Characteristics of Al-Si Alloys with High Melting Point Elements for High Pressure Die Casting

materials

Article Characteristics of Al-Si Alloys with High Melting Point Elements for High Pressure Die Casting

Tomasz Szymczak 1,* , Grzegorz Gumienny 1,* , Leszek Klimek 2 , Marcin Goły 3 , Jan Szymszal 4 and Tadeusz Pacyniak 1

1 Department of Materials Engineering and Production Systems, Lodz University of Technology, 90-924 Lodz, Poland; [email protected] 2 Institute of Materials Science and Engineering, Lodz University of Technology, 90-924 Lodz, Poland; [email protected] 3 Department of Physical & Powder Metallurgy, AGH University of Science and Technology, 30-059 Krakow, Poland; [email protected] 4 Department of Technical Sciences and Management, University of Occupational Safety Management in Katowice, 40-007 Katowice, Poland; [email protected] * Correspondence: [email protected] (T.S.); [email protected] (G.G.); Tel.: +48-426312276 (T.S.); +48-426312264 (G.G.) Received: 9 October 2020; Accepted: 29 October 2020; Published: 29 October 2020

Abstract: This paper is devoted to the possibility of increasing the mechanical properties (tensile strength, yield strength, elongation and hardness) of high pressure die casting (HPDC) hypoeutectic Al-Si alloys by high melting point elements: chromium, molybdenum, vanadium and tungsten. EN AC-46000 alloy was used as a base alloy. The paper presents the eﬀect of Cr, Mo, V and W on the crystallization process and the microstructure of HPDC aluminum alloy as well as an alloy from the shell mold. Thermal and derivative analysis was used to study the crystallization process. The possibility of increasing the mechanical properties of HPDC hypoeutectic alloy by addition of high-melting point elements has been demonstrated.

Keywords: crystallization; microstructure; mechanical properties; thermal and derivative analysis

1. Introduction High melting point elements, such as Cr, Mo, V and W are rarely used as additions in Al-Si alloys. Research papers described two main reasons for using these additions in Al-Si alloys. The first reason is the enhancement of the precipitation hardening effect [1–4] while the second one is a reduction of the detrimental effect of iron on mechanical properties [4–9]. However, the use of Cr, Mo, V and W in Al-Si alloys is very limited considering the nature of their interaction with the main alloy constituents, i.e., Al and Si. The phase diagrams Al-Cr [10,11], Al-Mo [12], Al-V [11] and Al-W [11] show the lack of solubility or very limited solubility of high-melting point elements in aluminum. According to [10], Cr is not soluble in Al, while according to [11], the solubility of Cr in Al is negligible and amounts to 0.71 wt % at 661.5 ◦C. Tungsten is not soluble in Al [11]. Similar behavior shows molybdenum [11,12], while the solubility of vanadium in aluminum is maximum 0.6 wt % (0.3 at %) [11]. Very limited solubility of the above-mentioned elements in aluminum causes the precipitation of a number of intermetallic phases. Also, Si-V [13], Cr-Si [11], Mo-Si [14] and W-Si [15] phase diagrams show that the tested elements practically do not dissolve in silicon. In accordance with these phase diagrams, Cr, Mo, V and W also tend to form numerous intermetallic phases with silicon. For example, four intermetallic phases can occur in the Si-V diagram, i.e., SiV3, Si3V5, Si5V6 and Si2V[13]. On the other hand, the analyzed elements offer excellent mutual solid-state solubility. Cr-Mo, Cr-V, Mo-V, Mo-W and V-W

Materials 2020, 13, 4861; doi:10.3390/ma13214861 www.mdpi.com/journal/materials Materials 2020, 13, 4861 2 of 13 phase diagrams presented in [11,16–19] show that they form unlimited solutions. In [20], Cr-W phase diagram has been described. It shows that Cr and W form a solid solution (αCr, W) with unlimited mutual solubility of both elements. At 1677 ◦C, it decomposes into two solid solutions (α1)-rich in chromium and (α2)-rich in tungsten. The presented data show that in Al-Si alloys containing Cr, Mo, V and W, intermetallic phases with Al and/or Si can crystallize. In multicomponent Al-Si alloys, the possibility of the formation of more complex phases containing other constituents, like Cu, Mg or Ni should be taken into account. Intermetallic phases can significantly increase the brittleness of Al alloys and reduce their strength and elongation. The risk of the precipitation of intermetallic phases in aluminum alloys containing Cr, Mo, V and W increases with the decreasing rate of heat transfer from the casting. Therefore, these additives are not applicable to Al-Si alloys produced in sand and ceramic molds. Additionally, in the case of hypoeutectic alloys, the formation of intermetallic phases in a higher temperature than the temperature of α(Al) phase crystallization changes the additives concentration ahead of the dendritic crystallization front. As a consequence of the high heat transfer during solidification in metal molds, the α(Al) phase can be supersaturated with high-melting point elements. High pressure die casting (HPDC) is a technology widely used in industry and characterized by intensive heat transfer from the casting. Therefore, in relation to hypoeutectic aluminum alloys containing Cr, Mo, V and W, precisely this technology should be seen as an opportunity to effectively increase the mechanical properties of castings made from the above mentioned alloys. The possibility of increasing the properties of HPDC Al-Si alloy by supersaturation of α(Al) phase with high melting point elements is the most innovative aspect of this paper. The aim of the paper is to confront the phenomena occurring during the crystallization of Al-Si alloy with different content of high-melting point elements with their mechanical properties in the context of developing the probable mechanism of changes in the properties of the alloy. The crystallization process was investigated by thermal and derivative analysis. Optical and scanning microscopy were used to study the alloy microstructure. A point analysis of the chemical composition of selected phases in the microstructure was also performed with the use of an EDS (Energy Dispersive Spectroscopy) detector. The static tensile test and the Brinell hardness test were used to determine the mechanical properties

2. Materials and Methods A typical hypoeutectic Al-Si alloy for high pressure die casting, i.e., EN-AC 46000 (EN-AC AlSi9Cu3 (Fe)), was used for the tests. It is included in the PN-EN 1706 standard. The chemical composition of the base alloy is shown in Table1. The range of the chemical composition is derived from 15 specimens of the base alloy. The reason for such a number of specimens is the testing of 15 diﬀerent combinations of high melting point elements. The number of Cr, Mo, V and W combinations means all statistically possible connections that can be used for the four analyzed elements. Therefore, each time new base alloy was smelted from ingots of EN AC-46000 alloy. The individual connections Cr, Mo, V and W have been shown in the following description of a melting technology.

Table 1. Chemical composition of the base EN AC-46000 alloy.

Chemical Composition, wt % Si Cu Zn Fe Mg Mn Ni Ti Cr Al 8.69–9.35 2.09–2.43 0.90–1.07 0.82–0.97 0.21–0.32 0.18–0.25 0.05–0.13 0.042–0.049 0.023–0.031 rest

The base alloy was melted in a gas-heated shaft furnace with a maximum charge capacity of 1.5 tonnes (StrikoWestofen, Gummersbach, Germany). After smelting, it was reﬁned inside the shaft furnace. An Ecosal Al 113S solid reﬁner was used for this treatment. After tapping the alloy from the furnace into a ladle, it was deslagged with an Ecremal N44 deslagging agent. After deslagging, the liquid alloy was transferred to a holding furnace set up next to an Idra 700S horizontal cold chamber Materials 2020, 13, 4861 3 of 13

Materials 2020, 13, 4861 3 of 14 pressure die casting machine (Idra, Travagliato, Italy). In the holding furnace, AlCr15, AlMo8, AlV10 and AlW8 master alloys were added into the alloy. The temperature in the holding furnace was 750 C. appropriate for the test casting process. High melting point elements, i.e., chromium, molybdenum,◦ After dissolution of the master alloys, the temperature of the liquid metal was reduced to a value vanadium and tungsten, were added as single elements, in double combinations (CrMo, CrV, CrW, appropriate for the test casting process. High melting point elements, i.e., chromium, molybdenum, MoV, MoW and VW), in triple combinations (CrMoV, CrMoW, CrVW and MoVW), or all the vanadium and tungsten, were added as single elements, in double combinations (CrMo, CrV, CrW, additives were added simultaneously. For individually added Cr, Mo, V and W, their content was MoV, MoW and VW), in triple combinations (CrMoV, CrMoW, CrVW and MoVW), or all the additives kept in the range of 0.0–0.5 wt %, and it was increased in 0.1 wt % steps. If more than one high melting were added simultaneously. For individually added Cr, Mo, V and W, their content was kept in the point element was added, all the elements were used in equal amount. For double combinations, the range of 0.0–0.5 wt %, and it was increased in 0.1 wt % steps. If more than one high melting point content of additives was in the range of 0.0–0.4 wt %, and in subsequent melts it was increased in 0.1 element was added, all the elements were used in equal amount. For double combinations, the content wt % steps. In the case of triple and quadruple combinations, the content of elements was in the range of additives was in the range of 0.0–0.4 wt %, and in subsequent melts it was increased in 0.1 wt % of 0.00–0.25 wt %, and in subsequent melts it was increased in 0.05 wt % steps. From the resultant steps. In the case of triple and quadruple combinations, the content of elements was in the range of alloys, covers of roller shutter housings with a predominantly 2 mm wall thickness were made by the 0.00–0.25 wt %, and in subsequent melts it was increased in 0.05 wt % steps. From the resultant alloys, high pressure die casting process. covers of roller shutter housings with a predominantly 2 mm wall thickness were made by the high For each chemical composition tested, three specimens were taken from one high pressure die pressure die casting process. casting to carry out the tensile test. The specimens had a flat shape and 2 × 10 mm rectangular cross- For each chemical composition tested, three specimens were taken from one high pressure die section. This cross-section is recommended by PN-EN 1706 standard for testing the strength of high casting to carry out the tensile test. The specimens had a flat shape and 2 10 mm2 rectangular pressure die casting. An Instron 3382 machine (Instron, Norwood, MA, USA)× was used to perform cross-section. This cross-section is recommended by PN-EN 1706 standard for testing the strength of the tensile test at a speed of 1 mm/min. The test enabled the determination of the tensile strength Rm, high pressure die casting. An Instron 3382 machine (Instron, Norwood, MA, USA) was used to perform the yield strength Rp0.2 and the elongation A. Alloy hardness was measured by the Brinell method the tensile test at a speed of 1 mm/min. The test enabled the determination of the tensile strength R , using an HPO-2400 hardness tester (WPM LEIPZIG, Leipzig, Germany) according to PN EN ISOm the yield strength R and the elongation A. Alloy hardness was measured by the Brinell method 6506. The diameter ofp0.2 the ball was 2.5 mm, the load was 613 N, and the static load holding time was using an HPO-2400 hardness tester (WPM LEIPZIG, Leipzig, Germany) according to PN EN ISO 6506. 30 s. The diameter of the ball was 2.5 mm, the load was 613 N, and the static load holding time was 30 s. Thermal and derivative analysis was used as a tool to study the solidification process. Thermal Thermal and derivative analysis was used as a tool to study the solidification process. Thermal and derivative curves were recorded with a PtRh10-Pt thermocouple enclosed by a quartz tube and derivative curves were recorded with a PtRh10-Pt thermocouple enclosed by a quartz tube placed placed in the thermal center of a probe made of a resin coated sand. Its dimensions are shown in in the thermal center of a probe made of a resin coated sand. Its dimensions are shown in Figure1. Figure 1. The alloy was superheated to 1000 °C before the probe was filled with liquid alloy. The alloy was superheated to 1000 ◦C before the probe was filled with liquid alloy.

Figure 1. Dimensions of the resin coated sand probe (units: mm). Figure 1. Dimensions of the resin coated sand probe (units: mm). Metallographic specimens for microstructure examinations were prepared on specimens taken Metallographic specimens for microstructure examinations were prepared on specimens taken from castings made in the resin coated sand probe and by high pressure die casting. The surface from castings made in the resin coated sand probe and by high pressure die casting. The surface of of the specimens was etched with a 2% HF acid solution. The microstructure was examined using the specimens was etched with a 2% HF acid solution. The microstructure was examined using Nikon Nikon Eclipse MA200 optical microscope (Nikon, Tokyo, Japan) with 100 and 1000 magniﬁcation Eclipse MA200 optical microscope (Nikon, Tokyo, Japan) with ×100× and ×1000× magnification for for castings made in the resin coated sand probe and by high pressure die casting process, respectively. castings made in the resin coated sand probe and by high pressure die casting process, respectively. The applied magniﬁcation ensures correct visibility of all alloy phases in the examined area. The applied magnification ensures correct visibility of all alloy phases in the examined area. Point microanalysis of the element concentration was performed using the Pioneer EDS detector cooperating with the HITACHI S-3000N scanning electron microscope (Hitachi, Tokyo, Japan) and the VENTAGE software from NORAN. The specimens for scanning under the scanning microscope were cut from the test castings using a hand hacksaw and water cooling. The shell mold casting

Materials 2020, 13, 4861 4 of 13

Point microanalysis of the element concentration was performed using the Pioneer EDS detector cooperating with the HITACHI S-3000N scanning electron microscope (Hitachi, Tokyo, Japan) and the VENTAGE software from NORAN. The specimens for scanning under the scanning microscope were cut from the test castings using a hand hacksaw and water cooling. The shell mold casting specimens had the shape of a cube with a side of 10 mm. Specimens taken from high pressure die castings had the shape of a cuboid with sides 2/10/20 mm, and after cutting they were mounted in a conductive phenolic resin. After mounting, the specimen was cylindrical with a diameter of 36 mm and a height of Materials 2020, 13, 4861 4 of 14 12 mm. The specimens were then ground using sandpaper and polished using diamond suspensions with a gradation of 9 to 1 µm. specimens had the shape of a cube with a side of 10 mm. Specimens taken from high pressure die The area of the spot examination of the elements concentration is shown in the figures of the castings had the shape of a cuboid with sides 2/10/20 mm, and after cutting they were mounted in a microstructure made with the use of a scanning microscope. These images show the intermetallic conductive phenolic resin. After mounting, the specimen was cylindrical with a diameter of 36 mm phases within which the analyzes were performed. For the purpose of this study, phase precipitations and a height of 12 mm. The specimens were then ground using sandpaper and polished using of relatively large sizes were selected in order to avoid the analytical beam going beyond the area of diamond suspensions with a gradation of 9 to 1 µm. the tested phase. The area of the spot examination of the elements concentration is shown in the figures of the microstructure3. Results and Discussionmade with the use of a scanning microscope. These images show the intermetallic phases within which the analyzes were performed. For the purpose of this study, phase precipitations of relativelyFigure2 largecompares sizes thewere thermal selected and in derivative order to avoid analysis the curvesanalytical of the beam EN going AC-46000 beyond base the alloy area and of thealloy tested containing phase. 0.5 wt % Mo and Cr, Mo, V and W added in an amount of 0.25 wt % each. Studies have revealed a three-stage solidification process of the base EN EN-46000 alloy from the 3.resin Results coated and sand Discussion probe. The first stage taking place at the highest temperature is the crystallization of α(Al) dendrites solid solution. This process is marked by the thermal effect CsAB. The primary Figure 2 compares the thermal and derivative analysis curves of the EN AC-46000 base alloy crystallization of α(Al) dendrites directly from the liquid results from Al-Si phase diagram shown in and alloy containing 0.5 wt % Mo and Cr, Mo, V and W added in an amount of 0.25 wt % each. Figure3. 1000 1

K dt/dτ = f `(τ) 0 800

H L -1 C/s o C , o

600 t = f(τ) -2 τ t, dt/d

-3 400 -4 base alloy 0.5 wt % Mo 0.25 wt % Cr, Mo, V, W 200 -5 0 100 200 300 400 500 600 700 800 900 1000 τ , s (a)

Figure 2. Cont.

MaterialsMaterialsMaterials 20202020, , 20201313,, 4861 4861, 13, 4861 5 of 145 5 of of 14 13

1000 0.40 1000 Cs A B E 0.40 CsCs A' A A B B E 0.20 τ τ Cs A' A B Edt/d = f `( ) 0.20 0.00 900 dt/dτ = f `(τ) 900 -0.200.00 -0.40-0.20 800 C/s -0.60-0.40o , C o 800 τ d t, C/s t/ -0.80 o

-0.60d , C o τ 700 d t, t, Cs -1.00 A' A" A B E -0.80 t/ d

700 -1.20 Cs A' A" A B E -1.00 600 t = f(τ) -1.40 -1.20 -1.60 600 t = f(τ) -1.40 500 -1.80 100 200 300 400 500 -1.60 500 τ, s -1.80

Figure 2. Thermal100 and derivative 200 curves of the base 300 alloy (black), 400alloy containing 5000.5 wt % Mo (red) τ and alloy containing Cr, Mo, V and W in an amount, s of 0.25 wt % each (blue).

Studies have revealed a three-stage solidificati(b) on process of the base EN EN-46000 alloy from the resin coated sand probe. The first stage taking place at the highest temperature is the Figure 2. Thermal and derivative curves of the base alloy (black), alloy containing 0.5 wt % Mo (red) Figurecrystallization 2. Thermal of α and(Al) derivative dendrites curvessolid solution. of the base This alloy process (black), is marked alloy containing by the thermal 0.5 wt effect % Mo C (red)sAB. and alloy containing Cr, Mo, V and W in an amount of 0.25 wt % each (blue). (a) and their enlarged andThe alloyprimary containing crystallization Cr, Mo, ofV andα(Al) W dendritesin an amount directly of 0.25 from wt %the each liquid (blue). results (a) andfrom their Al-Si enlarged phase fragment (b). fragmentdiagram shown(b). in Figure 3. 1500 Studies have revealed a three-stage solidification process of the base 1414EN oEN-46000C alloy from the resin coated sand1400 probe. The first stage taking place at the highest temperature is the crystallization of α(Al)1300 dendrites solid solution. This process is marked by the thermal effect CsAB. The primary crystallization1200 of α(Al) dendrites directly from the liquid results from Al-Si phase L diagram shown in Figure1100 3. 1000

900 L + β(Si) 800

C o o

660.452 C

, t 700 L + α(Al) o 600 577 C 12.6oC 500 α β 400 (Al) (Si) α(Al) + β(Si) 300 200 100 0 0 102030405060708090100

Al, wt %

Figure 3. Al-Si phase diagram (data from [11,21,22]).

The presented Al-Si phase diagram shows that after the crystallization of α(Al) dendrites, the crystallization of α(Al) + β(Si) eutectic mixture takes place. The tested AlSi9Cu3(Fe) alloy is

Materials 2020, 13, 4861 6 of 14

Materials 2020, 13, 4861 Figure 3. Al-Si phase diagram (own study based on [11,21,22]). 6 of 13

The presented Al-Si phase diagram shows that after the crystallization of α(Al) dendrites, the multicomponent,crystallization of therefore α(Al) + moreβ(Si) complexeutectic eutecticmixture mixturestakes place. crystallize The tested in it. InAlSi9Cu3(Fe) these mixtures, alloy apart is frommulticomponent,α(Al) and β(Si) therefore phases, theremore arecomplex also intermetallic eutectic mixt phases.ures crystallize The crystallization in it. In these of multicomponentmixtures, apart Al-Sifrom alloys α(Al) with and this typeβ(Si) ofphases, complex there eutectic are mixtures also intermetallic was described phases. in detail The in [22 crystallization–24]. In accordance of withmulticomponent the knowledge Al-Si contained alloys with therein, this thetype authors of complex present eutectic the furthermixtures crystallization was described process in detail of in the analyzed[22–24]. alloy.In accordance with the knowledge contained therein, the authors present the further crystallization process of the analyzed alloy. When dendrite crystallization is complete, the complex α(Al) + Al15(Fe,Mn)3Si2 + β(Si) When dendrite crystallization is complete, the complex α(Al) + Al15(Fe,Mn)3Si2 + β(Si) (ternary) (ternary) and α(Al) + Al2Cu + AlSiCuFeMnMgNi + β(Si) (quaternary) eutectic mixtures crystallize. Thermaland α(Al) eff ects+ Al caused2Cu + AlSiCuFeMnMgNi by the crystallization + β(Si) of (quaternary) the above mentioned eutectic mixtures eutectic crystallize. mixtures haveThermal been markedeffects caused with the by symbols the crystallization BEH and HKL,of the above respectively. mentioned The eutectic designation mixtures of AlSiCuFeMnMgNi have been marked phasewith shouldthe symbols be treated BEH as and conventional, HKL, respectively. because itThe does designation not mean anof AlSiCuFeMnMgNi intermetallic phase phase containing should all be the treated as conventional, because it does not mean an intermetallic phase containing all the elements elements included, but a number of phases that may contain these elements. The chemical composition included, but a number of phases that may contain these elements. The chemical composition of this of this phase depends on the diversified (as a result of microsegregation) content of elements in phase depends on the diversified (as a result of microsegregation) content of elements in different different areas of the residual liquid. It can be Mg2Si phase or the phases from Al-Cu-Ni, Al-Fe-Mn-Si areas of the residual liquid. It can be Mg2Si phase or the phases from Al-Cu-Ni, Al-Fe-Mn-Si and Al- and Al-Cu-Mg-Si systems. The microstructure of the base alloy and containing Cr, Mo, V and W is Cu-Mg-Si systems. The microstructure of the base alloy and containing Cr, Mo, V and W is presented presented in detail in [25]. This paper presents only those aspects of microstructure formation that are in detail in [25]. This paper presents only those aspects of microstructure formation that are related related to the mechanism of its strengthening presented by the authors. to the mechanism of its strengthening presented by the authors. When the additives in an amount not exceeding 0.1 wt.% Cr, 0.4 wt % Mo or W, and 0.5 wt % When the additives in an amount not exceeding 0.1 wt.% Cr, 0.4 wt % Mo or W, and 0.5 wt % V Vare are added added as as single single elements elements into into the the base base alloy, alloy, there there are areno new no new thermal thermal effects eff onects the on thermal the thermal and andderivative derivative curves curves and andnew newphases phases are not are formed not formed in the microstructure. in the microstructure. In this range In this of the range content, of the content,the above the mentioned above mentioned elementselements tend to join tend the to phas joines the already phases existing already in existingthe base inalloy. the These base alloy.are Thesemainly are iron-rich mainly iron-rich intermetallic intermetallic phases phasesoccurring occurring in both in botheutectic eutectic mixtures, mixtures, designated designated as as AlAl1515(Fe,Mn,M)(Fe,Mn,M)33SiSi22,, wherewhere MM isis anyany high melting point point element element or or any any combination combination of of them. them. A A single single additionaddition of of chromium chromium in in an an amount amount ofof 0.20.2 wtwt %% and molybdenum or or tungsten tungsten in in an an amount amount of of 0 0.5%5% leadsleads to to primary primary crystallization crystallization of of the the Al Al1515(Fe,Mn,M)(Fe,Mn,M)33Si2 phase.phase. Vanadium Vanadium in in the the tested tested content content range range diddid not not cause cause this phasethis phase to crystallize. to crystallize. As a result As of thea result crystallization of the crystallization of the primary Alof15 (Fe,Mn,M)the primary3Si 2 phaseAl15(Fe,Mn,M) on the derivative3Si2 phase curve on the a thermal derivative effect curve is created, a thermal which ef doesfect is not created, have a which distinct does local not maximum. have a Andistinct example local of maximum. such an eff Anect example is visible of on such the an derivative effect is visible curve ofon an the alloy derivative containing curve 0.5 of wtan alloy % Mo (Figurecontaining2–red 0.5 color) wt % and Mo is (Figure described 2–red as color) C sA’. and Increasing is described theamount as CsA’. ofIncreasing high melting the amount point elementsof high producesmelting apoint thermal elements effect produces with a clearly a thermal visible effect local wi maximum.th a clearly This visible thermal local emaximum.ffect is shown This in thermal Figure2 (blueeffect color), is shown for an in alloy Figure containing 2 (blue color), 0.25 wt for % Cr,an Mo,alloy V containing and W. It is0.25 described wt % Cr, as CMo,sA’A”. V and The W. amount It is anddescribed size of thisas C phasesA’A”. increasesThe amount with and the size increase of this in phase the content increases of highwith meltingthe increase point in elementsthe content in of the alloy.high Figuremelting4 shows point theelements microstructure in the alloy. of the Figure base 4 alloy shows and the containing microstructure 0.25 wt of % the Mo, base V and alloy W and each withcontaining marked 0.25 component wt % Mo, phases. V and W each with marked component phases.

(a) (b)

Figure 4. The microstructure of: (a) EN AC-AlSi9Cu3(Fe) base alloy, (b) alloy containing Mo, V and W in an amount of 0.25 wt. % each. Materials 2020, 13, 4861 7 of 14

MaterialsFigure2020, 134. ,The 4861 microstructure of: (a) EN AC-AlSi9Cu3(Fe) base alloy, (b) alloy containing Mo, V and 7 of 13 W in an amount of 0.25 wt. % each.

InIn Figure Figure4 b4b Al Al1515(Fe,Mn,M)(Fe,Mn,M)33Si22 phasephase is is characterized characterized by by a a skeletal skeletal morphology. morphology. For For example, example, FigureFigure5 5shows shows the the chemical chemical composition composition of of the the analogous analogous phase phase in in thethe alloyalloy containingcontaining 0.40.4 wtwt %% Cr.Cr.

Figure 5. An example of the area with Al15(Fe,Mn,M)3Si2 phase in the alloy containing 0.4 wt % Cr Figure 5. An example of the area with Al (Fe,Mn,M) Si phase in the alloy containing 0.4 wt % Cr from the shell mold and the results of the 15point measurement3 2 of the chemical composition within it. from the shell mold and the results of the point measurement of the chemical composition within it.

ThereThere is is a a high high concentration concentration of of Al Al (57.7 (57.7 wt wt %) %) and and an an increased increased concentration concentration of of Fe Fe (17.7 (17.7 wt wt %) %) in thein the phase; phase; Si (10.8 Si (10.8 wt wt %); %); Cr Cr (7.53 (7.53 wt%) wt%) and and Mn Mn (5.9 (5.9 wt wt %). %). ItsIts phasephase chemical composition composition indicates indicates that it is Al15(Fe,Mn,Cr)3Si2 phase. The occurrence of this phase in alloys containing Cr has been that it is Al15(Fe,Mn,Cr)3Si2 phase. The occurrence of this phase in alloys containing Cr has been confirmed in [26–28]. The chemical analysis of Al15(Fe,Mn,M)3Si2 phase detected in alloys containing confirmed in [26–28]. The chemical analysis of Al15(Fe,Mn,M)3Si2 phase detected in alloys containing additionally Mo, V and W showed the presence of all these elements. Thus, the crystallization of these additionally Mo, V and W showed the presence of all these elements. Thus, the crystallization of intermetallics causes the depletion of the remaining liquid in high melting point elements. A these intermetallics causes the depletion of the remaining liquid in high melting point elements. relatively large amount of the primary Al15(Fe,Mn,M)3Si2 phase causes the depletion of the liquid in A relatively large amount of the primary Al (Fe,Mn,M) Si phase causes the depletion of the liquid in high melting point elements leading to crystallization15 the3 2 classic plate α(Al) + β(Si) eutectic mixture high melting point elements leading to crystallization the classic plate α(Al) + β(Si) eutectic mixture instead of the ternary one. Thus, high melting point elements first increase their concentration ahead instead of the ternary one. Thus, high melting point elements first increase their concentration ahead of the crystallization front of α(Al) phase, but then reduces it as a result of the primary crystallization of the crystallization front of α(Al) phase, but then reduces it as a result of the primary crystallization of Al15(Fe,Mn,M)3Si2 phase. The possibility of supersaturation of α(Al) dendrites is related to the of Al (Fe,Mn,M) Si phase. The possibility of supersaturation of α(Al) dendrites is related to the concentration15 of the3 2 dissolved substance ahead of the dendrite crystallization front [29] and the rate concentration of the dissolved substance ahead of the dendrite crystallization front [29] and the rate of heat transfer during the solidification process. In [30,31], general relationships (1) and (2) were of heat transfer during the solidification process. In [30,31], general relationships (1) and (2) were developed to link the crystal nucleation rate and growth rate, respectively, with the supersaturation developed to link the crystal nucleation rate and growth rate, respectively, with the supersaturation of of the crystal with the substance on the crystallization front: the crystal with the substance on the crystallization front: B = kb × Ωb, (1) B = k Ωb, (1) b × V = kv × Ωv, (2) v V = kv Ω , (2) where: × where:B-crystal nucleation rate, B-crystalV-crystal nucleation growth rate, rate, V-crystalΩ-supersaturation, growth rate, kb and kv-original constant rate of crystal nucleation and growth, respectively, Ω-supersaturation, b and v-supersaturation exponent for crystal nucleation and growth, respectively. kb and kv-original constant rate of crystal nucleation and growth, respectively, and v-supersaturationFrom the above mentioned exponent forrelationships crystal nucleation it follows and that growth, an increase respectively. in the crystal nucleation rate andFrom growth the rate above increases mentioned the degree relationships of supersaturat it followsion. that It an isincrease also generally in the crystalknown nucleation that that the rate andhigher growth rate rate of heat increases transfer the degreeleads to of the supersaturation. higher rate of It the is also nucleation generally and known growth that of that the the crystal. higher rateTherefore, of heat two transfer factors leads can to accelerate the higher supersaturation–a rate of the nucleation high andconcentration growth of of the high crystal. melting Therefore, point elements on the dendritic crystallization front and the high rate of heat transfer during crystallization. two factors can accelerate supersaturation–a high concentration of high melting point elements on the dendritic crystallization front and the high rate of heat transfer during crystallization. Based on 84 thermal and derivative tests for the base alloy as well as all variants of the high melting point Materials 2020, 13, 4861 8 of 14 Materials 2020, 13, 4861 8 of 13

Based on 84 thermal and derivative tests for the base alloy as well as all variants of the high melting elementspoint elements used, the used, average the average heat transfer heat tran ratesfer during rate the during crystallization the crystallization of α(Al) phaseof α(Al) was phase calculated. was calculated. The value obtained was 0.13 °C/s. This, relatively low, heat transfer rate in castings from The value obtained was 0.13 ◦C/s. This, relatively low, heat transfer rate in castings from shell molds createsshell molds rather modestcreates possibilitiesrather modest of αpossibilities(Al) supersaturation of α(Al) withsupersaturation high melting with point high elements. melting However, point theelements. extensive However, thermal andthe derivativeextensive teststhermal showed and an derivative increase in tests the crystallizationshowed an increase start temperature in the crystallization start temperature Cs as a result of addition high melting point elements and increasing Cs as a result of addition high melting point elements and increasing their content in the alloy. This was thetheir case content for all in analyzed the alloy. combinations This was the case of these for al elements.l analyzed Figurecombinations6 shows of the these e ﬀ ectelements. of high-melting Figure 6 elementsshows the on theeffect crystallization of high-melting start temperatureelements on∆ tCs,the forcrystallization individually start and simultaneouslytemperature ΔtCs, added for Cr, individually and simultaneously added Cr, Mo, V and W. Mo, V and W.

(a) (b)

Figure 6. Changes in the crystallization temperature ΔtCs caused by the variable content of Cr, Mo, V Figure 6. Changes in the crystallization temperature ∆tCs caused by the variable content of Cr, Mo, or W added: (a) as single elements and (b) jointly; M–any high melting point element tested, i.e., Cr, V or W added: (a) as single elements and (b) jointly; M–any high melting point element tested, i.e., Cr, Mo, V or W. Mo, V or W.

FigureFigure6 a6a shows shows that that all all individually individually addedadded highhigh meltingmelting pointpoint elements increase increase the the liquidus liquidus temperature. This is mainly due to the pre-dendritic crystallization of Al15(Fe,Mn,M)3Si2 phase, which temperature. This is mainly due to the pre-dendritic crystallization of Al15(Fe,Mn,M)3Si2 phase, occurs when 0.2 wt % Cr and 0 5 wt.% Mo and W are added (thermal effects CsA’ or CsA’A”). which occurs when 0.2 wt % Cr and 0 5 wt.% Mo and W are added (thermal effects CsA’ or CsA’A”). Crystallization of this phase is responsible for a rapid increase in the temperature ΔtCs. However, for Crystallization of this phase is responsible for a rapid increase in the temperature ∆tCs. However, each high melting point element analyzed, the increase in Cs temperature was also obtained in the for each high melting point element analyzed, the increase in Cs temperature was also obtained in the range of their concentration that did not cause pre-dendritic crystallization of Al15(Fe,Mn,M)3Si2 range of their concentration that did not cause pre-dendritic crystallization of Al15(Fe,Mn,M)3Si2 phase. phase. This tendency is clearly visible when V, Mo and W are added. Vanadium did not cause pre- This tendency is clearly visible when V, Mo and W are added. Vanadium did not cause pre-dendritic dendritic crystallization of Al15(Fe,Mn,M)3Si2 phase, while Mo and W did not cause this phase to crystallization of Al (Fe,Mn,M) Si phase, while Mo and W did not cause this phase to crystallize to crystallize to 0.4 wt15 %. In the above-mentioned3 2 range of high melting point elements, the increase of 0.4 wt %. In the above-mentioned range of high melting point elements, the increase of ∆tCs is relatively ΔtCs is relatively mild. When Cr, Mo, V and W are simultaneously added (Figure 6b), up to 0.05 wt mild. When Cr, Mo, V and W are simultaneously added (Figure6b), up to 0.05 wt % each, the increase % each, the increase in ΔtCs is also initially mild, but starting with the 0.10 wt %, a rapid jump and in ∆tCs is also initially mild, but starting with the 0.10 wt %, a rapid jump and faster growth occur. faster growth occur. This rapid increase in ΔtCs is due to the pre-dendritic crystallization of the This rapid increase in ∆tCs is due to the pre-dendritic crystallization of the Al15(Fe,Mn,M)3Si2 phase. Al15(Fe,Mn,M)3Si2 phase. The increase in the liquidus temperature at the content of high melting point The increase in the liquidus temperature at the content of high melting point elements insufficient elements insufficient to start the primary crystallization of Al15(Fe,Mn,M)3Si2 phase indicates transfer toof start a certain the primary amount crystallization of these elements of Al to15 α(Fe,Mn,M)(Al) dendrites.3Si2 phaseMuch indicatesbetter conditions transfer for of supersaturation a certain amount α α ofof these the α elements(Al) solidto solution(Al) dendrites.with high melting Much better point conditionselements are for provided supersaturation by the high of the pressure(Al) soliddie solutioncasting (HPDC) with high technology, melting pointwhere elements the rate of are heat provided transfer byfrom the the high casting pressure is much die higher. casting The (HPDC) rate technology,of heat transfer where during the rate crystallization of heat transfer of the from α(Al) the dendrites casting is in much HPDC higher. alloy The was rate determined of heat transfer from duringrelationship crystallization (3) presented of the in [32]:α(Al) dendrites in HPDC alloy was determined from relationship (3) presented in [32]: SDAS=39,4 × V^(-0,317), (3) SDAS=39.4 Vˆ( 0.317), (3) × − where:where: SDAS-secondary dendrite arms spacing in α(Al) phase, µm; SDAS-secondary dendrite arms spacing in α(Al) phase, µm; V–the rate of heat transfer during the crystallization of α(Al) dendrites, °C/s. V–the rate of heat transfer during the crystallization of α(Al) dendrites, ◦C/s. The measured average SDAS in the examined HPDC alloy was 10.65 µm. The rate of heat transfer calculated for SDAS from relationship (3) amounts to ~55 ◦C/s. This value is over 400 times higher Materials 2020, 13, 4861 9 of 14 Materials 2020, 13, 4861 9 of 14 The measured average SDAS in the examined HPDC alloy was 10.65 µm. The rate of heat Materials 2020, 13, 4861 9 of 13 transferThe calculatedmeasured foraverage SDAS SDASfrom relationship in the examined (3) amounts HPDC to alloy ~55 °C/s.was 10.65This valueµm. Theis over rate 400 of timesheat transferhigher than calculated the value for SDASobtained from for relationship alloy from (3)shel amountsl mold. toThe ~55 high °C/s. rate This of valueheat transferis over 400 from times the thanhigherHPDC the thanalloy value theproduces obtained value obtainedmore for alloyrefined for from alloy microstructure shell from mold. shell compared Themold. high The rate withhigh of therate heat microstructure of transfer heat transfer from obtained thefrom HPDC the in alloyHPDCshell producesmold. alloy Figure produces more 7 refinedshows more the microstructurerefined microstructure microstructure compared of HPDC compared with EN the AC-46000 microstructurewith the alloy microstructure containing obtained Cr, inobtained shellV and mold. W,in Figureshelleach mold.in7 showsan amount Figure the microstructure7of shows 0.25 wt the %. microstructure Th ofe constituent HPDC EN of AC-46000phasesHPDC areEN alloyalsoAC-46000 marked. containing alloy containing Cr, V and W,Cr, eachV and in W, an amounteach in an of 0.25amount wt %. of The0.25 constituentwt %. The constituent phases are phases also marked. are also marked.

(a) (b) (a) (b) Figure 7. The microstructure of: (a) base alloy and (b) alloy containing Cr, V and W in an amount of FigureFigure0.25 wt 7.7. % TheThe each. microstructuremicrostructure of: ( a)) base base alloy alloy and and ( (bb)) alloy alloy containing containing Cr, Cr, V V and and W W in in an an amount amount of of 0.250.25 wtwt %% each.each. The microstructure of the die cast base alloy consists of α(Al) phase dendrites and eutectic mixture.TheThe microstructure microstructureThe eutectic ofmixture of the the die die castconsists cast base base alloyof αalloy(Al) consists consistsand of βα(Si)(Al) of solidα phase(Al) solutionsphase dendrites dendrites and and eutecticrelatively and eutectic mixture. fine Themixture.intermetallic eutectic The mixture phases.eutectic consists Diffraction mixture of α (Al)consists tests and have βof(Si) αsh(Al) solidown and solutions the βpresence(Si) andsolid relatively of solutions intermetallic ﬁne and intermetallic phases:relatively Al phases. 2fineCu, DiintermetallicAlﬀ2ractionCuMg and tests phases. other have phases shownDiffraction that the presencecrystallize tests have of in intermetallic sh Al-Fe-Si,own the Al-Cu-Fe phases:presence Aland of2Cu, Mn-Ni-Siintermetallic Al2CuMg phase andphases: systems. other Al phases 2Cu,The thatAladdition2CuMg crystallize of and a certain inother Al-Fe-Si, phasesamount Al-Cu-Fe that of highcrystallize and melting Mn-Ni-Si in poinAl-Fe-Si, phaset elements Al-Cu-Fe systems. causes and The the additionMn-Ni-Si pre-dendritic of phase a certain crystallization systems. amount The of highadditionof the melting Al 15of(Fe,Mn,M) a pointcertain elements 3amountSi2 phase. causesof With highthe themelting pre-dendriticincreasing point contentelements crystallization of causesabove mentionedthe of thepre-dendritic Al15 elements,(Fe,Mn,M) crystallization the3Si amount2 phase. Withofof thethis the Al phase15 increasing(Fe,Mn,M) also increases. content3Si2 phase. ofFigure With above 7bthe mentioned shows increasing this elements, contentphase assume ofthe above amount the mentioned form of of this polygons elements, phase alsoor the stars. increases.amount The Figureofsize this of7 bphasethe shows precipitates, also this increases. phase depending assume Figure the on7b form theshows ofcontent polygons this ofphase high or stars.assume melting The the point size form of elements, theof polygons precipitates, is from or dependingstars.a few Theto ~ onsize50 theµm. of contentthe Analysis precipitates, of of high the chemical meltingdepending pointcomposition on elements,the content of the is of frompre-dendritic high a melting few to Al ~50point15(Fe,Mn,M)µ m.elements, Analysis3Si 2is phase from of the hasa few chemical shown to ~ composition50that µm. it canAnalysis “assimilate” of the of pre-dendriticthe chemical all high compositionmelting Al15(Fe,Mn,M) point of elem the3Si2 pre-dendriticentsphase tested. has shown Figure Al15(Fe,Mn,M) that 8 presents it can3Si “assimilate” 2the phase results has ofshown all EDS high meltingthatpoint it analysiscan point “assimilate” elements carried tested.out all withinhigh Figure melting the8 areapresents point of the elem the pre-dendritic resultsents tested. of EDS phase Figure point in analysis8 thepresents alloy carried containingthe results out within Cr,of EDS Mo, the areapointV and of analysis theW in pre-dendritic an carried amount out of phase 0.25within inwt.% the the alloyeach. area containingof the pre-dendritic Cr, Mo, V phase and W in in the an alloy amount containing of 0.25 wt.% Cr, Mo, each. V and W in an amount of 0.25 wt.% each.

(a) (b) Figure 8. An example of Al15(FeMnM)3Si2 phase in HPDC alloy containing Cr, Mo, V and W in an (a) (b) amountFigure of8. An 0.25 example wt % each of andAl15(FeMnM) the results3Si of2 phase a point inanalysis HPDC alloy of the containing chemical compositionCr, Mo, V and within W in it. an

Figureamount 8. of An 0.25 example wt % each of Al (a15) (FeMnM)and the results3Si2 phase of a pointin HPDC analysis alloy of containing the chemical Cr, composition Mo, V and withinW in an it Theamount(b). EDS of analysis0.25 wt % shows each (a) that and the results tested of phase a point is analysis most e offfective the chemical in assimilating composition Cr within (6.44 it wt %), slightly (b). less effective in Mo (3.72 wt %) and V (2.96 wt %), and the least effective in W (0.99 wt %). The total concentration of Cr, Mo, V and W in this phase is ~14 wt %. The “assimilation” of chromium, Materials 2020, 13, 4861 10 of 13

molybdenum, vanadium and tungsten by the Al15(Fe,Mn,M)3Si2 phase causes the depletion of these elements in the liquid. Since the Al15(Fe,Mn,M)3Si2 phase crystallizes directly in front of the α(Al) dendrites, in the liquid ahead of the dendritic crystallization front, similar changes occur in the concentration of Cr, Mo, V and W as in the alloy from the shell mold. Chromium, molybdenum, vanadium and tungsten in the range of concentration that did not cause the pre-dendritic crystallization of Al15(Fe,Mn,M)3Si2 phase leads to an increase in the concentration of these elements ahead of the crystallization front of α(Al) dendrites. Further increase in the content of above mentioned elements to a level that can trigger the crystallization of the pre-dendritic Al15(Fe,Mn,M)3Si2 phase reduces the Cr, Mo, V and W concentration ahead of the crystallization front of α(Al) dendrites. The described changes in the concentration of Cr, Mo, V and W ahead of the crystallization front of α(Al) dendrites can significantly affect the mechanical properties of HPDC alloy. The relatively high cooling rate during the crystallization of α(Al) dendrites, (~55 ◦C/s), leads to supersaturation of the α(Al) phase with chromium, molybdenum, vanadium and tungsten, and may increase the mechanical properties of HPDC alloy. This is confirmed by the results of the statistical analysis of the effect of Cr, Mo, V and W on the tensile strength Rm; yield strength Rp0.2; elongation A and Brinell hardness of HPDC alloy presented in [33–35]. Statistical analysis was performed at the significance level p (α) = 0.05. The analysis of variance (ANOVA) test for the main effects was used to assess the influence of high melting point elements on the mechanical properties. The values of strength properties in [33–35] were presented in the standardized and dimensionless form. The results of the statistical analysis showed that each high melting point element can cause an increase in Rm,Rp0.2; A and HB. The data presented in these papers shows an unambiguously analogous nature of the impact of the tested high melting point elements on the properties of this alloy. For the purposes of this paper, standardized quantities were decoded to the form of real values, which can be expressed in the units of measurement proper for the tested mechanical properties. Figure9 presents an example of the e ffect of high melting point elements on the Rm,Rp0.2; A and HB. The error bars shown in the graphs represent the values of the Standard Error of the Mean (SEM). The small SEM values shown in (Figure9) prove the correctness of the results, which indicate the effect of high melting point elements on the tested properties. Figure9a shows that Cr at the level of 0.05 wt % allows to obtain the tensile strength R m = 274.4 MPa. It is a value by ~13% higher in relation to the base alloy. Other high melting point elements make it possible to obtain a high Rm value with a content 0.05–0.10 wt %. 0.05 wt % vanadium has the greatest effect on the increase in Rm. This elements results in Rm = 277 MPa, which means an increase by ~14% relative to the base alloy. The highest yield strength is obtained with the addition of high melting point element in an amount of 0.05 wt %. The element most effective in increasing the value of Rp0.2 is vanadium (Figure9b). Its presence produces Rp0.2 = 126.8 MPa, which means an increase by ~12% relative to the base alloy. The highest elongation is usually obtained when the amount of high melting point element is 0.15 wt %. The greatest impact on the increase in elongation has 0.15 wt % Mo. The elongation is then 5.30%, which gives a 40% increase relative to the base alloy. The data presented in Figure9c shows that chromium in an amount of 0.15 wt % is also very effective in increasing the elongation to A = 5.21%. Relative to the base alloy, it is a 36% increase. To obtain high values of the Brinell hardness, the optimal content of high melting point elements is 0.05 wt % for chromium, molybdenum and vanadium, and 0.5 wt % for tungsten. The impact of V and W on HB levels is shown in Figure9d. The greatest impact on HB increase has 0.5 wt.% W (117 HB). The relative increase is rather small and amounts to 5%. Materials 2020, 13, 4861 11 of 13 Materials 2020, 13, 4861 11 of 14

(a) (b)

FigureFigure 9.9. Examples of the effect effect of selected selected high high meltin meltingg point point elements elements on on the the mechanical mechanical properties: properties: (a) the effect of Cr on Rm level; (b) the effect of V on Rp0.2 level; (c) the effect of Cr on A level, (d) the (a) the effect of Cr on Rm level; (b) the effect of V on Rp0.2 level; (c) the effect of Cr on A level, (d) the eeffectffect ofof VV andand WW onon HBHB level.level.

m TheFigure highest 9a shows values that of Cr R matand the level Rp0.2 ofare 0.05 the wt result % allows of a relativelyto obtain the high tensile supersaturation strength R = of 274.4α(Al) solidMPa. solution It is a value with by these ~13% elements. higher in The relation effect ofto thisthe phenomenonbase alloy. Other on the high strength melting properties point elements is most beneficial,make it possible when the to obtain high melting a high pointRm value elements with a are content added 0.05–0.10 in an amount wt %. insu 0.05ffi wtcient % vanadium to start the has primary the greatest effect on the increase in Rm. This elements results in Rm = 277 MPa, which means an increase crystallization of Al15(Fe,Mn,M)3Si2 phase. The supersaturation of α(Al) dendrites is also beneficial forby the~14% hardness relative ofto alloythe base containing alloy. 0.05 wt % Cr, Mo or V. The highest hardness obtained at 0.5 wt % The highest yield strength is obtained with the addition of high melting point element in an W was due to the presence of relatively large precipitates of Al15(Fe,Mn,W)3Si2 phase. The largest increaseamount inof 0.05 elongation wt %. The accompanying element most the effective addition in increasing of high melting the value point of elementsRp0.2 is vanadium in an amount (Figure of 0.159b). wtIts %presence was the produces result of R primaryp0.2 = 126.8 crystallization MPa, which means of relatively an increase small by and ~12% compact relative precipitates to the base of alloy. The highest elongation is usually obtained when the amount of high melting point element is Al15(Fe,Mn,M)3Si2 phase, which reduced the concentration of high melting point elements as well as 0.15 wt %. The greatest impact on the increase in elongation has 0.15 wt % Mo. The elongation is then Fe and Mn ahead of the crystallization front of α(Al) dendrites. Crystallization of Al15(Fe,Mn,M)3Si2 5.30%, which gives a 40% increase relative to the base alloy. The data presented in Figure 9c shows phase in alloy with high melting point elements can reduce the supersaturation of α(Al) dendrites that chromium in an amount of 0.15 wt % is also very effective in increasing the elongation to A = compared with the alloy without them. This leads to an increase in the alloy plasticity. 5.21%. Relative to the base alloy, it is a 36% increase. Statistical analysis in [33–35] gives no evidence supporting the occurrence of a synergistic effect To obtain high values of the Brinell hardness, the optimal content of high melting point elements between simultaneously introduced high melting point elements and alloy properties. However, is 0.05 wt % for chromium, molybdenum and vanadium, and 0.5 wt % for tungsten. The impact of V the results of mechanical tests carried out on various combinations of the co-introduced high melting and W on HB levels is shown in Figure 9d. The greatest impact on HB increase has 0.5 wt.% W (117 point elements indicate the possibility of obtaining much higher values of R ,R and A than the HB). The relative increase is rather small and amounts to 5%. m p0,2 values obtained in a statistical analysis carried out for the same additives but introduced as single The highest values of Rm and Rp0.2 are the result of a relatively high supersaturation of α(Al) solid elements. The largest increase in R was obtained with the addition of vanadium and tungsten added solution with these elements. Them effect of this phenomenon on the strength properties is most in an amount of approximately 0.2 wt % each. The value of R was then 299 MPa, which means that it beneficial, when the high melting point elements are addedm in an amount insufficient to start the was higher by ~50% relative to the base alloy. The same addition of V and W also caused the largest primary crystallization of Al15(Fe,Mn,M)3Si2 phase. The supersaturation of α(Al) dendrites is also (over twofold) increase in an elongation. The maximum increase in R , i.e., by ~21%, was due to the beneficial for the hardness of alloy containing 0.05 wt % Cr, Mo or V.p0.2 The highest hardness obtained use of Cr and W added in an amount of 0.1 wt.% each. at 0.5 wt % W was due to the presence of relatively large precipitates of Al15(Fe,Mn,W)3Si2 phase. The largest increase in elongation accompanying the addition of high melting point elements in an amount of 0.15 wt % was the result of primary crystallization of relatively small and compact precipitates of Al15(Fe,Mn,M)3Si2 phase, which reduced the concentration of high melting point elements as well as Fe and Mn ahead of the crystallization front of α(Al) dendrites. Crystallization of

Materials 2020, 13, 4861 12 of 13

4. Conclusions From the data presented in this paper, the following conclusions emerge:

1. Proper amount of high melting point elements added into HPDC hypoeutectic alloy as well as alloy from shell molds results in pre-dendritic crystallization of the Al15(Fe,Mn,M)3Si2 phase. 2. High intensity of the heat transfer during HPDC process enables supersaturation of α(Al) dendrites with high melting point elements.

3. The addition of Cr, Mo, V and W as well as pre-dendritic crystallization of the Al15(Fe,Mn,M)3Si2 phase change concentration of the above mentioned elements ahead of the crystallization front of α(Al) dendrites, resulting in its various supersaturation during HPDC process. 4. A significant increase in mechanical properties of the tested hypoeutectic alloys was obtained by supersaturation of the α(Al) phase with high melting point elements during HPDC process. 5. Changes in mechanical properties of the tested alloys were largely affected by supersaturation of the α(Al) phase, and also by the size of precipitates and content of the pre-dendritic Al15(Fe,Mn,M)3Si2 phase. 6. In the future, to prove the correctness of the presented hypothetical mechanism of strengthening of HPDC alloy by supersaturation of α(Al) phase with high melting point elements, analyzes should be carried out to show the presence of these elements in α(Al) phase with their different contents.

Author Contributions: Conceptualization, T.S., G.G. and T.P.; methodology, T.S., G.G. and T.P.; software, L.K. and J.S.; validation, T.S. and G.G.; formal analysis, T.S., G.G. and J.S.; investigation, T.S. G.G., L.K. and M.G.; resources, T.S.; data curation, T.S.; writing—original draft preparation, T.S. and G.G.; writing—review and editing, T.S. and G.G.; visualization, T.S.; supervision, T.S. and G.G.; project administration, T.P.; funding acquisition, T.P. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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