Fatigue Property of Electroless Ni–P Plated A7075-T6511 Alloy Affected by Plated Film Composition

Ryohei Shinno; Makoto Hino; Ryoichi Kuwano; Koji Monden; Masaaki Sato; Yukinori Oda; Naoki Fukumuro; Shinji Yae; Keitaro Horikawa; Teruto Kanadani

doi:10.2320/matertrans.MT-L2022013

Abstract

In this study, with the aim of improving the fatigue characteristics of the A7075 aluminum alloy, A7075-T6511 alloy rod specimens were plated with electroless Ni–P with different compositions, and the fatigue characteristics were evaluated by a rotary bending fatigue test. The fatigue strength of the specimen plated with low-P type was higher than that of the un-plated specimen. However, the fatigue strength of the specimen plated with high-P type was significantly lower than that of the un-plated specimen, and the fatigue strength of the specimen plated with medium-P type was also lower though the extent was not so significant. It was speculated that these reductions in fatigue strength were due to hydrogen embrittlement by the hydrogen introduced into the specimen during the plating.

This Paper was Originally Published in Japanese in J. JILM 71 (2021) 534–538.

Fig. 4 Relation between stress amplitude (σ_a) and number of cycles to failure (N) for the four types of specimens.

1. Introduction

In the field of transportation, the weight reduction of various equipment components, such as the automobile equipment, is necessary to reduce CO₂ emissions and fuel consumption. To deal with this pressing problem, the application of aluminum alloys, which are lightweight materials, is expanding.¹⁾ In particular, A7075 aluminum alloy, known as extra super duralumin, has a high strength exceeding 600 MPa and can realize a significant weight reduction. However, owing to its inferior fatigue properties, it is not currently used in applications involving tensile fatigue loading.²⁾ Therefore, the application scope of 7000 series alloys can be expanded by improving their fatigue properties. It has been reported that the fatigue properties of various aluminum alloys, including A7075, which is the subject of this study, can be improved by electroless Ni–P plating.³⁾ This plating technology provides a hard film of over 550 HV and uniform thickness for complex shapes.⁴^–⁶⁾ In particular, because of the superior corrosion resistance provided by electroless Ni–P plating, the alloy can obtain excellent fatigue properties even under a corrosive environment with a 3% sodium chloride solution.⁷⁾

However, in the electroless Ni–P plating of Al–Si and Al–Cu–Mg aluminum alloys containing micron-sized precipitates, it has been observed that the hydrogen absorbed in the material during plating adversely affects the fatigue strength.⁸^,⁹⁾ Because A7075 aluminum alloy exhibits high hydrogen embrittlement sensitivity,¹⁰⁾ there is a concern that the hydrogen absorbed in this material by electroless Ni–P plating may adversely affect its fatigue strength.

In this study, various electroless Ni–P plating compositions were applied to A7075 aluminum alloy to improve its fatigue properties, which were evaluated by rotating bending fatigue tests. Furthermore, the effect of the hydrogen absorbed into the substrate during the plating process on the fatigue properties was analyzed.

2. Experimental Procedure

A commercially available A7075-T6511 aluminum alloy (10 mm diameter bar, hereinafter referred to as A7075 alloy) was used. Table 1 lists the chemical composition of this alloy. Specimens with the shape described in a previous report¹¹⁾ were machined and then subjected to testing. Four types of electroless Ni–P plating were applied to the machined specimens according to the conditions indicated in Table 2 to obtain an average thickness of 10 µm. For the electroless plating, commercially available plating solutions (low-phosphorus: KHN-HK; medium-phosphorus: KTB-HK; high-phosphorus: KRB-HK; and Ni–P-PTFE: FUL-HKPTFE25 vol%, manufactured by C. Uyemura & Co., Ltd.) were used to improve the adhesion of the plating film, and double zincate treatment was performed as a pre-treatment process.¹²⁾ Surface and cross-sectional observations were carried out at the radius of the specimens before and after plating, and the rotating bending fatigue test (rotation speed: 3150 rpm (52.5 Hz)) was conducted as previously described¹¹⁾ to obtain the S–N curve. In addition, the fractured surface after the fatigue test was observed by scanning electron microscopy (SEM). It has been shown in previous experiments that the hydrogen content of each electroless Ni–P plating becomes almost constant after 14 days from the end of plating.¹³⁾ Thus, all specimens were left in the desiccator for more than 14 days after the end of plating before the experiments.

Table 1 Chemical composition of the A7075-T6511 aluminum alloy. (mass%)

Table 2 Plating conditions for the four electroless Ni–P plating types.

3. Results and Discussions

3.1 Status of plating film

In the rotating bending fatigue test, the highest bending stress occurs at the periphery, and cracks usually initiate at the periphery and propagate toward the center. Therefore, the surface morphology of the machined R part has a significant effect on the fatigue properties. A surface SEM image of R part of the specimen is shown in Fig. 1. Only the tool marks due to the cutting process were observed and no other scratches or swelling was found.

Fig. 1

Secondary electron images (SEIs) of the specimen surface at R part before plating at two magnifications. (a) ×30. (b) ×1k.

Figure 2 displays the SEM images of the surface of R part after electroless Ni–P plating. The morphologies of the plating films of low-, medium-, and high-phosphorus types are all smooth. The waviness characteristic of the electroless Ni–P plating was observed, and the effect of the different phosphorus contents in the films on the surface morphology was minor. However, the high phosphorus + PTFE composite plating (hereinafter referred to as Ni–P-PTFE), which was obtained by mixing fine PTFE particles in the high-phosphorus plating bath, had fine PTFE particles dispersed and co-deposited on the surface. This was very different from the plating film without the fine PTFE particles. Figure 3 depicts a cross-sectional image of the Ni–P-PTFE plating, where the PTFE particles are evenly dispersed in the coating.

Fig. 2

Surface SEIs of the four types of plated films. (a) Low P. (b) Medium P. (c) High P. (d) High P+PTFE.

Fig. 3

Cross-sectional SEI of the High P+PTFE film.

Table 3 presents the hardness values of the various electroless Ni–P plating films. The hardness was determined from the cross-sectional direction of the film, with a thickness of 20 µm, and measured using a micro-Vickers hardness tester (test load: 0.245 N, holding time: 10 s). The hardness of the low-phosphorus type is 670 HV, which is the hardest among the plating films used in the experiments. Those of the medium- and high-phosphorus types are 555 HV and 550 HV, respectively, with no significant differences between them. On the other hand, the Ni–P-PTFE is the softest among all the plating films with a 250 HV hardness. This is caused by the dispersion and co-deposition of soft PTFE particles in the film, as shown in Fig. 3.

Table 3 Vickers hardness of the four plating films.

3.2 Fatigue properties

Figure 4 shows the S–N curves obtained from the rotating bending fatigue tests of the unplated and various electroless Ni–P plated specimens, excluding that of the Ni–P-PTFE plated specimen. In the low stress range, the specimens did not break even after 10⁷ rotations; therefore, the test was terminated at that point, and the stress amplitude at that moment was used to evaluate the fatigue strength. The fatigue strength of the unplated specimens, which was calculated using the average value of the censored specimens, was 230 MPa. The tensile properties of the A7075 alloy are presented in Table 4. The tensile strength is 620 MPa, which is high, but the fatigue strength is only 37.1% of the tensile strength.

Fig. 4

Relation between stress amplitude (σ_a) and number of cycles to failure (N) for the four types of specimens.

Table 4 Mechanical properties of the A7075-T6511 aluminum alloy.

The fatigue strength of the A7075 alloy specimens with various types of Ni–P plating varies greatly depending on the phosphorus content in the plating film. The hardness of the Ni–P plating films also varies with the phosphorus content, but all of them are hard films exceeding 550 HV and have high toughness.¹⁴⁾ As previously mentioned, in the rotating bending fatigue test, the maximum stress occurs at the periphery, and the crack usually propagates from the starting point at the periphery, leading to fracture. Therefore, the fatigue strength was improved by applying a hard film on the surface to suppress crack initiation. The improvement in fatigue properties by electroless Ni–P plating on various aluminum alloys has been reported.⁴^–⁷⁾ The fatigue strength of the low-phosphorus Ni–P plated specimen is 270 MPa, which is approximately 40 MPa higher than that of the untreated specimen. Thus, the low-phosphorus plating film is effective in improving the fatigue strength of the A7075 alloy, which is consistent with previous reports.⁴^–⁷⁾ In contrast, the fatigue strength of the medium-phosphorus Ni–P plated specimen is only 190 MPa, which is approximately 40 MPa lower than that of the untreated specimen. This indicates that the medium-phosphorus Ni–P plating film has an adverse effect on the fatigue strength. In addition, the fatigue strength of the high-phosphorus Ni–P plated specimen is 40 MPa, which is much lower than that of the untreated specimen. Both the medium- and high-phosphorus Ni–P plated specimens are harder than the base A7075 alloy and are expected to have a higher fatigue strength similar to the low-phosphorus plated specimen.⁷⁾ However, it is speculated that the hydrogen absorbed in the substrate during plating is responsible for the decrease in fatigue strength in spite of the hard plating.⁹⁾ The effect of hydrogen embrittlement of the electroless Ni–P plating with different phosphorus contents on high-strength steels was investigated using three-point bending slow strain rate tensile tests.¹³⁾ It was found that the degree of hydrogen embrittlement of the low-phosphorus type was lower than that of the medium- and high-phosphorus types, which is consistent with the fatigue test results in this study.

Figure 5 illustrates the S–N curves of the untreated, high-phosphorus Ni–P plated, and Ni–P-PTFE plated specimens. The fatigue strength of the Ni–P-PTFE plated specimen is 160 MPa, which is 120 MPa higher than that of the high-phosphorus Ni–P plated specimen owing to the incorporation of the PTFE particles. Because PTFE is a resin material and much softer than the Ni–P plating film, the hardness of the plating film is reduced to less than half of that of the untreated film, as indicated in Table 3, when PTFE particles are composited in the plating film. Therefore, it is expected that incorporating the PTFE particles will have a negative effect on the fatigue strength; however, the opposite occurs. This can be explained as follows. It is well known that plating high-strength steel with zinc induces hydrogen embrittlement. However, it has been shown that fine silica particles can act as a hydrogen release path and suppress hydrogen embrittlement by compositing them into the zinc coating.¹⁵⁾ Similarly, in the Ni–P-PTFE composite plating, it is inferred that the composite PTFE particles act as a hydrogen release path and suppress the decrease in fatigue strength due to hydrogen.

Fig. 5

Relation between stress amplitude (σ_a) and number of cycles to failure (N) for the three types of specimens.

3.3 Fracture surface observation

It was found that the high-phosphorus Ni–P plating significantly reduced the fatigue strength. To clarify the cause of this reduction, SEM observation of the fracture surface after the fatigue test was conducted. Figure 6 shows the SEM images of the fracture surface at and near the initiation point for the high-phosphorus Ni–P and Ni–P-PTFE specimens with the highest number of cycles to fracture. In the fracture surface of the high-phosphorus Ni–P plated specimen, voids of several hundred nanometers are observed just below the plating film at the initiation point enclosed by the dashed line. In contrast, in the fracture surface of the Ni–P-PTFE plated specimen, no voids are found under the plating film at the initiation point. The fatigue strength of the high-phosphorus Ni–P plated specimen is significantly reduced to 40 MPa. It is inferred that the stress concentration due to void formation reduces the fatigue strength by more than 200 MPa.

Fig. 6

SEIs of fracture surface. (a) Low magnification image of the high P specimen tested at stress amplitude of 33.4 MPa and fractured at 1.28 × 10⁷ cycles, indicating crack initiation site, (b) Close-up view of the crack initiation site of (a), (c) Low magnification image of the high P + PTFE specimen tested at stress amplitude of 154.6 MPa and fractured at 4.33 × 10⁶ cycles, (d) Close-up view of the crack initiation site of (c).

In terms of void formation, the following hydrogen vacancy cluster theory has recently been proposed as one of the causes of hydrogen embrittlement.¹⁶^–¹⁸⁾ In a H₂ atmosphere, several vacancies trapping hydrogen are generated, and these hydrogen vacancy clusters move around in the metal substrate to form voids. Hence, hydrogen embrittlement, similar to ductile fracture, starts from the voids and leads to fracture. Regarding the voids observed at the initiation point of the high-phosphorus Ni–P plated specimen, based on the hydrogen vacancy cluster theory mentioned above, it is considered that the hydrogen absorbed in the substrate by the plating process forms voids and reduces the fatigue strength. There are many pores in the Ni–P-PTFE coating, which are caused by the loss of PTFE particles, resulting in a porous coating structure. Therefore, it is deduced that the hydrogen absorbed in the substrate by the plating is released and voids are not formed. Thus, the fatigue strength is improved despite the soft coating. The reason that the fatigue strength of the low-phosphorus specimen is higher than that of the untreated specimen is probably because the hydrogen diffusion coefficient of its coating is higher than those of the medium- and high-phosphorus types,¹⁹⁾ which prevents the formation of hydrogen voids at the initiation point and suppresses crack initiation by the hard film, as shown in Fig. 7. In the future, we will study the void formation in more detail.

Fig. 7

SEIs of fracture surface. (a) Low magnification image of the low P specimen tested at stress amplitude of 302.9 MPa and fractured at 6.64 × 10⁴ cycles, indicating crack initiation site, (b) Close-up view of the crack initiation site of (a).

4. Conclusion

The effects of various electroless Ni–P plating compositions on the fatigue properties of A7075-T6511 aluminum alloy were investigated. It was found that the fatigue strength is significantly different depending on the phosphorus content of the plating film. The fatigue strength of the low-phosphorus Ni–P plated specimen is approximately 40 MPa higher than that of the untreated specimen. This composition is therefore useful for improving the fatigue strength of A7075 alloy. In contrast, the fatigue strength of the high-phosphorus Ni–P plated specimen is much lower than that of the untreated specimen, and that of the medium-phosphorus Ni–P plated specimen is approximately 40 MPa lower than that of the untreated specimen. Based on the hydrogen vacancy cluster theory, it was inferred that the hydrogen absorbed during the plating process forms voids, and that the fatigue strength is reduced by the stress concentration originating from these voids. The fatigue strength of the high-phosphorus Ni–P plating composite with PTFE microparticles is much higher than that of the plating without PTFE microparticles because the formation of voids is suppressed by the PTFE microparticles in the coating, which act as a hydrogen release path.

Acknowledgment

This work was supported by the Light Metal Educational Foundation 2019 integrated advanced research.

REFERENCES

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