Effect of Fe Addition on Heat-Resistant Aluminum Alloys Produced by Selective Laser Melting

Round 1

Reviewer 1 Report

This work addresses the effect of Fe addition on Al alloys produced by SLM. It is relatively an interesting work and can be considered for publication after addressing the following comments:

-          The quality and resolution of the images are not good. Please improve all of them.

-          Please add units to your equation 1 and explain it much better and clear. Please use this work as reference in this regard: International Journal of Machine Tools and Manufacture 133 (2018) 85-102.

-          Please compare your tensile properties with the previous works.

-          Please explain further why ductility of the samples are different.

Author Response

Dear Reviewer

We appreciate to your quick response and the precise comments. We have done revisions according to your comments. The revised points are summarized below. Our responses are described in bold. Changed parts are shown in red in the manuscript.

1.        The quality and resolution of the images are not good. Please improve all of them.

I improved the resolution of all figures.

2.        Please add units to your equation 1 and explain it much better and clear. Please use this work as reference in this regard: International Journal of Machine Tools and Manufacture 133 (2018) 85-102.

We thank you for your precise suggestions. I added the paper introduced by you as a reference [24] and added the following explanation on line 73 regarding equation 1.

The energy density is the amount of energy of the irradiation beam per unit volume applied to the powder material, and is an important condition for producing a sound SLM body. Also, preheating of the substrate is also necessary to suppress destruction of the SLM body during SLM. The conditions summarized in Table 2 are values found to be optimal from prior examinations for producing the SLM bodies of the present alloy.

3.        Please compare your tensile properties with the previous works.

Table 3 was added for a comparison between this study and previous works. In addition, the following sentences are added to line 140 as a comparison using Table 3.

The UTS and the elongation to fracture (EL) at 300°C of the sample prepared in this study and the novel cast Al alloys recently reported [25-28] were summarized in Table 3. The UTS at 300°C of the Fe added SLM alloys is comparable to that of novel cast Al alloys whose strength is improved by the addition of Sr, Ti, Zr, V. However, it should be noted that UTS at high temperature depends on the strain rate. Since the strain rate in this study is larger than the literature value, it is considered that UTS is evaluated somewhat higher. The elongation to fracture of the SLM-annealed Base + 3Fe and Base + 5Fe alloy at 300°C were 20% and 24%, respectively, which were larger than that of the cast-T6 Base alloy or other novel cast Al alloys [25,26].

4.        Please explain further why ductility of the samples are different.

The following explanation was added on line 193 about the difference in ductility between cast and SLM materials.

This is also clear from that the UTS of the Base alloys produced by SLM and by cast are almost same in annealed condition. The ductility of the SLM-T6 Base alloy was improved compared with the cast Base alloys, though both alloys contain coarse Si particles. The Si particles in the SLM material that exhibited good ductility had an equiaxed shape, whereas that of the cast material had a Chinese script shape or rod shape. Therefore, it is considered that the equiaxial shape of the Si particles contributed to the improvement of the ductility of the SLM material.

Reviewer 2 Report

This is an interesting work that fits well within the scope of this Journal. New data are presented, whereas the experimental approach used was solid. However, to my point of view, there are some points that need to be addressed prior to publication. My comments/suggestions are given hereafter:

1. In line 37, you mention that the mechanical properties at higher temperatures will help to improve the efficiency and decrease the emissions of automotive engines. To the best of my knowledge, this is dependent on the friction and wear of the system, which are purely surface and not bulk properties.  

2. In lines 52-53, you clearly mention the research aim of your work. However, you should also mention what is the added value of this work. For example is it intended for a specific application/component or it is aiming in a more fundamental research on this material and process?

3. Within the text you should mention that the ‘%’ is ‘wt.%’ (it is only mentioned in Table 1).

4. Al-Si alloys are typically considered as non-heat treatable alloys. Why did you perform a hardening T6 treatment?

5. In figure 2, I noticed that after the T6 process you performed a long annealing (?) treatment at 300 °C for 10 hours. What is the reason for this? The reason why I am asking this is because the maximum precipitation hardening is achieved after T6 treatment. A post heat treatment can result in a loss of mechanical strength due to increase in the size of precipitates and grain growth.

6. Can you please increase the resolution of figures 2 and 3?

7. How many tests did you perform per material? If you performed multiple tests, then consider adding a table in figure 3, to summarize the result values and show their spread.

8. From figure 2b it is interesting that the UTS is significantly lower. In this work you attribute these differences to the Si cell structures. What about the effect of precipitate/intermetallic size and distribution and grain size? They also can have a significant effect on the mechanical properties.

Author Response

Dear Reviewer

We appreciate to your quick response and the precise comments. We have done revisions according to your comments. The revised points are summarized below. Our responses are described in bold. Changed parts are shown in red in the manuscript.

1.        In line 37, you mention that the mechanical properties at higher temperatures will help to improve the efficiency and decrease the emissions of automotive engines. To the best of my knowledge, this is dependent on the friction and wear of the system, which are purely surface and not bulk properties.

As you pointed out, wear resistance is an important mechanical property for piston materials. In addition, the pistons of engines that use relatively high speeds, such as automobiles, need to withstand the high pressures of combustion gases while saving as much material as possible to reduce inertia. Therefore, high temperature strength and creep strength are required as bulk properties for piston materials. The following sentence is added to line 34 to explain this.

An important mechanical property for engine piston materials that are required to be lightweight to reduce inertia is high temperature strength to withstand high pressure from combustion gases, even for small parts, as well as wear resistance.

2.        In lines 52-53, you clearly mention the research aim of your work. However, you should also mention what is the added value of this work. For example is it intended for a specific application/component or it is aiming in a more fundamental research on this material and process?

This study is a basic study to improve high temperature strength by Fe addition by applying the rapid solidification of SLM. However, since the temperature of the heat treatment and tensile test of the prepared material (i.e. 300°C) were set considering to be applied to the engine piston materials. So, the following sentence was added to line 56.

on the conditions assuming application to engine piston material.

3.        Within the text you should mention that the ‘%’ is ‘wt.%’ (it is only mentioned in Table 1).

The following description has been added to line 61.

(in this paper, all percentages for the chemical composition are wt.% unless otherwise stated)

4.        Al-Si alloys are typically considered as non-heat treatable alloys. Why did you perform a hardening T6 treatment?

In AlSi-12CuNiMg (AC8A), which is the Base alloy in this work, Cu and Mg are added at 1%, so T6 treatment causes age hardening. When this alloy is practically applied to piston materials, it is used after T6 treatment. Therefore, we prepared a T6-treated sample for comparison with conventional materials. This statement was added to line 84 to clarify the intention of T6 heat treatment.

In AlSi-12CuNiMg (AC8A), which is the Base alloy in this work, Cu and Mg are added at 1%, so T6 treatment causes age hardening. When this alloy is practically applied to piston materials, it is used after T6 treatment. Therefore, we prepared a T6-treated sample for comparison with conventional materials.

5.        In figure 2, I noticed that after the T6 process you performed a long annealing (?) treatment at 300 °C for 10 hours. What is the reason for this? The reason why I am asking this is because the maximum precipitation hardening is achieved after T6 treatment. A post heat treatment can result in a loss of mechanical strength due to increase in the size of precipitates and grain growth.

As mentioned before, the material we developed assumes application to piston materials that are used for a long time at high temperatures around 300°C. Therefore, the microstructure is required to be stable at the temperature used. As you pointed out, annealing after T6 causes mechanical strength to decrease due to microstructural changes. Annealing at 300°C is intended to prevent the change in strength during use at high temperature by causing this microstructural change in advance.

In addition, SLM materials are rapidly solidified, so they are extremely supersaturated solid solutions in the as-built state. Elements dissolved in supersaturation precipitate during high temperature holding, causing dimensional change of the SLM body. For engine pistons, such dimensional changes cause serious problems. Therefore, suppression of the dimensional change is also the purposes of 300°C annealing.

To clarify our intention of 300°C annealing, the following sentence was added to line 87.

A piston material that is used at a high temperature of about 300°C for a long time is required to have a stable microstructure at the temperature used. In addition, the rapidly solidified SLM material is a highly supersaturated solid solution (SSSS) in the as build state. Elements dissolved in supersaturation precipitate during high temperature holding, causing dimensional change of the SLM body. In the case of engine pistons, this causes serious problems. Therefore, Annealing was performed at 300°C in order to make the strength change and the dimensional change less likely to occur when used for a long time at high temperature by causing precipitation from a SSSS and growth of precipitates in advance.

6.        Can you please increase the resolution of figures 2 and 3?

I improved the resolution of all figures.

7.        How many tests did you perform per material? If you performed multiple tests, then consider adding a table in figure 3, to summarize the result values and show their spread.

Each tensile test was performed only once. Therefore, we can not show the spread or repeatability of the data.

8.        From figure 2b it is interesting that the UTS is significantly lower. In this work you attribute these differences to the Si cell structures. What about the effect of precipitate/intermetallic size and distribution and grain size? They also can have a significant effect on the mechanical properties.

I think that this reviewer's comment means about Figure 3(b). The following responses are given for each of the intermetallic particles and the grain size.

Intermetallic particles

In this work, when T6 treatment is applied to the SLM material, the high temperature strength is reduced. As described in lines 190 to 205, this strength reduction is due to the coarsening of intermetallic compound particles rather than the Si cell structure during heat treatment at 510°C-2 h. Therefore, as you point out, the size of the intermetallic compound in SLM-annealed +3Fe, +5Fe and its spatial distribution are very important to understand the high temperature mechanical properties of our alloy. As mentioned at the end of the text, we are currently working on this point and will present in a separate paper in the future.

Grain size

The grain size of SLM materials is about 20 μm regardless of the heat treatment conditions. Therefore, the decrease in strength of the SLM material due to T6 heat treatment is not due to the change in grain size. The following description about grain size is added to Materials and Methods (line 110).

The grain size of the SLM material after heat treatment was about 20 μm regardless of the heat treatment conditions.

Round 2

Reviewer 1 Report

The revised version is suitable for publication.