Effect of Injected Oxygen Amount on the Gas Porosity and Mechanical Properties of a Pore-Free Die-Cast Al–Si–Cu Alloy

Round 1

Reviewer 1 Report

(1) Fig. 6 has to be modified because the words in the figures are too smaller to read. 

(2) Figs. 9 and 10 show the microstructure of the center of the sample cross section, however, the microstructure near the cast surface is great different to the center in high pressure die castings. So, please show the microstructures in the center and near the cast surface, and compare them.

(3) The elements distribution in Fig. 10  is not clear especially for oxygen, So, please improve the figure quality,

(4) The description of weibull modulus is not clear. Please give more description and explanation.  

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

1) The authors have done a great deal of work on the study of the technique for reducing the porosity in the alloys under study, however, the scientific novelty of this work remains unclear.
2) I wonder where Pb came from in Table 1
3) What is the reason for the following observation? "On the other hand, under 1.68 L conditions, the defects slightly increased." it is logical to assume that the number of defects should decrease.
4) Please provide more details on how the DAS was measured.
5) Please demonstrate this conclusion more clearly. It is not observed in the figures. "However, unlike Figure 10 (a), in Figure 10 (b), both 2–3 μm and nanoscale oxides are dispersed throughout."
6) Please fill in the definition of the abbreviation column "SD" in table 5. There is no information about this in the text.
7) With the indicated deviation, most of the conclusions are not significant: "In particular, the best results (tensile strength, yield strength, and elongation = 331.98 Mpa, 246.27 Mpa, and 4.56%, respectively) were observed for the 1.26 L specimen." UTS 304 ± 78 differs significantly from 331 ± 50 And YS 239 ± 30 is not much different from 246 ± 25?
8) For the reason stated above, the conclusions require correction.
9) Please explain what is the practical meaning of reducing porosity if the mechanical properties remain within the error limits.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

The manuscript on the oxygen used in high pressure die casting of aluminum to eliminate porosity. This concept has been known and in the manuscript they have made trials to optimize the process parameters and to see the effect of oxygen and effect of heat treatment for pore free die casting. 

In the manuscript, other methods have been mentioned but the results of other methods and similar techniques used researches outcome have not been included. We would like to see the discussion with other findings. Discussion is a must to see the contribution of the study. 

There is one sentence that is questionable is " However, the heat treatment and weldability of such casting is limited. ???? It can be problematic due to having large porosities and their amount. 

No standard deviation is given in calculations/ observations in Tables.

It would be better to see the picture of the casting. Is there any blister formation on the surface of the specimen after T6 heat treatment.

In this concept, one of the other concept of bi-film formation has to be discussed?

It seems that there are microshrinkage in the SEM pictures. It should also be considered. 

In XRD card numbers should be given. Since there are some symbols given it is misleading to people that there are some phases such as Al2O3 but only Al and Si seems to be available. And, it is hard to detect with XRD if you do not have certain content in the structure.

I understand that, according to prediction made in the manuscript the phases are getting smaller in size (nanoscale) due to having oxide in the melt that may help for grain refining due to the oxide acting as nucleation sites. In Figure 10 a and b there is oxide in the structure and I assume that if the phases are getting in nanoscale why the increase in mechanical properties are small. Table 5 needed standard deviation as well.

In Figure 10 it is hard to say from EDS analysis that nanosize Al2O3 formed. EDS is not a technique we can proof that. Al is al over the specimen which we expect as well as O because of aluminum is oxidize in atmosphere and forms aluminum oxide. This Al2O3 only can be detected if you have XRD attachment in SEM. If you are sure that there are Al2O3 in the structure focus on the area (high mag needed) and do EDS and than make atomic calculation and assume again that it could be Al2O3. Or do advanced characterization.

What is the contribution of Al2Cu in the alloy after heat treatment? Is there a significant change in their size as in the case of Al2O3. What is the phase structure in Al2O3?

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

The manuscript presents one comprehensive and interesting research regarding pore free die casting process. It should be published and falls within the scope of the Metals journal. However, although the research is impressive, the written manuscript is somewhat lacking. I believe that the following concerns should be addressed prior to publication:

1. The manuscript should be proofread before publication. The readability of the text should be improved. An example (Abstract, line 14): „Pore-free (PF) die casting is another effective process which reduces gas porosity in castings.“ By stating that this is ANOTHER effective process implies that before mentioned welding and heat treatment are used to reduce gas porosity… I presume the authors' line of thought was that the pore-free die casting process enables welding and heat treatment of parts. But this is not stated, nor understood from the abstract (it is clear from the introduction, but the abstract should be able to stand alone). 
   Page 7, line 231 is another good example: “We predicted that with the increase in the injected oxygen amount, the oxygen substitution rate of the nitrogen present in the cavity and sleeve would also increase, resulting in a decrease in the internal nitrogen. However,…“
   Here „however“ implies that measured nitrogen levels were unexpected. But, manuscript continues: „… as the injected oxygen amount increased, the internal oxygen slightly increased. In particular, the internal oxygen measured at 1.68 L was approximately four times greater than that at 0 L.“ There is no mention of the nitrogen!
   The whole manuscript is filled with similar constructions. Some of them are mentioned later in the review, but not all of them. Thorough proof reading should be done.

2. Please check the validity of all references and whether they substantiate the statement made in the sentence they are mentioned. An example (Page 1, line 33): ”In particular, as the demand for eco-friendly and electric vehicles increases, welding and heat treatment are becoming very important to meet the weight reduction, complexity (integral molding), and high functionality of die castings[1–4]”. Ref. 1 deal with the influence of pores and primary silicon particles on fracture morphology of hypereutectic Al-Si alloys. It does not deal with welding, heat treatment, weight reduction, eco-friendly vehicles, integral molding, nor any other claim made in that sentence. Similar can be said for References 2 and 3 – they show that conventional HPDC parts have porosity and that it prevents heat treatment due to blistering. But, as currently written, the sentence should be substantiated with references to why heat treatment and welding are important. The sentence can be made without references since the statement is well known and obvious to all researchers and engineers that deal with HPDC. But if references are included, then they should be appropriate. This is just the first example, and similar referencing problems are found throughout the paper.
   The references are often given at the end of the paragraph. This is not an adequate method for making references, but even in this case, references are often inadequate. An example: the first paragraph of the second page deals with the influence of the vacuum level on the porosity of the castings. Not a single cited reference (9-11) deals with determining gas quantities in vacuum-assisted HPDC. Articles 10 and 11 do not even mention the word vacuum in the text, and ref. 9 only states the use of vacuum in the experimental setup, but investigates the influence of heat treatment, not porosity.
   Please check all the references, and make appropriate changes!
3. Page 2, paragraph 3: First sentence implies smooth-nonturbulent filling during HPDC if one uses vacuum. To my knowledge, the filling is always turbulent, and even atomized filling is often preferred. Vacuum only reduces air entrapment events since the mold atmosphere is rare. Please change the text by explaining your position more clearly, or remove all instances where the smooth filling is mentioned, including Fig. 1c. In my opinion, Fig. 1c would be more revealing if turbulent flow with entrained oxides is depicted, as a suggestion to authors to implement if they wish.
   In the following two sentences of this paragraph authors give oxide sizes and wt.%. Please provide references to these measurements.
4. Materials and methods: Please provide the total volume of the casting so that future researchers could use the data to express porosity as a % and more easily use these results to compare them with other investigations (in this paper the porosity is expressed as mm3 and this info is difficult to compare to other studies).
5. Materials and methods: Please state the total number of castings produced. Weibull analysis implies that there was a large number of castings (seems like at least 20 tensile tests, implying at least 5 castings per variation). Page 3, line 105 states: “A Micro Focus 3D CT system (Nikon, XTH320L) was employed to analyze the gas porosity of each specimen before and after heat treatment.” However, the discussion given in chapter 3 mentions values only of one casting (therefore only 4 tensile tests) for every variation in oxygen injection amount. I presume only one casting per variation has been CT scanned. If so, please correct the text to reflect this info (one cannot state that every specimen was measured). If not, please provide average values and standard deviations (or confidence intervals) of the measurements.
6. Page 3, Line 100: “Thus, the mold and sleeve volumes were calculated, and the injected oxygen amount was calculated at 0 L (0%), 0.42 L (50%), 0.84 L (100%), 1.26 L (150%), and 1.68 L (200%), based on the volume of the internal empty space” Please provide information whether the volume is related to the oxygen supplied only prior to molten metal pouring, or it is related to the supply of oxygen during the whole process up until the pouring hole is covered. Additionally, please provide the info regarding the purging time and whether some actions were taken to prevent most of the oxygen from exiting the sleeve through the pouring hole, and not through the mold cavity.
7. Table 2: Please change the sample designations to reflect designations used in the rest of the manuscript (0L, 0.42L, etc.). PF0, PF10, PF20, etc. are used only in this table.
8. Page 3, Line 111: “This analytical method, established by C. E. Ransley et al., creates a high vacuum inside the device…” Please provide reference for Ransley et. al methodology.
9. Page 5, line 171: “This increase was attributed to the effect on the vacuum in the cavity of the injected oxygen. Thus, gas entrapment during molten metal injection was very small.” Materials and Methods did not state the use of vacuum, although the vacuum is mentioned in the introduction when explaining the pore free die casting (Page 2, paragraph 3). If the vacuum was used, it should be stated. If authors imply that the increased oxygen in the atmosphere and reaction with molten metal was sufficient to create a vacuum, please provide evidence (measurements) or adequate references (or rephrase the text to note that this explanation is only an assumption and then please elaborate the mechanism of vacuum formation more thoroughly). The absence of detectable pores can be explained by the loss of volume of the entrained bubbles due to cooling and oxygen consumption through melt oxidation. But this mechanism does not require the existence of a vacuum inside the cavity.
10. Page 6, line 180: “472 mm2“. I presume that the unit should be mm3
11. Page 6, Line 207: “Thus, the gas in the product was 8.410 cc/10 0g, which was less than the 10 cc/100 g attained from welding and heat treatment.“ Does this imply that 100 cc/100g is from welded parts? Reference? If it is implying that it is the minimum value for successful welding and heat treatment, again a reference is needed, but also a rephrasing of the sentence.
12. Page 8, Line 254: „In particular, at 1.26 L injected oxygen, only micropores that could not be confirmed by CT analysis were confirmed.“ I did not understand the sentence. Please rephrase since I have not understood the meaning. I presume that not a single pore visible on the optical micrograph was detected by CT?
13. “This decrease in DAS was attributed to the effect of the injected oxygen on the vacuum inside the cavity” Please elaborate on this statement and provide some references. Why would applied vacuum cause a decrease in SDAS?
14. Page 10, explanation of Fig 10: Discussion of the results is not persuasive. Please rephrase. EDS mapping shown in Fig. 10b does suggest the presence of more oxygen than in Fig. 10a. However, it also shows more Fe, Mn, and Cu. It could be related to absolute count numbers (longer sampling times), or localized differences due to sample inhomogeneity. For example, I do not see a notable difference in peak intensities in XRD measurements. Although it is reasonable to think that PF castings are filled with dispersed oxides (due to the quick reaction between molten Al and O, and significant turbulence/atomization during HPDC), those oxides should be in nanometer scales (as written by the authors). I do not think that SEM/EDS is capable to detect such features. 
15. Page 11, discussion of the mechanical testing results: There should be a more thorough analysis of the results. The discussed differences in mechanical properties are not clearly statistically significant. Let us roughly calculate 95% confidence intervals (therefore Z value is 1.96) for data presented in table 5. The margin of error is ±ZxSD/sqrt (n). For as-cast samples, the elongation SD is roughly 1. Therefore, for 20 samples margin of error is 0.44, and for T6 treated castings margin of error is 1.3 (for SD = 3). Confidence intervals overlap even for elongation measurements! This does not prove that the difference is statistically insignificant, but data requires further analysis (for example Mann-Whitney U test).

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

1) the DAS measurement method is not entirely clear. the presented images do not show the difference that the authors talk about. please provide more detailed information to support your calculations.
2) Thank you for your additional measurement error calculations. however, the reported error is quite insignificant and all values ​​fall within the confidence interval for all measurements. in other words, there is no difference in mechanical properties and there is no practical point in applying your machining methods.

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

The analysis of the mechanical testing data is still somewhat lacking, but this is true for most of the manuscripts published in the field. Limited number of experiments do not allow the adequate statistical significance of the average UTS and elongation to be achieved However, Weibull plots do demonstrate the improvement of the PF route over conventional HPDC (although this could be further improved by including the confidence bounds in graphs). I would once again suggest the authors to improve the analysis, but I think that it can be left as is.

The manuscript can be accepted for publication after the following minor issues are addressed (mostly related to still inadequate references):

• The references were reorganized but are, in my opinion, still inadequate. As an example, I will analyze the first three sentences that have references in them.

(Page 1, line 33): ”In particular, as the demand for eco-friendly and electric vehicles increases, welding and heat treatment are becoming very important to meet the weight reduction, complexity (integral molding), and high functionality of die castings[1–3]”. Given references show that conventional HPDC parts have porosity and that it prevents heat treatment due to blistering (so they are appropriate to be used instead of references 6-8 that I mention later in this analysis), and that HPDC is used to make automotive parts. But, as currently written, the sentence should be substantiated with references to why heat treatment and welding are important. The sentence can be made without references since the statement is well known and obvious to all researchers and engineers that deal with HPDC. But if references are included, then they should be appropriate.  

Similar can be said for the next sentence that has citations: “HPDC has several advantages such as high productivity, smooth surface, and the ability to afford thin castings with accurate dimensions[4,5]” Neither Ref. 4 nor 5 give any information regarding comparison of HPDC with other processes. The statement is common knowledge and is not under dispute. However, if you use a reference it should be able to back up the claim given in the sentence it is referenced in.

Next sentence that has citations: “In particular, heat treatment by blistering on the surface of thin castings is very difficult[6–8].” Reference 6 and 7 can be used (barely) since they do mention blistering caused by heat treatment (although research dealing specifically with blistering would be better). Reference 8 is dealing neither with blistering nor heat treatment. In the whole referenced manuscript neither term is not even mentioned! It shows that porosity reduces mechanical properties, but that is not the claim of the sentence it was used in.  

Please recheck all references and make appropriate changes! 

• There is an error in Ref. 20. Please change 0 to o in S0lid, and 1 to l in A1uminum. Additionally, pagination is not 682-699, but 599-620.
• Please change Materials and methods to reflect number of measurements stated in your response to previous review. You have stated only one sample per variation for CT, microstructure and XRD analyses. The manuscript still states: A Micro Focus 3D CT system (Nikon, XTH320L) was employed to analyze the gas porosity of EACH SPECIMEN before and after heat treatment.“; If I understood the response correctly, the analysis was performed on only one casting per variation, not on all castings. Or, you should state obtained standard deviations in the Results and discussion section.
• Page 11, line 305: “DAS showed no significant difference between 7 and 9 μm in all conditions”. Maximum DAS was 8.73 rounded correctly to 9. Minimum DAS was 7.63 and rounded incorrectly to 7, it should be 8.
• In several places Mpa should be changed to MPa.

Author Response

1. The references were reorganized but are, in my opinion, still inadequate. As an example, I will analyze the first three sentences that have references in them.

(Page 1, line 33): ”In particular, as the demand for eco-friendly and electric vehicles increases, welding and heat treatment are becoming very important to meet the weight reduction, complexity (integral molding), and high functionality of die castings[1–3]”. Given references show that conventional HPDC parts have porosity and that it prevents heat treatment due to blistering (so they are appropriate to be used instead of references 6-8 that I mention later in this analysis), and that HPDC is used to make automotive parts. But, as currently written, the sentence should be substantiated with references to why heat treatment and welding are important. The sentence can be made without references since the statement is well known and obvious to all researchers and engineers that deal with HPDC. But if references are included, then they should be appropriate.  

Similar can be said for the next sentence that has citations: “HPDC has several advantages such as high productivity, smooth surface, and the ability to afford thin castings with accurate dimensions[4,5]” Neither Ref. 4 nor 5 give any information regarding comparison of HPDC with other processes. The statement is common knowledge and is not under dispute. However, if you use a reference it should be able to back up the claim given in the sentence it is referenced in.

Next sentence that has citations: “In particular, heat treatment by blistering on the surface of thin castings is very difficult[6–8].” Reference 6 and 7 can be used (barely) since they do mention blistering caused by heat treatment (although research dealing specifically with blistering would be better). Reference 8 is dealing neither with blistering nor heat treatment. In the whole referenced manuscript neither term is not even mentioned! It shows that porosity reduces mechanical properties, but that is not the claim of the sentence it was used in.  

Please recheck all references and make appropriate changes! 

Response: The references has been reorganized in its entirety.

2. There is an error in Ref. 20. Please change 0 to o in S0lid, and 1 to l in A1uminum. Additionally, pagination is not 682-699, but 599-620.

Response: The references has been revised.

3. Please change Materials and methods to reflect number of measurements stated in your response to previous review. You have stated only one sample per variation for CT, microstructure and XRD analyses. The manuscript still states: A Micro Focus 3D CT system (Nikon, XTH320L) was employed to analyze the gas porosity of EACH SPECIMEN before and after heat treatment.“; If I understood the response correctly, the analysis was performed on only one casting per variation, not on all castings. Or, you should state obtained standard deviations in the Results and discussion section.

Response: I revised the contents.

4. Page 11, line 305: “DAS showed no significant difference between 7 and 9 μm in all conditions”. Maximum DAS was 8.73 rounded correctly to 9. Minimum DAS was 7.63 and rounded incorrectly to 7, it should be 8.

Response: I revised the sentence.

5. In several places Mpa should be changed to MPa.

Response: I revised the contents.

Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

1) I have no idea how to measure DAS in the presented figure 9, but we will leave this remark on the conscience of the authors.
2) in the presented version of the article in Figure 10, it is required to correct the quality of the dimension mark.
3) usually, the parameters of hardness and standard deviation are indicated in whole numbers in this order of magnitude

Author Response

1) I have no idea how to measure DAS in the presented figure 9, but we will leave this remark on the conscience of the authors.

Response: Thank you for the comment.

2) in the presented version of the article in Figure 10, it is required to correct the quality of the dimension mark.

Response: Thank you for the comment. I revised the contents.

3) usually, the parameters of hardness and standard deviation are indicated in whole numbers in this order of magnitude

Response: Thank you for the comment. I revised the contents.