Effects of Surface Treatment on Fatigue Property of A5052-H14 and A2017-T4Aluminum Alloys+1

Ryota Kido1, Ryoichi Kuwano1, Makoto Hino1,+2, Keisuke Murayama2, Seigo Kurosaka2, Yukinori Oda2,Keitaro Horikawa3 and Teruto Kanadani4,+3

1Department of Mechanical Systems, Graduate School of Engineering, Hiroshima Institute of Technology, Hiroshima 731-5193, Japan2C. UYEMURA & Co., Ltd., Hirakata 573-0065, Japan3School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan4Okayama University of Science, Okayama 700-0005, Japan

In this study, the effect of anodization and electroless Ni­P plating on the fatigue strength of commercial A5052-H14 and A2017-T4aluminum alloys was investigated. The coated aluminum alloys were tested using a rotary bending fatigue testing machine. Anodization led toa slight increase in the fatigue strength of the A2017-T4 alloy of approximately 10% because of the suppression of the generation of fatiguecrack, and anodization with a 5-µm thickness for A5052-H14 also led to a slight increase in the fatigue strength. However, anodization with a20-µm thickness for A5052-H14 led to reduced fatigue strength because of the pits that formed in the film. In addition, electroless Ni­P platingdrastically improved the fatigue strength of the A5052-H14 alloy by suppressing the generation of fatigue crack. It also improved the fatiguestrength of the A2017-T4 alloy in the high-stress region. However, the fatigue strength in the low-stress region was the same as that of thenon-coated specimens. This fatigue strength should have originated from the hydrogen embrittlement by the hydrogen introduced into thespecimen during the plating. [doi:10.2320/matertrans.MT-M2020280]

(Received August 31, 2020; Accepted October 5, 2020; Published November 13, 2020)

Keywords: aluminum alloy, fatigue property, surface treatment, hydrogen embrittlement

1. Introduction

In the field of transportation, weight reduction of variousequipment components (such as automobile equipment) isa necessary measure to reduce CO2 emissions and fuelconsumption.1) For this reason, the use of lightweightaluminum alloys is expanding. Aluminum alloys can alsobe used in other various functions such as decorativeness,corrosion resistance, and abrasion resistance on aluminumsubstrate surfaces. Currently, there are various aluminumsubstrate surface treatments aimed at improving the surfacefunction of aluminum substrates. Anodization and platingare widely used as aluminum substrate surface treatments asthey impart corrosion and wear resistance properties.2)

However, it has been reported that the anodic oxidationtreatment (anodization) of an aluminum alloy causes it to loseits fatigue strength.3) Electroless Ni­P plating on Al­Sialuminum alloys containing micron-sized precipitates is theother widely used aluminum substrate surface treatment,however, hydrogen absorbed by the materials during platinghurts their fatigue strength.4) When applying aluminum alloysto structures, their mechanical properties including fatiguestrength are important, and it is useful to clarify the effectof these surface treatments on mechanical properties.

In this study, we investigated the effects of anodicoxidation and electroless Ni­P plating on the fatigueproperties of 5000 and 2000 series aluminum alloys, whichare typical non-heat-treated and heat-treated. Also, weinvestigated the effect of hydrogen embrittlement on fatigueproperties for each surface treatment.

2. Experimental Procedure

In this experiment, commercially available A5052-H14aluminum alloy (10mm diameter bar, herein referred to asA5052 alloy) and A2017-T4 aluminum alloy (10mmdiameter bar, herein referred to as A2017 alloy) were used.Table 1 shows the chemical composition of each alloy. Thealloys were first cut into a specific shape as shown in aprevious report;5) they were then plated (anodized sulfateand electroless Ni­P plating were conducted on each sample)under the conditions shown in Table 2. For electroless Ni­Pplating, a commercially available plating solution (KTB-HK,manufactured by UYEMURA & Co., Ltd.: phosphoruscontent of 8 to 9mass%) was used. The pretreatment inreadiness of anodized sulfate and electroless Ni­P platingwas performed per standard procedures. Double zincatetreatment was performed on electroless Ni­P from via filmadhesion.6) The samples before and after the surfacetreatment were observed from the surface and cross-sectionat the R part of each specimen, and the rotational bendingfatigue test (rotation speed 3150 rpm (52.5Hz)) shown in theprevious report5) was performed to obtain the S-N curve. Toobserve the impact of hydrogen embrittlement on electrolessNi­P plating, a three-point bending test was performed usingSK85 steel (70 © 4 © 0.5mm) that had been subjected tohigh heat treatment, as previously reported.7)

3. Results and Discussions

3.1 Tensile propertiesTable 3 shows the tensile properties of A5052 and A2017

alloys. A5052 alloy when non-heat-treated exhibited 0.2%proof stress and tensile strength, the heat-treated A2017alloy exhibited similar properties, but only halfway. Theirelongations were, however almost similar. A2017 alloy is

+1This Paper was Originally Published in Japanese in J. Japan Inst. Met.Mater. 84 (2020) 74­79.

+2Corresponding author, E-mail: hino@me.it-hiroshiam.ac.jp+3Professor Emeritus, Okayama University of Science

Materials Transactions, Vol. 62, No. 1 (2021) pp. 69 to 74©2020 The Japan Institute of Metals and Materials

stronger and tougher owing to the precipitation of its G.P.zone corresponding to Al2Cu and Al2CuMg by solutiontreatment and subsequent natural aging.

3.2 Surface condition of R partIn the rotating bending fatigue test, bending stress at the

outer periphery was maximized, causing cracking to startfrom the outer periphery and progress toward the center. Asthe surface morphology of the R part is greatly affected thefatigue characteristics, the surface of the R part after lathingto the test piece shape was observed by SEM. Figure 1 showsthe observation results. In both A5052 and A2017 alloys,only lathing tool marks were observed, other scratches werehardly observed. The surface roughness (Ra) of the A5052and A2017 alloys were 1.30 µm and 1.25 µm, respectively,and there was no significant difference in the surface state.

Figure 2 shows the SEM photographs of the surface at theR part after anodized sulfate (film thickness 5 µm). In bothA5052 and A2017 alloys, tool marks based on lathing were

observed even after plating with anodized sulfate. Fine pitswere formed in each of the anodic oxide films, and the pitsof all alloys increased with increasing film thickness. TheA5052 alloy had larger and more pits than the A2017 alloy.

Pits occurring on the film may cause stress concentrationand maybe a starting point for fatigue fracture. Therefore,the pit portion was observed cross-sectionally by FIB-SEM.Figure 3 shows the observation results of the 5 µm-thickanodic oxide film on A2017 alloy. Since the anodic oxidefilm also formed pits, it was presumed that the pits wereformed owing to the dropout of intermetallic compoundssuch as Al3Fe in the substrate during pretreatment orelectrolysis with anodized sulfate. An anodic oxide filmwas formed on the pits of both alloys.

Figure 4 shows the SEM photograph of the surface at theR part after electroless Ni­P plating. Both A5052 and A2017alloys showed a tool mark based on lathing after plating. Thesurface roughness (Ra) of the A5052 alloy with the filmthickness of 3 µm and 13 µm were 1.80 µm and 2.03 µm,respectively, and those of the A2017 alloy film thicknesses of3 µm and 13µm were 1.73 µm and 2.02 µm, respectively. Thesurface roughness (Ra) of each of the treated alloys afterplating was higher than that of the untreated ones. It waspresumed that the three stages of plating pretreatment i.e.alkali degreasing, pickling, and double zincate treatmenttook place and that the substrate was etched and the surfaceroughness increased. The surface roughness of each alloyslightly increased with the increase in film thickness, but nopits were observed in the anodized sulfate film.

Table 1 Chemical composition of A5052-H14 and A2017-T4 aluminum alloys. (mass%)

Table 2 Conditions for surface treatment.

Table 3 Mechanical properties of A5052 and A2017 aluminum alloys.

Fig. 1 SEM images of each specimen surface at R part.

R. Kido et al.70

Figure 5 shows the cross-sectional observation results ofa 13 µm-thick electroless Ni­P plated A5052 alloy. Thethickness of the plating film was uniform, and no gap wasobserved at the interface between the A5052 alloy substrateand the plating film, and the plating film was completelyadhered to. This is because the double zincate treatment isapplied to the plating pretreatment. The plating on the A2017alloy was the same as on the A5052 alloy.

3.3 Fatigue propertiesFigure 6 shows the S-N curves of the A5052 and A2017

alloys obtained by the rotating bending fatigue test onspecimens with different thicknesses of the anodized sulfate.In the low-load range, no fracture was observed even when

Fig. 2 SEM images of anodized specimen surface at R part. (a) A5052 alloy (b) A2017 alloy.

Fig. 3 Cross-sectional SEM images of anodized layer for A2017 alloy.

Fig. 4 SEM images of electroless Ni­P plated specimen surface at R part. (a) A5052 alloy (b) A2017 alloy.

Effects of Surface Treatment on Fatigue Property of A5052-H14 and A2017-T4 Aluminum Alloys 71

the number of rotations reached 107 cycles, so the test wasended at that point, and the fatigue strength (arrow in thefigure) at that point was measured. The untreated A5052 andA2017 alloys had a fatigue strength of 90MPa and 170MPa,respectively. Fatigue strength of the A2017 alloy almostdoubled that of the A5052 alloy. The tensile strength of theA2017 alloy shown in Table 3 also almost doubled that ofthe A5052 alloy. Summarily, A2017 alloy exhibited bettermechanical properties than the A5052 alloy.

The effect of treatment on the fatigue strength of theanodized sulfate on the A5052 alloy varied depending on

film thickness. In the high-load region of the treated A5052alloy film, the number of cycles that caused fractures to a5 µm thick film was lower than those of the untreated one.In the low-load range, the number of cycles increased,and the fatigue strength also increased by about 20%. It isspeculated that the anodized film, which is harder than theA5052 alloy substrate, increased by suppressing theoccurrence of fatigue cracks on the surface in the low-loadregion. The fatigue strength of a film thickness of 20 µmwas slightly lower than that of the untreated film, and thistendency was especially noticeable in the high-load region,and the variation was large. It is speculated that the pitsformed in the film shown in Fig. 2 expand as the filmthickness increases and becomes the starting point of fatiguefracture, resulting in film cracking and a decrease in fatiguestrength.

On the other hand, on A2017 alloy with anodized sulfate,the fatigue strength increased for both the film thicknesses of5 µm and 20µm, although there were variations. Since thepits formed in the A2017 alloy film were smaller comparedto those of the A5052 alloy, it is considered that the decreasein fatigue strength was due to film cracking of the A5052alloy with a film thickness of 20 µm.

Figure 7 shows the S-N curves obtained by the rotatingbending fatigue test for the specimens of A5052 and A2017alloys which were subjected to electroless Ni­P plating withdifferent film thicknesses. Regarding electroless Ni­P platingon the A5052 alloy, the fatigue strength of the film with a

Fig. 6 Cross-sectional SEM images of electroless Ni­P plating for A5052 alloy.

Fig. 5 Cross-sectional SEM images of electroless Ni­P plating for A5052alloy.

Fig. 7 Relation between stress (·) and number of cycles to failure (N) for electroless Ni­P plated specimens.

R. Kido et al.72

thickness of 5 µm was 140MPa, which was 1.5 times morethan that of the untreated one. The fatigue strength of thefilm with a thickness of 13 µm was 170MPa, which wasabout 1.9 times more than that of the untreated one, and thisvalue was the same as that of untreated A2017 alloy,indicating that electroless Ni­P plating on A5052 alloy wasextremely effective for fatigue properties. These improve-ments in fatigue strength may be due to the Ni­P plating film.It is 580HV hard and has high toughness and, therefore, iscapable of suppressing fatigue cracks on the surface.

On the other hand, the fatigue strength of electroless Ni­Pplating on A2017 alloy was significantly different from thatof the A5052 alloy. The number of cycles that led to thefracture in the high-load region of treated A2017 alloy wasmore than that of the untreated one, and this tendency wasmore remarkable at the film thickness of 13 µm than at thefilm thickness of 3 µm. However, the fatigue strength after107 cycles decreased to the same level as that of the untreatedspecimen, and the fatigue strength of all specimens wasaround 170MPa. Thus, it was revealed that the effect onfatigue strength greatly differs depending on the alloy typeeven when the same plating film is used. The reason whythe fatigue strength of the electroless Ni­P plating on theA2017 alloy was almost the same in treated and untreatedwas that the fatigue strength of the electroless Ni­P on theAl­1.2mass% Si alloy was similar to that of the plating.3) Itis speculated that the hydrogen occluded in the aluminumsubstrate causing embrittlement.

3.4 Evaluation of hydrogen embrittlement by a three-point bending test

Regarding the hydrogen embrittlement of electroless Ni­Pplating, according to Takata’s report8) by the delta gaugemethod, the hydrogen embrittlement rate was 1.5% or less,and it was classified as a plating method that did not easilycause hydrogen embrittlement across various plating films. Ithas also been reported that fatigue strength is improved 1.5times by performing electroless Ni­P on AA2618-T61 alloy,which is the same Al­Cu alloy as A2017 alloy.9) To confirmwhether the change in fatigue strength of the A2017 alloyobtained in this experiment is due to hydrogen absorbed byelectroless Ni­P plating, hydrogen brittleness was evaluatedby a three-point bending test. It is generally known thathydrogen embrittlement susceptibility to metallic materialsincreases with stress concentration and low strain rate.10) Weevaluated the hydrogen brittleness of high-strength steelwith the zinc-based electroplating on the SSRT three-pointbending test and clarified its usefulness.7) A 5µm-thick, high-strength steel (SK85, hardness 640HV) film was subjectedto the SSRT three-point bending test via electroless Ni­Pplating. Hydrogen embrittlement is characterized by strainrate, and embrittlement is promoted at low strain rates.10)

Figure 8 shows the relationship between the displacementrate and the breaking stress. The breaking stress decreasesas the displacement rate decreases, and this result indicatesthat electroless Ni­P plating induces hydrogen embrittlement.The breaking stress increased slightly when the displacementrate decreased below 0.01mm/min. This is probably becauselike Zn­Ni alloy electroplating,7,11) diffusible hydrogen,which induces hydrogen embrittlement during the test,

penetrated the electroless Ni­P plating film and was releasedfrom the substrate. Thus, the involvement of hydrogenembrittlement was suggested as a factor that hindered theimprovement of fatigue strength in electroless Ni­P platingon A2017 alloy. Especially in the high-load region, thenumber of cycles leading to fracture was higher than that inthe untreated region. However, the result that the numberof cycles until fracture in the low-load region was reduced tothe same level as that of untreated one was in agreement withthe strain rate dependency of embrittlement at lower strainrates, which was a characteristic of hydrogen embrittlement.Furthermore, the fatigue strength of anodized sulfate onA2017 alloy was improved by about 10% over that ofuntreated one, which is also based on electrolysis at the anodewhere hydrogen is not involved. It supports that the decreasein the fatigue strength of electroless Ni­P plating is due tohydrogen embrittlement.

It has been found that hydrogen absorbed by platingreduces fatigue strength in electroless Ni­P plating on Al­Sialloy containing micron-sized precipitates.4) It was suggestedthat hydrogen absorption by electroless Ni­P plating hurtsfatigue strength even for finer precipitates such as 2000 seriesheat-treated alloys. On the other hand, hydrogen embrittle-ment did not occur in the 5000 series non-heat treatmenttype alloy even if the same plating was performed. In thefuture, we plan to study in detail the relationship betweenmechanical properties, including fatigue properties due tohydrogen absorption by plating, and the microstructure dueto heat treatment.

4. Conclusion

The following conclusions were arrived at after examiningthe effects of anodization and electroless Ni­P plating onthe fatigue properties of commercial A5052 and A2017aluminum alloys.(1) Anodized sulfate for A5052 alloy suppressed fatigue

crack initiation and improved fatigue strength whenthe film thickness was thin, however, when the filmthickness was large, pits formed in the film causing filmcracking and the fatigue strength decreased. On theother hand, anodized sulfate on A2017 alloy suppressed

Fig. 8 Effect of the displacement rate on the breaking stress of SK85 steelsubstrate with electroless Ni­P coating.

Effects of Surface Treatment on Fatigue Property of A5052-H14 and A2017-T4 Aluminum Alloys 73

fatigue crack initiation and improved fatigue strength.This is because the pits generated in the A2017 alloyare suppressed more than those in the A5052 alloy.

(2) Electroless Ni­P plating on A5052 alloy suppressesfatigue crack initiation and significantly improvesfatigue strength. In addition, the fatigue strength ofthe 13 µm thick film was improved to the same levelas that of an unplated A2017 alloy. On the other hand,in electroless Ni­P plating on A2017 alloy, the numberof cycles leading to fracture increased in the high-loadregion, but in the low-load region, it was about thesame value as the untreated one. It was speculated thatthis phenomenon was caused by hydrogen embrittle-ment I.e. hydrogen occluded during plating.

Acknowledgments

Authors would like to express sincere thanks to the lightmetal educational foundation.

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