Evaluation of pore re-opening after HIP in LPBF Ti–6Al–4V

Introduction

Additive manufacturing (AM) of titanium Ti–6Al–4V (Ti64) parts has many potential benefits over conventional processing routes that can save costs. However, the overall cost related to AM limits the use of this technology in some applications. Owing to the high cost of machine hours, faster processing in combination with hot isostatic pressing (HIP) post-process has been proposed to reduce the part building time and consequently the cost-per-part. This approach could have cost benefits especially for industries where HIP is performed as a risk-mitigation measure regardless of starting pore content.

HIP is often performed as a post-processing step for AM Ti64 parts to improve material properties for demanding applications such as medical implants and aerospace components. HIP is a process, in which components are exposed to a high temperature (up to 2000°C) and isostatic gas pressure (up to 200 MPa) simultaneously. This enables densification of the material and modification of the microstructure.

HIP effectively reduces the amount and size of irregular pores and hence improves the properties of the final components [1]. Pores occurring in laser powder bed fusion (LPBF), such as round keyhole and irregular lack-of-fusion (LoF) pores, can be closed during HIP. This has a positive effect on mechanical properties, especially on the fatigue properties of the material [2]. Porosity level of LPBF Ti64 material is generally well below 1% already in as-manufactured condition. In terms of HIP:ing, this means that the material is already in the last stage of densification and predominant material transport mechanism is solid-state diffusion. Plastic yielding and creep have only a minor effect on densification [3].

To achieve shorter build times, it is usually necessary to alter the LPBF process parameters (for example the layer thickness, scanning speed, or hatch distance) to allow faster processing, and this often results in increased pore content. As LPBF processes run under inert gas atmosphere, the pores formed in the microstructure can contain entrapped inert gas and unmelted particles. The most commonly used inert gas is argon (Ar), which is insoluble in titanium [4]. Thus, Ar could remain trapped in the solid material in the high pressure and temperature applied during HIP, even if compressed in volume. Depending on the possible heat treatments that follow HIP, and the service temperature of the final part, this entrapped gas can expand at elevated temperature and re-open the seemingly closed porosity.

Although some literature on the pore re-opening exists, the risks with faster processing in combination with HIP post-process are not fully understood. In electron beam melting (EBM), porosity has been successfully healed during HIP, but has reappeared after exposure to elevated temperatures [5–7]. Similar trends have been reported also for LPBF processes [8,9].

The aim of the present study was to investigate the changes in pore content and morphology in HIP:ed LPBF Ti–6Al–4V parts after exposure to different elevated temperature aging treatments. The LPBF Ti64 parts were produced using a high productivity process. Industrial standard HIP-densification treatment for Ti64 alloy is performed with parameters 920°C, 100 MPa, 2 h. This treatment was, however, originally developed for densification of Ti64 castings. As the microstructure of rapidly solidified LPBF Ti64 material differs considerably from a cast material, optimised HIP parameters (820°C, 140 MPa, 2 h) are beneficial for the material properties. The lower temperature of the optimised HIP parameters prevents the coarsening of the original fine microstructure of LPBF material. The resulting fine α/β -lamellar microstructure provides excellent static and dynamic mechanical properties [10]. The experimental part of this study includes both standard and optimised HIP parameters. Additionally, the gas content of the samples was analysed to improve the understanding of the inert gas behaviour in the application parts.

Experimental

Post-treatment

After manufacturing, the samples were divided into two HIP treatments: (i) industry standard parameters and (ii) optimised HIP parameters [10]. The HIP treatments were followed by aging the samples at elevated temperatures for varying times, as shown in Table 1. Lower aging temperature of 400°C was chosen to represent the maximum service temperature of Ti64 alloy, whereas the higher temperature of 900°C was chosen more according to research aspects. Analysis of the samples was performed after each processing step (manufacturing, HIP and aging treatments). The samples were coated with anti-scaling coating (ATP-707) to prevent excessive oxidation of samples during the aging treatments. After aging treatments, the coating was scale-like and it mostly peeled off during preparation of the samples. Loose coating residues were removed.

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Table 1

Summary of the processing steps for the HIP samples.

Reference	HIP treatments	Aging treatments
As manufactured	(i) 920°C, 100 MPa, 2 h (ii) 820°C, 140 MPa, 2 h	HIP (i) + 400°C for 81 h HIP (i) + 900°C for 81 h HIP (ii) + 400°C for 81 h HIP (ii) + 900°C for 81 h

Additionally, changes in pore content were studied for samples in heat-treated condition (HT), to establish the baseline for the pore re-growth behaviour at elevated temperatures (aging). The heat treatment was performed at 800°C for 2 h followed by same aging treatments as the HIP:ed samples, as shown in Table 2. The samples were coated similarly to the HIP samples before aging treatments. After each processing step (manufacturing, HT and aging treatments), the samples were analysed.

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Table 2

Summary of the processing steps for heat-treated samples.

Reference	Heat treatments	Aging treatments
As manufactured	800°C, 2 h	HT + 400 °C for 81 h HT + 900 °C for 81 h

Results and discussion

After manufacturing, the areal pore percentage of the samples was in the range of 0.01–0.03%, maximum observed pore diameter 51–271 µm and pore count approximately 180–370 (pcs) in OM using the image-based analysis method. Density of the material was measured to be 4.42 g cm⁻³ corresponding to ∼99.8% density [12]. Heat treatment (800°C, 2 h) did not have an effect on the measured pore content or density. HIP:ing effectively reduced the areal pore content to 0.001%, maximum pore size to <30 µm and pore count of the samples to <100 pcs for both used HIP parameters. The measured densities were in the range of 4.42–4.43 g cm⁻³ for all the samples.

Microstructures of the samples after each processing step for standard HIP test series are presented in Figure 1. In as-manufactured condition, the material consists of metastable martensitic α’-phase formed during rapid cooling in LPBF process (Figure 1(a)). In the heat treatment, martensite decomposes to a fine lamellar α/β-structure (Figure 1(b)). Same phase transformation from α’-martensite to α/β-structure takes place also during HIP:ing (Figure 1(c)). During aging treatments, some coarsening of the microstructure was observed (Figure 1(d,e)). However, microstructure remains lamellar α/β even after high-temperature aging treatment (900°C, 81 h).

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Figure 1

Microstructure of LBPF Ti64 material after different post- and aging treatments: (a) as-manufactured, (b) heat treated (800°C, 2 h), (c) HIP:ed (920°C, 100 MPa, 2 h), (d) HIP:ed (920°C, 100 MPa, 2 h) + Aged (400°C, 81 h), (e) HIP:ed (920°C, 100 MPa, 2 h) + Aged (900°C, 81 h).

Figure 2 shows the results for the image pore classification (in three classes: spherical pore ≥ 15 µm, irregular lack-of-fusion LoF ≥ 15 µm and <15 µm pore) for the heat-treated samples after each processing step. There is slight increase of the smallest <15 µm pores during heat treatment. Presumably very small pores with size under detection limit expand during heat treatment, because of expansion of the Argon gas inside them like reported also in Reference [5]. For the samples aged at 400 and 900°C for 81 h, the pore content decreases with especially irregular pore content decreasing significantly for the higher temperature aging. This is possibly explained by solid-state sintering of the material during the relatively long exposures to elevated temperature.

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Figure 2

Image pore classification results for the heat-treated samples after each processing step.

Image pore classification results for the HIP:ed samples after each processing step are shown in Figure 3. Both HIP treatments effectively reduced pore content of the samples, with especially spherical pore content decreasing remarkably during HIP:ing. Aging treatment of 81 h at 400°C does not seem to cause any remarkable pore re-growth. In contrast, the overall pore amount seems to further decrease. In this case, the decrease is not likely caused by sintering, as the samples have been already exposed to HIP treatment. This phenomenon requires more research. For the higher temperature (900°C) aging treatment of 81 h, a slight increase in the pore content was observed compared to HIP:ed condition. Especially small (<15 µm) pores start to re-grow or emerge.

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Figure 3

Image pore classification results for the HIP:ed samples after each processing step for: (a) standard HIP parameters, and (b) optimised HIP parameters.

SEM images of the typical pore types found in the samples after each processing step are collected in Figure 4. In as-manufactured condition, (Figures 4(a,b)) sharp corners were found from LoF-type pores. These sharp corners can serve as initiation points for fatigue cracks. Some minor rounding of pore corners occurs during heat treatment (Figure 4(c,d)). During aging, rounding continues and LoF pores transform to more oval shape (Figure 4(e,f)). Some so-called faceting was also observed in inner surfaces of pores. Facets form during high-temperature aging most likely as the surfaces start to grow inwards towards certain low energy lattice directions. After HIP:ing, only a few pores can be found (Figure 4(g,h)). Pores are very small and transformed to a rounded or circular shape. After HIP:ing and low-temperature aging (Figure 4(i,j)), only a few rounded defects can be found. Remaining pores are predominantly small <5 µm circular pores. After HIP:ing and high-temperature aging, pore size grows, but remain rounded in shape (Figure 4(k,l)). No cracking or formation of new sharp corners was found after high-temperature aging. Faceting effect was clearly observable.

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Figure 4

SEM images of typical pores found on the samples after each processing step. In as-manufactured condition (a) pore, diameter 16 µm, and (b) lack-of-fusion, max diameter 16 µm. After heat treatment (800°C, 2 h), (c) pore, diameter 3 µm, and (d) lack-of-fusion, max diameter 38 µm. After heat treatment (800°C, 2 h) and aging (900°C, 81 h) (e) pore, diameter 6 µm, and (f) irregular pore, size 5 × 8 µm. After HIP:ing (920°C, 100 MPa, 2 h) (g) angular pore, max diameter 3 µm, and (h) pore, max diameter 1,2 µm. After HIP:ing (920°C, 100 MPa, 2 h) and aging (400°C, 81 h) (i) irregular pore, max diameter 4 µm, and (j) pore, diameter 1,3 µm. After HIP:ing (920°C, 100 MPa, 2 h) and aging (900°C, 81 h) (k) pores, max diameter 20 µm, and (l) pore, max diameter 11 µm.

Figure 5 shows a graphical representation of all pores observed in the µ-CT as seen in a single plane, with the dashed line showing the border of the part. The µ-CT results confirm that pores were effectively closed by HIP using both standard and optimised parameters. Pore re-growth was minimal in all samples both after exposure to 400 and 900°C for 81 h. Especially after exposure to 400°C (Figure 5(a,c)), the pores seem to be the same as observed before the aging treatment. Clustered pores at the or near the centre of the sample are most likely artefacts caused by the µ-CT on the axis of rotation of the scanned samples.

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Figure 5

µ-CT results in XY plane in different colours after each processing step for (a) standard HIP + Age at 400°C for 81 h, (b) standard HIP + Age at 900°C for 81 h (c) optimised HIP + Age at 400°C for 81 h, (d) optimised HIP + Age at 900°C for 81 h. Blob sizes are in relation to measured pore volume.

According to the test results, it seems that there is some difference in pore re-growth behaviour between the two HIP treatments studied. However, this conclusion cannot be made based on the current results, but further research would be needed. In the performed tests, comparability inside one test series is specially emphasised and comparability between different test series has been secondary. Results show mainly the trends in porosity behaviour during HIP and aging treatments. It is possible that differences in HIP parameters between the two HIP treatments can induce differences in pore re-growth behaviour. The difference of 100°C in HIP-temperature is remarkable in terms of diffusion-driven material densification.

Gas analysis results are shown in Figure 6 as average of three measurements after each processing step. According to the measurements, Ar content seems to increase during HIP:ing. This is potentially caused by Ar-gas penetration into the surface area of the material and pores just below the surface. Similar results of Argon surface pick-up during HIPping have been reported earlier for LPBF Ni-alloys [8,13]. In these studies, the effect is explained by oxidation of surfaces during HIP caused by impurities in pressurising Ar-gas. Oxidation together with surface deformation during HIP induces more and more oxidation and eventually entrapment of Ar-gas into the oxidised surface. Longer HIP-cycle time and higher surface roughness also increased the Ar-pick-up. This is possible explanation for the results of this study also. Additionally, as indicated theoretically in reference [14], Ar-gas in high temperature and pressure can possibly slowly diffuse through grain boundaries and lattice defects. During low-temperature aging (400°C for 81 h) Ar-level does not change significantly. However, during high-temperature aging (900°C for 81 h) Ar-level drops back near to as-manufactured level (0.12 ppm). One explanation for this behaviour can be the higher amount of surface material scaling off after higher temperature aging. In addition, the Ar content in all the processed samples were equal or higher compared to the as-manufactured reference. This suggests that the Ar-gas does not diffuse out of the bulk material during HIP:ing or during aging treatments.

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Figure 6

Averages of measured argon (Ar) contents of the samples after each processing step.

Results of the second gas analysis test series in Figure 7(b) show that more area exposed to Ar-gas during HIP:ing resulted in higher Ar content of the material, which is in line with other studies [13]. After removal of 0.35 mm from the surface of the sample, Ar-level decreased back at the level of the as-manufactured material (0.12 ppm). After aging treatment, Ar-level decreases from that of only HIP:ed material to a level of 0.20–0.40 ppm. It seems that formation and cracking of thin alpha case/surface oxide layer promotes removal of HIP-induced Ar from the material surface. Additionally, these results indicate that Ar-gas from the bulk of the material cannot diffuse out during HIP:ing or during aging treatments, as indicated also in referred study [13].

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Figure 7

(a) Surface modifications used for samples for the second test series of Ar content analysis. (b) Averages of Argon content results for HIP:ed and HIP:ed + aged samples after different surface modifications.

Conclusions

Compared to as-manufactured and EOS standard post-process heat treatment for LPBF Ti–6Al–4V material (2 h at 800°C, ambient pressure), Hot Isostatic Pressing (HIP) effectively decreases pore content. Based on the results, it was concluded that for LPBF Ti64 material, pore re-growth after HIP:ing was minimal:

Based on OM and µ-CT measurements, aging treatment of samples at 400°C for 81 h does not increase pore content after HIP:ing.

At moderate starting pore levels, pore re-growth was not seen be a problem at typical service temperatures of Ti64 alloy (<400°C) or during normal heat treatment after HIP (<<81 h at 900°C).

Pore morphology was seen to change during HIP:ing generally towards more rounded shape. Sharp edges of irregular LoF pores smoothened or changed to angular oval shape. Oval pores shrank and changed their shape towards spherical. These morphology changes continued during high-temperature aging treatments. No cracks or new sharp-edged pores were observed after aging treatments. Only minor changes in pore morphology occurred during standard heat treatment.

It was concluded that Ar-gas entrapped in the bulk of the material during LPBF process cannot escape from the material during HIP:ing or during subsequent aging treatments, as the Ar content does not drop below that of as-manufactured material. Ar content of the material was seen to increase during HIP:ing, possibly caused by surface oxide induced Ar-gas entrapping effect.