Influence of steel preheat temperature and molten casting alloy AlSi9Cu3(Fe) impact speed on wear of X38CrMoV5-1 steel in high pressure die casting conditions

Author’s Accepted Manuscript Influence of steel preheat temperature and molten casting alloy AlSi9Cu3(Fe) impact speed on wear of X38CrMoV5-1 steel in high pressure die casting conditions Zvonimir Dadić, Dražen Živković, Čatipović, Ivo Marinić-Kragić

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S0043-1648(18)30716-6 https://doi.org/10.1016/j.wear.2019.02.008 WEA102787

To appear in: Wear Received date: 14 June 2018 Revised date: 4 February 2019 Accepted date: 4 February 2019 Cite this article as: Zvonimir Dadić, Dražen Živković, Nikša Čatipović and Ivo Marinić-Kragić, Influence of steel preheat temperature and molten casting alloy AlSi9Cu3(Fe) impact speed on wear of X38CrMoV5-1 steel in high pressure die casting conditions, Wear, https://doi.org/10.1016/j.wear.2019.02.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Original scientific paper

Influence of steel preheat temperature and molten casting alloy AlSi9Cu3(Fe) impact speed on wear of X38CrMoV5-1 steel in high pressure die casting conditions Zvonimir DADIĆ, Dražen ŽIVKOVIĆ, Nikša ČATIPOVIĆ, Ivo MARINIĆ-KRAGIĆ

Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Split, Ruđera Boškovića 32, 21000 Split, Republic of Croatia [email protected] [email protected] [email protected] [email protected]

Abstract This paper is a part of experimental research in the area of high pressure die casting (HPDC) mould wear. Influence of mould preheat temperature and molten aluminium alloy impact speed on total mould wear was researched using novel laboratory die casting testing equipment. Testing parameters were set to simulate HPDC of aluminium alloy AlSi9Cu3(Fe). The specimens were made from X38CrMoV5-1 hot work steel (H11). Experiment was designed using central composite design. Following the experiment design, 13 specimens were heat treated and the surface was modified by “Tenifer” nitrocarburizing. Specimen wear was measured by a Mettler B5 scale (Küsnacht, Switzerland) and shown graphically by 3D scan before and after the experiment; the 3D scans were overlapped to determine main wear areas. Response surface was acquired. Most influential tribological wear mechanisms were determined by ANSYS CFX 17.2 analysis. It was found that preheat temperature and molten aluminium alloy impact speed directly affects total wear of the mould surface. An increase of preheat temperature decreases total wear, while an increase of molten aluminium alloy impact speed increases total wear. Conditions for minimum and maximum wear were quantified; most significant wear was observed on sharp edges of nitrided mould material specimens. ANSYS CFX 17.2 hard particle erosion rate simulation suggested erosion occurrence mostly at impact angles perpendicular to the specimen surface. Other ANSYS CFX 17.2 simulation suggested occurrence of cavitation erosion. SEM analysis indicated an occurrence of intermetallic compounds between molten alloy and hot work steel. Graphical Abstract:

Keywords: preheat temperature; casting alloy impact speed; wear; thermal fatigue; erosion; high pressure die casting; ANSYS CFX

1

Introduction

Motivation for this research came from a visit to a high pressure die casting (HPDC) manufacturing plant “LTH Metal Cast” (Benkovac, Croatia). Analysing the HPDC process it was obvious that most problems and additional expenses came from the everlasting problem of HPDC mould wear. Further research indicated that there is a direct connection between HPDC working parameters (i.e. preheat temperature and casting alloy pressure) and mould wear [1-3]. Purpose of this work was to research the relation between the HPDC parameters and mould wear. HPDC is a near-net shape manufacturing process in which the molten metal is injected into a metal mould at high speed and solidified under high pressure. It has been widely used in producing thin wall aluminium and magnesium alloy components with high dimensional accuracy, high production efficiency and considerable economic benefit for automotive and other industries [4,5].

Due to rapid fluctuations of temperature, pressure and melt velocity, die casting moulds are exposed to cyclic thermomechanical loading, which gradually leads to the occurrence of wear and damage [6]. The maintenance or replacement of these moulds is costly which makes this kind of research economically viable. One of the most important parameters in a die casting process is the temperature of the die [1,2]. The mould is filled with molten aluminium alloy at elevated temperatures, usually over 670 °C. When the molten casting alloy and surface of the mould meet, expansion of the mould material occurs. The heat is then conducted by the mould material and the mould surface is cooled. Heat is dissipated by the cooling medium flowing through the mould cooling channels and by the waterbased lubricant which is sprayed on the moulds surface [7]. The greater the temperature gradient between molten casting alloy and moulds surface, the sooner initial cracks will appear [8]. These thermal loads support crack propagation. The propagation of initial crack leads to the separation of material particles from the surface [9]. Temperature gradient can be reduced by increased mould preheat temperature [10]. In this paper, the influence of preheat temperature on mould wear was researched. The aluminium alloys commonly used in automotive parts such as fuel pump and throttle housings, EGR valves, support brackets, et cetera usually contain hard particles, e.g. silica and alumina. Pressuring the molten casting alloy into the mould, speed of molten metal can reach up to 60 m/s. Aluminium flow into the mould at high temperature and high speed induces severe wear, due to erosion effects [3]. Speed of molten metal at point of contact with the mould (impact speed) has a significant influence on mould wear [11]. Influence of impact speed on total mould wear was researched in this paper. The aluminium from the casting alloy tends to form intermetallic compounds with iron from steel die surface which leads to mould wear. For this reason, many studies have been conducted to build coatings to increase mould lifespan [3,1215]. In this paper, SEM analysis was done to determine if the nitrided layer was damaged and if there was a possibility of formation of intermetallic compounds. Previous research, regarding HPDC mould wear and increasing the lifespan of mould, showed great benefits when using hard coatings [3,12-16]. Nunes et al. tested different Ti-Al-N coatings and showed great benefits of using surface coatings in opposition to uncoated hot work steel [3]. Markežić et al. used a plasma nitrided hot work tool insert. Experimental and numerical results showed correlation between die surface temperatures and die failure modes. Hardness drop, nitride diffusion layer removal and microstructural changes were observed [1]. Mitterer et al. found that hard coatings based on nitrides and carbides of transition metals may protect the steel surface from erosion and adherence of aluminium and improve resistance against thermal cracking [16]. Akhtar et al. found that nitriding H13 steel with sharp edges can lead to oversaturation of nitrogen or “corner effect” which leads to increase in hardness and makes the material brittle [17]. Further, this research investigated the effect of nitrided surface and sharp edges on total wear. 2

Materials and methods

There is no standardized test method for research of hot working tool steel wear [18]. Most of previous studies were focused on individual wear mechanisms [9]. Laboratory prototype of die casting testing equipment was designed for this purpose. This equipment allowed simulation of the most influential parameters in the die casting process. Influential wear mechanisms were incorporated, and their simultaneous effect was researched. To simplify, to reduce costs and accelerate the testing procedure, small specimens of mould material were immersed in molten aluminium alloy and then into a cooling lubricant medium. One testing cycle consisted of heating the specimen in molten aluminium alloy, then cooling in the lubricant. Schematic representation of the test equipment is shown in Figure 1. Testing equipment block diagram is shown in Figure 2.

Figure 1 Schematic representation of test equipment with characteristic positions of specimen carrier

Industry casting conditions were met by adjusting the immersion times. Testing temperatures were defined by thermal imaging data of moulds surface from the die casting plant “LTH Metal Cast”. To achieve noticeable wear by thermal fatigue, 10000 testing cycles were executed for every specimen. The main problem in some previous research [19] was adherence of the aluminium alloy to the mould material. This problem was solved by vibrations, centrifugal force and molybdenum disulphide lubrication [20]. Rapid rotational motion of the specimen was achieved by the servo motor Kollmorgen AKM54K-ACDNCA-00 (Radford, Virginia). Specimen carrier was 200 mm long, so a high circumferential speed could be achieved using a relatively small acceleration angle. Rotational motion of the servo motor was programmed in BASIC computer language in Kollmorgen Workbench software. The motor was controlled by Kollmorgen AKD-T01207-NBAN-E000 servo drive (Radford, Virginia). The casting aluminium alloy AlSi9Cu3(Fe) was melted by 650 W heating elements. Temperature of the molten aluminium alloy was controlled by a type K thermo-element (NiCr-Ni). Regulation of the temperature was done by Novus N480D controller (Portus Alegre, Brazil). Wear was determined by weighing the specimens with Mettler B5 scale with sensitivity of 0.1 mg, maximum capacity of 200 g, readability of 0.05 mg and precision of ±0.03 mg. Wear was also determined by 3D scanning with GOM Atos (Braunschweig, Germany) before and after the experiment.

Figure 2 Testing equipment block diagram

To avoid splashing of the molten aluminium alloy, specimen cross-section profile was shaped according to National Advisory Committee for Aeronautics (NACA). Profile NACA 0030 was used. The length of the specimen (profile) was 9 mm with thickness of 2.7 mm. The total height of the specimen was 58 mm. Thirteen specimens were made by CNC milling from hot work tool steel H11 (X38CrMoV5-1). Specimen before and after nitriding is shown in Figure 3. All specimens were heat treated. They were quenched from 1050 °C in oil and then tempered three times at 510 °C, 605 °C and 520 °C. This is a common procedure for pressure die cast mould material [21]. Hardness after heat treatment was 481 HV10. The surface was modified by “Tenifer” nitrocarburizing. The composition of the salt bath was 34-36% CNO and 2.5-3% CN at temperature of 580 °C. The holding time was 4 h. The thickness of nitrided layer was between 4 and 5 µm. The final surface hardness was 1100 HV30.

Figure 3 Specimen before (left) and after nitriding (right)

Before testing, specimens were cleaned in NaOH solution (20%), 3D scanned (GOM Atos) and weighed. NaOH was used for specimen cleaning since it dissolves the aluminium alloy while steel remains passive at 20% NaOH solution [22]. This was tested by immersing nitrided X38CrMoV5-1 specimen in 20% NaOH solution for 10 hours. Mass and hardness of the specimen remained unchanged. Casting aluminium alloy AlSi9Cu3(Fe) was used in molten state at constant temperature of 680 °C. The lubricant used for testing was molybdenum disulphide. MoS2 particles were transferred in oil that was dispersed in water (1:80 ratio).

For every specimen, 10000 test cycles were performed. Characteristic positions of specimen (A) and specimen carrier (B) during testing are shown in Figure 1. The experiment began with specimen preheating. Specimen was immersed in the lubricant (Fig. 1, positions 6-2-1-2), then in the aluminium alloy (Fig. 1, positions 2-5-4-5) and again in the lubricant which lowered the specimen temperature to preheat temperature. When specimen was preheated, first testing cycle began. One testing cycle consisted of heating the specimen in molten aluminium alloy and cooling in lubricant to preheat temperature. Immersion times depended on the required preheat temperature. During testing, impact speed was the circumferential speed at 200 mm from the centre of rotation (top of specimen) in the moment of contact with molten aluminium alloy (position 3). During the experiment, preheat temperature was the surface temperature of the specimen before it was immersed in molten aluminium alloy. Other temperatures were set to match the temperatures from the casting plant. Impact speed was the relative speed between specimen and molten aluminium at point of impact. Test parameters were defined using Design-Expert 7 software (State-Ease Inc.). Central composite design for two factors was used: preheat temperature of the mould (100-350 °C) and molten alloy impact speed (6-18 m/s). According to central composite design, 13 specimens are required. Experiment plan is shown in table 1.

Table 1 Central composite design experiment plan

No.

Preheat temperature, °C

Impact speed, m/s

1 2 3 4 5 6 7 8 9 10 11 12 13

225 100 350 48.22 225 225 225 100 401.78 225 225 350 225

12 6 18 12 12 3.51 12 18 12 20.49 12 6 12

Specimen temperatures were measured and controlled by ScanTemp 480 infrared thermometer (Dostmann electronic GmbH, Wertheim-Reicholzheim) and Flir T660 thermal camera (Wilsonville, Oregon, USA). Thermography image taken during testing is shown in Figure 4. Maximum temperature on Fig. 4 is marked with a red triangle which is located on the testing specimen.

Figure 4 Flir T660 thermography

Every 2000 cycles specimens were cleaned in NaOH solution (20% NaOH) and weighed. After 10000 cycles, specimens were cleaned by ASonic Pro 60 ultrasonic cleaner (Ljubljana, Slovenia) while immersed in NaOH solution (20 %). Cleaner power was 150 W and the frequency of 40 kHz was used. After the specimens were cleaned, they were weighed and 3D scanned.

3

Results

As a result of the experiments, according to central composite design, a response surface was acquired (Figure 5). Wear is shown by mass loss.

Figure 5 Response surface

Regression analysis data is shown in Table 2. ANOVA analysis is shown in Table 3. For acquired regression model, correlation coefficient is 0.9475. According central composite design, mass loss (m) is equal to:

(1) [g] with A as preheat temperature and B as impact speed of molten aluminium alloy.

Table 2 Regression analysis data

Standard deviation Mean Variation coefficient, % Predicted residual error sum of squares

0.00233791 0.01996154 11.712071

R-squared Adjusted R-squared Predicted R-squared

0.897804 0.846706 0.800966

8.5161∙10-5

Adequate precision

13.76138

Table 3 ANOVA analysis

Model A – Preheat temperature B – Impact speed A2 A2B Residual Lack of fit Pure error Corrected total

p-value

Sum of squares

df

Mean square

F-value

3.841∙10-4 1.695∙10-4 1.531∙10-4 3.897∙10-5 2.907∙10-5 4.373∙10-5 8.347∙10-6 3.538∙10-5 4.279∙10-4

4 1 1 1 1 8 4 4 12

9.604∙10-5 1.695∙10-4 1.531∙10-4 3.897∙10-5 2.907∙10-5 5.466∙10-6 2.087∙10-6 8.845∙10-6

17.57 31.01 28.02 7.13 5.32

Probability> F 0.0005 0.0005 0.0007 0.0283 0.0500

0.24

0.9046

Through regression analysis, regression model of predicted mass loss versus actual mass loss was acquired (Figure 6).

Figure 6 Predicted versus actual mass loss

Specimens were 3D scanned before and after the experiment. Models from 3D scanning were overlapped to easily determine specific wear areas (Figure 7). This alleviated defining the effect of each tribological wear mechanism responsible for mass loss.

Figure 7 Overlapped 3D scanned models of specimens 4, 6, 9, 10 and 13

4

Analysis

According to most previous research, thermal fatigue wear or so-called “heat checking” is one of the most important HPDC moulds wear mechanisms [9,20,23]. Temperature fluctuations cause the material on the surface to expand and contract. This can cause parts of the material to exceed the yield strength of the material near the surface. Local accumulation of plastic deformations leads to initial cracks. Propagation of initial cracks leads to separation of material particles from the surface. In this experiment, heat checking was noticed mostly on the sharp edges (Figure 8). Cracks often started at the edge and propagated perpendicular to the edge. There was significant spalling of the surface near the sharp edges. The reason for this could be oversaturation of nitrogen [17]. This is further analysed in the discussion section.

Figure 8 Wear near the sharp specimen edges 1, 2 and 3

Besides heat checking, erosion wear and adhesion wear were also expected. When simulating erosion by molten aluminium alloy, hard particles in the molten alloy (e.g. Al2O3, SiO2) had significant effect on the total wear. Therefore, the simulation was done using hard particles. For the numerical modelling of experimental setup, CFD techniques were applied using the commercial software ANSYS CFX 17.2. Different flow models can be used for the current problem but the most practical and most common belong to the group of two-equation turbulence flow models based on the Reynolds-Averages Navier Stokes (RANS) equations. In this paper, the k-ε two-equation RANS model was used [24] since it is robust, commonly used [25] and the current problem does not contain substantial separation which would require more advanced models. Two different computational domains were used in the paper because of different simulation requirements. In the first case, a cylindrical computational domain was selected as shown in Figure 9a. The diameter of the domain (x-z plane) was 500 mm, while the width (y-direction) was 30 mm from the airfoil symmetry plane. In most cases only one half of the airfoil was used with symmetry conditions applied on the symmetry plane. In the case which includes erosion modelling, both sides of the symmetry were used. The airfoil specimen (A) was fixed within the computational domain while the domain itself was rotating with the rotational speed corresponding to different experimental setup. The boundary conditions at the airfoil surface were no-slip wall. A pressure outlet with atmospheric pressure was set on the lateral surface of the cylindrical computational domain. In the case which includes erosion modelling, both sides of the symmetry were used as shown in Figure 9b. Additionally, the computational domain in this case requires a velocity inlet at which the solid particles initial positions are defined. The velocity at the inlet (vin) is linearly increased along the specimen height as it is also the case for the rotating domain. The pressure outlet (pout) was set on the opposite side while the remaining sides of the computational domain are set as free-slip wall.

Figure 9 CFD computational domain (A-airfoil specimen) used for: a) gaseous aluminium volume fraction and b) simulation of erosion rate density.

In order to select the appropriate mesh, grid independency study was conducted (see Figure 10). The basic grid was composed of tetrahedron-type elements, and the element size at the specimen surface was varied from 0.1 mm to 10 mm. Pressure values at points A and B (Figure 9) were monitored. The convergence of local pressure values (2% error) was obtained for the 0.25 mm mesh element size. This computational mesh contained about 4∙10 6 tetrahedron-type finite volumes or 2∙106 for the domain on one side of the symmetry plane. The mesh cross-section is shown in Figure 11. The y+ value was in the range (0.10) along the airfoil surface. Since scalable wall functions were used, low y+ values were not problematic.

Figure 10 Mesh convergence test

The use of the wall functions was acceptable since the flow separation was minimal i.e. fine resolution of the flow near the wall was not necessary.

Figure 11 Computational mesh near the airfoil

The fluid properties were selected for liquid aluminium at 700°C: density ρ=2350 kg/m3 and the dynamic viscosity η=0.00117 Pa s [26,27]. In the numerical models which included cavitation, Rayleigh Plesset model was used. The parameters of the model were the mean diameter (default value of 2∙10 -6 m was used), and the saturation pressure. For the saturation pressure, 1∙10-6 Pa was used, and it was calculated using [28], where the value was taken for aluminium at temperature of 700°C. In the case where the erosion was modelled, an additional inlet was added to the computational domain upstream from the airfoil specimen. The Finnie’s erosion model was used with the following parameters: mean particle diameter 30 μm, standard deviation 15 μm and number of particles at the inlet was 10 6 with random distribution with random distribution [29]. The model exponent n was set to n=2.3 and the reference speed was set to 952 m/s which is recommended for aluminium by ANSYS CFX documentation [30]. Remaining model parameters were set to the respective default value. ANSYS CFX 17.2 simulation was executed using molten metal impact speeds of: 6, 12 and 18 m/s, corresponding to the central composite plan (Table 1). Simulation showed that the most significant erosion wear by hard particles could be expected, on the front part of the specimen (Figure 12, A). When compared to real wear areas (Figure 7) it was determined that erosion wear appeared on the front part of the specimens where hard particles had a near perpendicular angle of impact on the surface of the specimens. Since the surface is nitrided and brittle, damage is more severe under normal impacts [31]. Significant wear was found on the top part of specimens. ANSYS CFX 17.2 volume fraction analysis for impact speed of 12 m/s determined occurrence of gaseous aluminium phase on specimen sides and on the top of the specimen (Figure 13). This indicates the possibility of cavitation erosion.

Figure 12 ANSYS simulation of erosion rate density for tip speeds: a) 18 m/s b) 12 m/s and c) 6m/s.

Figure 13 ANSYS CFX 17.2 simulation of gaseous aluminium volume fraction

Before cleaning the specimens, there were small quantities of adherent aluminium alloy. This could be due to mechanical anchoring or formation of intermetallic compounds. After the experiment was finished specimens were again immersed in aluminium alloy AlSi9Cu3(Fe) at 680 °C for 2 h. Specimens were cut at 10 mm from top and imaged by SEM (scanning electron microscope) to determine if there were fractures in the nitrided layer. Since the nitrided layer was found to be damaged at certain areas of the specimen, according to previous research [32] there is a possibility that intermetallic compounds were formed (Figure 14). For future research, EDX (energy-dispersive X-ray spectroscopy) analysis is necessary to determine if intermetallic compounds were formed on the material surface, between iron from the steel specimen and aluminium from the casting alloy.

Figure 14 Contact of casting alloy AlSi9Cu3(Fe) and mould material X38CrMoV5-1 in the area of the damaged nitrided layer

5

Discussion

Significant wear due to thermal fatigue was observed on the sharp edges of the nitrided specimens. Considering previous research [17], the reason could be the oversaturation of nitrogen on sharp edges. The simultaneous diffusion from two convergent directions on the sharp edge leads to oversaturation of nitrogen between the compound (“white”) layer and diffusion zone. Subsequently, iron nitride starts nucleating along grain boundaries in the diffusion zone [33]. This produces an iron nitride network formation on sharp edges [34]. The result is a very brittle edge that can prematurely fail by chipping or spalling during application. Evident chipping and spalling are shown in Figure 8. This shows the importance of geometrical shape of the HPDC mould. ANSYS CFX 17.2 analysis suggested existence of erosion on the front part of the specimen where the particle impact angle was almost perpendicular to the specimen surface. According to previous research considering hard particle erosion [35], the angle of impact is of major importance to the rate of mass loss by erosion. Material loss rate for ductile metals approaches maximum values at impact angles of 20° to 30° while loss rate for brittle materials reaches maximum values at impact angles around 90°. Another research [36] provided an explanation for crack development for brittle materials. Indenting particles cause elastic and plastic deformation of the brittle material. When the fracture threshold is exceeded, cracks develop. Final length is determined by the residual stress field resulting from the strain mismatch between the plastically deformed zone and its elastically deformed surroundings. Pressure drops on the top and on the sides of the specimen indicated the possibility of cavitation erosion. Cavitation is a result of formation and subsequent collapse of gas bubbles in the mould due to local differences in pressure. It is a common source of erosion in die casting, where molten metal speed is high at the casting gate (up to 60 m/s) [37]. It occurs when the vapour pressure of the melt is exceeded, usually in areas where molten metal makes sudden changes in direction. Cavitation erosion can be reduced or completely avoided by modifying geometrical features of the mould (larger radii etc.). 5

Conclusions

The purpose of this research was to determine the influence of preheat temperature of mould and molten aluminium alloy impact speed on total mould wear in high pressure die casting conditions. The approach used to investigate the problem was simulating high pressure die casting conditions from the plant “LTH Metal Cast”. Testing temperatures and impact speeds were defined and simulated using thermographic images of the mould during casting and simulation data used in the plant. Same type of mould material, casting alloy and lubricant was used for testing. A prototype wear testing apparatus was developed for this purpose. This apparatus will be used for future research since it allows testing of most

influential die casting parameters on wear. For example, preheat temperature, molten aluminium alloy impact speed, mould material, heat treatment, mould surface modifications, casting alloy, casting alloy temperature etc. As a result of the research presented in this paper, conclusions concerning the effects of preheat temperature and molten casting alloy impact speed on mould wear, within tested conditions, were as follows:  The preheat temperature directly affected the total wear of the mould material surface. By increasing the preheat temperature, total wear was decreased;  An increase to the preheat temperature above 200 °C caused a significant drop in wear for any impact speed of molten aluminium alloy;  An increase of molten aluminium alloy impact speed increased total wear;  Total wear is lowest (0.0108 g) at preheat temperature of 271.7 °C and molten aluminium alloy impact speed of 6 m/s;  Highest total wear (0.0291 g) is achieved at preheat temperature of 100 °C and molten aluminium alloy impact speed of 18 m/s;  Acquired mass loss model (1) indicated an interaction between the tested parameters, the preheat temperature and the molten aluminium alloy impact speed. It was also determined that nitriding the mould can make sharp edges susceptible to accelerated wear by thermal fatigue. With nitriding, sharp edges should be avoided by mould design to decrease oversaturation of nitrogen and premature wear. Within tested conditions, molten aluminium alloy speed had an almost linear relationship with total wear. For the lowest impact speed of 6 m/s, total wear increased at preheat temperatures over 300 °C. The reason for this could be faster propagation of cracks at elevated temperatures or more aggressive formation of intermetallic compounds. Since the nitrided layer was damaged, SEM analysis showed the possibility of intermetallic compounds formation (Figure 14). To determine if there was any adhesion wear, in future research, existence of intermetallic compounds should be determined by EDX analysis. ANSYS CFX 17.2 analysis determined erosion wear by hard particles on the front part of the specimens at simulated impact speeds. This was confirmed by 3D scanning. Wear areas were found on the front parts of the specimens. This indicated the substantial effect of hard particle erosion on total wear of HPDC mould. Acknowledgements Motivation for this paper came from collaboration with casting plant “LTH Metal Cast”. Moulds for HPDC are expensive and have a limited life span which makes this kind of research economically acceptable. This research was fully supported by the Croatian Science Foundation under the project 5371.

6

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Highlights -

Increase of preheat temperature was shown to decrease total mould wear Increase of molten aluminium alloy impact speed was shown to increase total wear Interaction between HPDC parameters was found Hard particles had most effect on erosion wear in HPDC conditions Most of heat cracking developed at sharp edges SEM analysis showed possible formation of intermetallic compounds