depicts the diagrams of the peel force as a function of the peeling distance for the specimens AF, MIT and AN of an HPDC AlSi10MnMg plate cast with a lubricant dilution ratio of 1:50. The appearance of the specimens after the peel tests is shown in Fig. 5b. For the specimen AN, a constant force of about 140 N during the peel test was recorded and the failure mode is cohesive at the interface between Al-sheet and adhesive, suggesting a good bonding property in the areas near the melt-ingate. The specimen AF exhibited an unstable peeling force in the test distance of about 160 mm, particularly, a peel force of about 30 N was observed at the peeling distance between 12 and 35 mm. This range corresponds to adhesive failure at the HPDC panel, Fig. 5b, where the adhesive was peeled away and attached to the Al-sheet. The rest of the specimen AF in the peel test range is also dominated by cohesive failure at the interface between Al-sheet and adhesive, besides several fabrication errors indicated by downward sharp peaks in the diagram of peel

Fig. 6. a) Peel force as a function of peel distance, b) photographs of failure modes after peel tests for the specimens AN, MIT and AF from an HPDC AlSi10MnMg plate cast using a polysiloxane-based lubricant with a dilution ratio of 1:50. The bonding area of the specimen AF covers the regions away from the melt-ingate (S1, S2 and S3), the specimen AN covers the regions near the melt-ingate (S4, S5 and S6) with the specimen MIT covering the middle parts between the AN and the AF regions of a plate.

The surface chemical inhomogeneity of HPDC Al-Si components is characterized by the non-uniform coverage of residual die lubricants and by the segregation of alloying elements. A higher degree of contamination (a thicker C-film) in the area away from the melt-ingate was found and the negative effect on the adhesive bonding capability is also observed by the adhesive failure in the

heavily contaminated region (S1 or S3). The enrichment of alloying elements in these regions will initialize corrosion processes at the adherends’ surface upon environmental attack and induces the performance degradation of adhesively bonded structures. In order to improve the initial bonding properties of HPDC die castings, surface cleaning treatments using physical and chemical processes, such as laser, plasma and chemical etchants were tested to remove surface contaminations1013 . We have found that degreasing using alkaline etchants is an effective and economic way of ensuring the initial bonding capability of HPDC castings and that additional chemical treatment involving chemical conversion coatings is necessary to improve the durability by inhibiting corrosion processes at the surface of HPDC adherends in adhesive joints. These aspects are out of the scope of this paper and will be published elsewhere. CONCLUSIONS Surface chemical compositions of high-pressure die cast Al-Si alloys were quantitatively analyzed regarding residual lubricants, alloying element distributions in selected regions of casting surfaces, showing a strong dependence on locations. The corrosion behaviors and adhesive bonding properties were also investigated at different regions of casting surfaces which show a correlation with local surface chemistry. The residual lubricants reflected by the averaged thickness of carbon films on casting surfaces exhibited a heterogeneous nature with a thinner film of carbon-containing species in the regions near the melt-ingate while a strong contamination occurred in the regions away from the melt-ingate. The thickness of residual lubricants and their distribution also depend on dilution ratios for a given lubricant, e.g. the Cfilm thickness is about 10 nm for a dilution ratio of 1:125, only half of that for a dilution ratio of 1:50. Adhesive failure which occurred on HPDC surfaces in adhesive joints is attributed to the high degree of contamination by residual lubricants in the region away from the meltingate. The segregation of Cu in the regions away from the melt-ingate results in a high corrosion rate of AlSi9Cu3Fe which will cause degradation of adhesive joints during long-term usage. These results indicate that surface pretreatment procedures are necessary to achieve a new surface chemistry for the improvement of the initial bonding strength and long-term stability of adhesively bonded high-pressure die castings.

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