Conventional Heat Treatment of Additively Manufactured Alsi10mg

Conventional heat treatment of additively manufactured AlSi10Mg

Atilla Sarentica

Materials Engineering, master's level 2019

Luleå University of Technology Department of Engineering Sciences and Mathematics Contents

1 Introduction 5 1.1 Background ...... 5 1.2 Aims and scope of the project ...... 6 2 Additive manufacturing 7 3 Powder bed fusion 8 4 Selective Laser Melting 9 5 Aluminium 12 5.1 Background and processing ...... 12 5.2 Alloying elements ...... 12 5.3 AlSi10Mg ...... 13 5.4 Solidiﬁcation and microstructure ...... 14 6 Powder manufacturing 17 6.1 Powder characteristics ...... 19 6.1.1 Particle size distribution ...... 19 6.1.2 Flowability ...... 20 6.1.3 Reﬂectivity and absoptivity ...... 20 6.1.4 Thermal conductivity ...... 20 6.1.5 Oxidation ...... 20 6.1.6 Humidity and hydrogen ...... 20 6.1.7 Particle shape and morphology ...... 21 7 Conventional Heat treatment 21 7.1 Annealing ...... 22 7.2 Solution Heat Treatment ...... 22 7.3 Quenching ...... 23 7.4 Precipitation Hardening (Age Hardening) ...... 24 7.5 T6 Heat treatment ...... 25 8 Mechanical properties of SLM components 26 9 Materials and Method 28 9.1 Samples ...... 28 9.2 Heat treatment ...... 29 9.3 Hardness measurement ...... 32 9.4 Microstructure ...... 33 9.4.1 Sample preparation ...... 33 9.4.2 Microstructural analysis ...... 33 9.5 Tensile test ...... 34 10 Results 35 10.1 Hardness measurement ...... 35 10.2 Tensile test ...... 37 10.2.1 Ultimate tensile strength (UTS) ...... 37 10.2.2 Elongation ...... 38 10.3 Microstructure ...... 39 10.3.1 As-cast condition ...... 39 10.3.2 Cast T6 ...... 40 10.3.3 As-built condition ...... 41 10.3.4 AM T6 ...... 44 10.3.5 AM T4 ...... 45 10.3.6 AM T5 ...... 46 11 Discussion 47 12 Conclusions 49 13 Future work 50 14 References 51 List of Figures 57 List of Tables 58 Abstract

Literature has shown that additively manufactured Al-alloy components tend to exceed in mechanical properties compared to the material manufactured by conventional methods. For cast aluminium, however, a post process in form of a heat treatment is usually necessary in order to further increase the mechanical properties of the alloy. As-cast conditions are rarely seen in service due to their poor properties and therefore a heat treatment is often performed. This report inves- tigates additively manufactured AlSi10Mg alloy when heat treated in comparison to the same alloy when cast.

Fifteen additively manufactured samples of the alloy were produced by the selective laser melting (SLM) process and six were cast. Heat treatment mainly consisted of the conventional T6 heat treatment, which includes solution heat treatment, quenching and artiﬁcial aging. However, T4 (solution heat treated, quenched and naturally aged) and T5 (artiﬁcially aged after manufacturing) were also investigated in this thesis. Hardness properties, tensile strength, elongation and microstructures were analyzed.

It was found that for the additively manufactured samples, the hardness values and the ultimate tensile strength decreased significantly in comparison to the as-built samples that were not heat treated. The elongation did however increase after heat treatment, where the T4 and T5 heat treated almost doubled in elongation in comparison to the as-built sample. For the cast samples, however, the results showed the complete opposite where the as-cast condition showed poor hardness at tensile strength values but increased significantly after heat treated, whilst the elongation was poor for both heat treated and as-cast condition. The microstructure for the cast samples showed Mg2Si precipitates dispersed throughout the aluminium matrix which caused the enhanced values in mechanical properties. For the additively manufactured samples, however, a different microstructure was obtained due to the different manufacturing method. SLM results in a rapid cooling rate which yield a fine microstructure around the heat affected zone, which consists of a coarse microstructure. This fine microstructure is the reason for the high mechanical properties in comparison to the heat treated and cast counter-parts. When solution heat treatment is performed, the microstructure changed completely and the fine regions disappeared, which explains the decrease in hardness and UTS values. The T5 heat treated sample did however show a similar microstructure as the as-built, but further investigations has to be done in order to conclude the decrease of mechanical properties for the T5 AM sample.

2 Acknowledgements

The author would like to express his gratitude to his supervisor Conny Svensson for his constant guidance, support and valuable discussions. I would like to thank my supervisor from LTU, Pia Åkerfeldt for good input during the project and for the feedback prior to the presentation of the thesis. I would also like to thank Alumbra AB, located in the same building as Saab Surveillance in Järfälla, for helping out with the post-milling procedures of my samples to obtain same dimensions for the cast and the AM samples as well as to obtain the required waist to perform tensile testing. Both Lasertech and Reftele Gjuteri has played a big part in this project by providing me the samples used for the thesis and I would hence like to express my gratitude to these companies. Also, thank you to the company Exova Elements in Linköping for conducting the tensile test. A special thank you to Karin Fröderberg, manager of the Material & Analysis department at Saab Surveillance for making this master’s thesis possible. Finally, I would like to thank all the colleagues and people at Saab for the constant assistance and valuable discussions through the project.

3 Abbreviations

AM Additive Manufacturing CAD Computer Aided Program PBF Powder Bed Fusion SLM Selective Laser Melting EBM Electron Beam Melting SLS Selective Laser Sintering BCC Body-centred cubic FCC Face-centred cubic SHT Solution Heat Treated GA Gas Atomization PA Plasma Atomization PREP Plasma Rotationg Electrode Process UTS Ultimate Tensile Strength HAZ Heat Aﬀected Zone

4 1 Introduction

Additive manufacturing (AM) has received great attention over the last decade due to its few limitations when it comes to design, making it a manufacturing method with a high design freedom. More industries are adopting AM and research is being done in order to replace components manufactured by conventional methods. However there are some limitations with AM and for the powder bed fusion techniques, one of the drawbacks is the build volume. It is not possible to manufacture large structures and components, and for the powder bed fusion techniques, the resulting component usually has a high surface roughness. Another flaw is that AM is time consuming compared to conventional methods and it may not be as cost-effective. In recent years, additive manufacturing has gain world wide attention due to its phenomenal ability to produce near-net-shape components with complex shapes [1, 2]. As a result of the vast research going into AM, the technique is be- coming cheaper by every second. Additive manufacturing has grown into a powerful tool and is used at many industries today, such as within the aerospace, automotive and medical industries to name a few. Complex shaped components such as air-inlets and components with inner channels require a lot of post work if manufactured by conventional methods, making it non-time efficient and expensive. AM offers a new method to manufacture the same components without the need of post-work which in some cases may even be cheaper and more time efficient compared to conventional manufacturing methods. There are several categories within AM but the principle with AM is that the components is produced by joining material in a layer-by-layer manner [1].

1.1 Background

This thesis is a part of the of the Materials Science & Engineering program at Luleå University of Technology, E7008T: Degree project in Materials Technology, Masters of Science in Engineering, conducted in the spring of 2019 at Saab Surveillance in Järfälla. The course contains a 20 week long thesis project and is a part of the project research at the Material & Analysis department at Saab Surveillance, supervised by Material Engineer Conny Svensson (Saab) and Associate Senior Lecturer Pia Åkerfeldt (LTU).

Cast AlSi10Mg alloys are characterized to have good mechanical property, good weldability and good corrosion resistance as well as having a high strength-to-weight ratio. This is usually due to the Mg2Si precipitates that forms during the solution heat treatment of the alloy [3]. These precipitates agglomerate when artificially aged, which strengthens the alloy even further. Additively manufactured material offers different microstructures and investigation of heat treatments for the same alloy, AlSi10Mg, has been conducted in order to compare the results with the cast parts. The same, conventional heat treatment has been conducted to the cast and AM materials. The main focus with the thesis is the T6 heat treatment (solution heat treated, quenched and artificially aged), whilst some investigation was also done regarding T4- and T5 heat treatments (T4: solution heat treated, quenched, naturally aged. T5: artificially aged after manufacturing).

5 1.2 Aims and scope of the project

The project consists of the following parts where the aim is to compare the mechanical properties of heat treated AM an cast AlSi10Mg alloys.

• The samples were delivered in as-built and as-cast conditions. • Sample preparations for analysis and tensile testing. • Pre-analysis of hardness properties and microstructure for as-built and as-cast samples.

• Perform T4, T5 and mainly T6 heat treatment on the samples. • Post-analysis of hardness properties and microstructure for the heat treated samples. • Tensile testing for both heat treated and as-built as well as as-cast samples. • Summarize and compare the results obtained during the project.

• Discussion regarding the results and future work that can be done.

6 2 Additive manufacturing

Additive Manufacturing (AM) refers to a process which digital 3D design data is used to build a component in layers by depositing material. The term ’3D printing’ is increasingly used as a synonym for AM. This type of manufacturing method has gained great attention over the last decades due to the possibilites of producing near net-shaped objects with complex geometries. The manufacturing method is seemingly new but it was introduced and it began development as early as during the late 80s and early 90s. However, it is due to the availability of Computational Aided Design (CAD) over the last decade that AM has received remarkable attention. AM usually consists of a computer with a 3D-modelling software such as CAD, machine equipment and layering material, resulting in an exceptional automation process. AM has a growing library of ’printable’ materials such as polymers, metals, ceramics, concrete and human tissue [1]. AM is applied in conjugation with rapid prototyping but has since grown into a powerful manufacturing tool within series production. The strength of AM lies in those areas where conventional manufacturing reaches its limitations. It provides a high degree of design freedom and is used for components where complexity, weight and stability is of high importance, resulting in a manufacturing method with minimal constraints. The benefit with AM is also that it offers an environmentally friendly option compared to conventional methods as a result of its very high material utilization, making it energy efficient [4]. Some industries prosper using AM as their manufacturing process for components. The aerospace industry was one of the first industries to adopt AM, where components are exposed to high performance standards. The buy-to-fly ratio is one of the most important aspects within the aerospace industry and AM delivers complex, consolidated parts with high strength properties and light weight and is hence a hot topic within the aerospace industries for both conventional and military applications. Additive manufacturing is also a hot topic in the medical industry. The industry utilizes high strength material using AM to produce life saving components. Some metals are bio-compatible which makes AM an exceptional tool due to its precise results for complex structures. Some applications within the medical industries includes orthopedic implant devices, dental devices, pre-surgery models from CT scans, some custom saw and drill guides as well as specialized instrumentation where complexity is of highest importance [5]. As a result of AM components being able to withstand harsh envi- ronments and extreme conditions due to the production of high strength materials, the process has received great attention in the energy sector. Some applications within the energy sector includes rotors, stators, turbine nozzles, down-hole tool components and pump manifolds for the gas and oil industry [6]. AM may also be found within several other industries such as the automotive industry and other transportation industries for vehicles as well as for consumer products. AM is still a new, hot topic in the industry and it is constantly being developed to produce environmentally friendly, high performance components for future generations and it will be exciting to see what holds for the future of additive manufacturing. There are several AM process available for the fabrication of metals, such as through binder jetting, metal extrusion, sheet lamination and directed energy deposition. One of the most common process within Metal-AM, however, is through powder bed fusion, either by selective laser melting (SLM) or electron beam melting (EBM) [7].

7 3 Powder bed fusion

In additive manufacturing, the two most important parameters are the raw material and the energy source used to form the component. In powder bed fusion (PBF) the energy source is through a laser- or an electron beam. Regarding powder bed fusion, SLM is the process most widely used within the AM industry and fabricates the component by fusing metal powder together using a laser beam. The first layer is placed on a moving platform where a high power density laser will melt the powder. After the scan is complete, the moving platform will be lowered so that the next layer may be scanned with the laser. This process is performed layer-by-layer until the desired component is formed. The technique is an advanced form of the selective laser sintering (SLS) process where one or more lasers are used to melt the powder bed. The system usually consists of a fiber laser with a capacity to selectively melt the powder. An inert gas in form of either Argon- or Nitrogen gas is present depending on the whether a reactive or a non-reactive material is manufactured, respectively [8]. This is a totally different environment in comparison to EBM where the building chamber environment consists of vacuum to prevent the electrons of the beam colliding with other particles and elements. Feature differences between SLM and EBM are represented in Table 2.

Table 2: Feature comparisons between SLM and EBM [8].

SLM EBM At least one ﬁber laser High power Electron beam Power source (200-1000 W) (3000 W) Build chamber environment Argon or Nitrogen Vacuum Method of powder preheating Platform heating Preheat scanning Powder pre-heating temperature 100-200 700-900 (◦C) Max. build volume 500 x 350 x 300 350 x 380 (mm) Maximum build rate 20-35 80 (cm3/hr) Layer thickness 20-100 50-200 (µm) Melt pool size 0.1-0.5 0.2-1.2 (mm) Surface ﬁnish - Roughness 4-11 25-35 (Ra value (µm)) Geometric tolerance ± 0.05-0.1 ± 0.2 (mm) Recommended powder size 20-45 45-106 (µm)

During the processes in PBF a pre-heating is performed in order minimize the cooling rate if desired where EBM scans with the beam during pre-heating whilst SLM pre-heats the building-plate [9]. Both SLM and EBM are versatile where many materials can be used for manufacturing, including

8 the production of amorphous materials as a result of the high cooling rates. Advantages with SLM, however, are the possibilities of tuning properties which inﬂuences the mechanical properties as well as the relatively low cost of SLM in comparison to EBM, resulting in SLM being the most used process of the two [7].

4 Selective Laser Melting

Selective Laser Melting (SLM) is a powder bed fusion process in additive manufacturing developed by Dr. M. Fockele and Dr. D. Schwarze of F & S Stereolithographietechnik GmbH, with Dr. W. Meiners, Dr. K. Wissenbach and Dr. G. Anders of Fraunhofer ILT in order to produce metal components of metallic powder using rapid prototyping [10]. SLM is a subcategory of Selective Laser Sintering (SLS) where the diﬀerence is that in SLM the metallic powder is fully melted whilst in SLS it is not. In SLS the powder is heated to a temperature so that the particles fuses together but melting is not achieved, making it possible to control the porosity of the material. The process of SLM consists of utilized CAD data guiding a high intensity laser which melts regions of metallic powder, layer by layer, until the desired component is created. A thin layer of powder is laid on a pre-heated platform and a high energy density laser is used to melt and fuse selected areas according to the processed data. Once the laser scan is complete, the platform will be lowered and a new layer of powder is placed, scanned with a laser for melting and fusion and this process is performed repeatedly until the last layer has been fused. Once the laser scanning process is completed, loose powders are removed from the building chamber and the component is revealed [10]. The process in the building chamber is presented in Figure 1. A gas of either Argon or Nitrogen is used during the whole process in order to provide an inert atmosphere which protects the heated powder from oxidation. The powder size regarding SLM can be up to 100 µm in diameter but often results in poor resolution and build tolerance. Too small powder sizes tend to agglomerate as a result of the van der Waals forces which results in a poor powder ﬂowability and hence a poor powder deposition. The recommended powder size for SLM is said to be between 20-45 µm [11].

Figure 1: Illustration of the SLM process [10].

9 Important process parameters in SLM are laser power, scanning speed, hatch spacing and the layer thickness. A balance between the parameters is desired to obtain a component of high quality where some tweaks regarding the mechanical properties may be adjusted. A combination of low laser energy, high scanning speed and low layer thickness results in a phenomenon known as ”balling” due to lack of wetting of molten pool within the layers [12]. The balling phenomenon is formed when the molten laser track possesses a shrinking tendency, which occasionally occurs when a low laser energy is used. Issues emerging when the balling phenomenon is present are that the surface roughness tend to increase, requiring the component to be polished for the increased surface roughness to be reduced [13]. This counters the design freedom of additive manufacturing as a whole and even though some kind of chemical milling may be used to reduce the surface roughness, new problems such are pores from the chemicals arise. A large number of pores tend to form between many discontinious metallic balls which results in an decrease of mechanical properties [14]. A high laser energy and low scanning speed, however, results in an extensive material evaporation and the keyhole phenomenon [15]. Hatch spacing has to be taken into account as well. Poor hatch spacing has a tendency to result in particles not fully fused together. The SLM process parameters are illustrated in Figure 2.

Figure 2: SLM process parameters [10].

The SLM process has a maximum build rate of 20-35 cm3/hr and the maximum part size (build volume) that can be produced is 500 x 350 x 300 mm which increases the part cost and limits the application for small sized parts. Machine manufactures and research institutes are constantly focusing on expanding the capabilities of their machines in order to apply SLM for larger part components in the future. One approach of doing this is to use several high power lasers for an increase in the build rate. In some cases, three to four lasers are used simultaneously to increase the build rate when increasing the build volume of the machine [16].

10 One of the largest beneﬁts with SLM is its wide library of materials that can be chosen for manufacturing. It can be used to manufacture parts from materials made out of stainless steel, tool steel, low-expansion alloys, intermetallic compounds, aluminium, copper, beryllium, Ni-based and Co-based superalloys and other hard metals [18]. All the materials that are used within SLM is represented in Figure 3, where it is possible to note that even non-metallic materials such as ceramics and diﬀerent composites can be manufactured using SLM. The main focus in SLM are metallic materials where steel and titanium accounts for over half of the SLM production [10]. The parameters for SLM will also vary depending on what material is used for manufacturing. For example, regarding aluminium alloys, it has been found that quality of the SLM component depends on the powder morphology and content. Results found that small and spherical powders with higher silicon content (AlSi10Mg alloy) gives a higher relative density compared to larger and irregularly-shaped powders [17].

Figure 3: Materials used for manufacturing within SLM [10].

Regarding the material used within SLM, they are usually manufactured through conventional methods such as extruding, casting and forging, but as a result of the high melting temperature for metals such as titanium, these methods introduce several problems. By additively manufacturing using Ti-powders in SLM, using a high power laser beam and an inert gas environment it is possible to curtail many of the issues. One drawback with the PBF methods is the resulting surface roughness obtained from manufacturing usually being high as a result of unmelted powder on the surface. Several post-treatments can be performed to reduce this if so desired such as chemical milling or polishing [19]. In addition to the previously mentioned balling phenomenon, another disadvantage regarding SLM is molten pool instabilities that may cause selective laser melt failures. The liquid- solid transition zone tend to experience shrinkage as a result of phase transition. There are also studies showing tearing and crack formations in copper alloyed materials due to residual stress formation due to shrinkage within the melt pool during manufacturing [20].

11 5 Aluminium

5.1 Background and processing

Aluminium (Al) alloys are known for their wide ranged of properties and is widely used within industries all over the world. Aluminium is characterized for its tensile strength, relative low density, ductility, formability, weldability as well as its corrosion resistance. Due to the low density of aluminium it is widely used in application where low weight is of highest priority. Regarding the aerospace and automotive industries, Al-alloys have been used ever since the introduction of these materials due to their high strength-to-weight ratio [21]. The metal can be remelted without losing durability or modifying its original properties, allowing aluminium to become a high-value recycled commodity. Due to this, together with the previously mentioned light-weight properties and its structural strength gives aluminium a unique role as a metal in infrastructure and applications of sustainable communities. Aluminium production was introduced late compared to other metals where aluminium was valued higher than gold before the 1850s due to the difficulties of refining it. Despite this, today aluminium is produced more than all of the non-ferrous materials combined [22]. Aluminium is produced with several steps, where the ore in form Bauxite (Al(OH)3) is grounded and mixed with sodium hydroxide and dissolved where the precipitation of the resulting solution is washed and heated to eliminate water. The remains is melted using an electrolytic process (Hall-Hérlout process) to obtain aluminium metal. The metal is then mixed with other metals to form alloys, where different elements are added depending on the desired properties [23]. Industries such as the aerospace and automotive industries are two examples that has adopted AM for producing Al-alloy components. However, processing Al-alloys with SLM can be challenging due to the material’s high reflectivity and thermal conductivity [24]. Studies have been conducted with parameter optimization to produce close to fully dense parts from Al-alloys using SLM which has curtailed many of the issues concerning Al use within SLM [25].

5.2 Alloying elements

Aluminium components are manufactured using several methods such as extrusion, forging, casting and through additive manufacturing. Additive manufactured aluminium alloys consists of casting alloy compositions such as AlSi10Mg, AlSi12 and AlSi12Mg, to name a few [26]. Elements such as Cu and Mg are commonly added to improve the strength properties of the alloy in both room- and elevated temperatures. It also allows the alloy to be heat-treated when these elements are present. What governs the mechanical properties of these alloys are the size, morphology and the distribution of microstructural features of particles added [27]. The most important alloying element for aluminium is silicon (Si), where the Si content often varies between 5 to 12 wt%. Si improves aluminium alloys as such that it has the ability to readily ﬁll dies as well as to solidify casting with no hot tearing or no hot cracking issues. High Si content present in the alloy results in a lower thermal expansion coeﬃcient. The addition of Mg in the alloy enhances the tensile properties at elevated temperatures, making the availability of the alloy extending to higher temperatures. This also makes the alloy heat-treatable to enhance the properties even more by reducing or completely

12 removing internal stresses. The presence of a very hard Si phase makes it diﬃcult to machine the material, however, and therefore casting is often used for Al-alloys containing Si. Additive manufacturing makes it possible to manufacture these materials with minimal wastage of and it is also possible to manufacture components with complex structures of alloys consisting hard-phase elements such as Si.

5.3 AlSi10Mg

Aluminium-Silicon alloys are characterized by good mechanical properties as well as good weldability and excellent corrosion resistance. They also have a high strength-to-weight ratio which makes it an attractive metal in the aerospace-, automotive and other domestic industries and the alloy also has a very high thermal conductivity. Alloying magnesium to the Al-Si alloys enables Mg2Si precipitation in the metal-matrix which increases the strength of the alloy as well as increasing other mechanical properties. Eutectic Al + Si phase present in the alloy may significantly affect its ductility and strength but it also makes this material difficult to machine. Taking that into consideration as well as the low shrinkage and relatively low melting temperature of AlSi10Mg alloys results in the material being mostly used within casting. The properties of the material, however, gives an opportunity to apply it in a manufacturing process based on additive manufacturing. The chemical composition of AlSi10Mg is represented in Table 3 where approximately 9-11% of Si content and 0.2-0.45% of Mg content is present in the alloy, together with small portions of Cu, Zn and Fe content. The maximum useful limit of Mg content in the alloy is approximately 0.7 wt% where higher content than that would result in softening the metal-matrix of the alloy [34].

Table 3: Chemical composition of the AlSi10Mg alloy.

Elements Al Si Cu Mn Mg Zn Fe wt % Balance 9-11 ≤ 0.05 ≤ 0.45 0.2-0.45 ≤ 0.10 ≤ 0.55

Table 4: Properties of as-built AlSi10Mg [41].

Properties Value Density 2.68g/cm3 Tensile Strength 430-470 MPa E-Modulus 50-70 GPa Yield Strength 215-245 MPa Thermal Conductivity 160W/m.K Speciﬁc Heat Capacity 860-960 J/(kgK) Relative density (AM) >99.5%

AlSi10Mg alloys are light density alloys with excellent mechanical properties. The density of the alloy is 2.68g/cm3 and can be compared with other known metals used in same applications, such

13 as pure iron that has a density of 7.874g/cm3. The alloys also have a high thermal conductivity and heat-treated alloys contains excellent hardness and strength properties. Some properties of AlSi10Mg is listed in Table 4.

5.4 Solidiﬁcation and microstructure

Solidification of pure metals in general is rarely encountered in practice. Instead, several elements are usually present within the alloy at different temperature and content of the elements. Regarding aluminium alloys, the most common being Al-Si, contains a so called ”anomalous” solidification form rather than a normal solidification form when it comes to eutectic solidification. An eutectic solidification is when a binary eutectic composition of two solid phases form cooperatively from the liquid, i.e. L → α + β. An anomalous structure occurs when one of the solid phases is capable of faceting, meaning that it has a high entropy of melting [28]. The phase diagram for Al-Si is represented in Figure 4.

Figure 4: Phase diagram of Al-Si [29].

For Al-Si alloys this is the case due to Si being present in the alloy which results in coarse flakes of Si in the microstructure that promotes brittleness within the alloy. For the alloy to have any great use the properties of these brittle Si-flakes has to be improved. This can be done by adding a small quantity of a ternary element to modify the microstructure. This will move the eutectic point to a higher silicon concentration and a lower temperature and it also modifies the growth of eutectic silicon in such way that irregular spherical shaped Si particles will be formed with improved properties is established rather than the brittle Si-flakes. Al-Si alloys solidify by a primary precipitation of dendrites. As the liquid begin to cool and goes below the freezing point, small

14 crystals of solid starts to grow within the liquid [29].

During the process of additive manufacturing, rapid solidification yields an expected fine microstructure. The control of the solidification is important to control the resulting mechanical properties of the part. When the component is produced, each laser scan differs with a 67◦ angle in order to obtain homogeneous properties throughout the material. This type of scan is known as a Meander scanning strategy, where each layer is scanned with a 67◦ angle in regards to the previous layer. This results in a complex pattern of variations in apparent melt pool width and overlap on a vertical section through the built part [30]. However, residual stresses is a drawback when it comes to additive manufacturing. It is a natural result of the rapid heating and cooling that is inherent to the laser powder bed fusion process. When a new layer of powder is embedded on top of the previous one, both layers will be melted and fused together. The heat will flow from the hot weld pool down into the solid metal below and the solidify [31]. The microstructures observed will have an ellipsoid shape in the horizontal cross-section and half-cylindrical in the vertical section. These will be equal to the laser spot diameter and are hence laser tracks on the specimen containing the microstructure. Figure 5 illustrates half-cylindrical shapes of the laser tracks obtained as the microstructure during additive manufacturing. Various research studies has been carried out to get a more concrete understanding additively manufactured, as-built microstructures. Regarding aluminum, these microstructures consists of fine dendritic microstructures of sub-micron size that is specific to the SLM process as a response to the instant melting and rapid solidification during the laser irradiation [32].

Figure 5: Illustration of the microstructure obtained after selective laser melting [32].

When it comes to cast aluminium, A356 and A357 alloys are two of the most widely used casting alloys within the automotive and aerospace industries. The casting process used is what strongly

15 influences the final properties of the material. Some other influencing factors are the chemical additions made to control eutectic structure, primary silicon and grain structure, and molten metal treatment to reduce hydrogen gas content and remove inclusions. When it comes to the grain structure of ingots and cast aluminium alloys, small equiaxed grain structure is usually preferred [28]. These grain structures improves casting attributes such as resistance to hot tearing and mass feeding and it also enhanced the mechanical properties and surface finish characteristics. The increase in soundness (measure of impurities and/or discontinuities) is a big influence in obtaining improved properties. An important contributor to this is the overall increase in the homogeneity of an equiaxed casting compared to a nonuniform columnar grain structure. This is because a more homogeneous structure results in less segregation causing the casting to respond better to heat treatments, and defects such as porosity and intermetallic constituents are more uniformly distributed and therefore less harmful [28]. Figure 6 shows a schematic of a cast grain structure with equiaxed grains and columnar grains.

Figure 6: Microstructure obtained of cast ingots [28].

When AM is used to manufacture components, the component will be subjected to a complex thermal cycle. This cycle involves a rapid heating above the melting temperature due to the ab- sorption energy of the laser or electron beam and its transformation into heat, followed by a rapid solidiﬁcation. This cycle is continuous because several layers are manufactured on top of each other and fused together. As a result of this, AM components tend to have meta-stable microstructures and non-equilibrium compositions of the resulting phases and therefore metal components manufactured through AM shows a unique, ﬁne-grained microstructure with a grain size at nanoscale.

16 The fine microstructure is dependent on the machine used in manufacturing and can easily be al- tered by changing the influencing parameters such as the energy density, thickness of each layer, pre-heating of the building plate as well as the building chamber environment. The geometry of the AM component may also affect the microstructure where different microstructures can be observed at different parts of the component, especially between the bulk material and the surface [35].

Table 5: Typical conditions selected for SLM with resulting grain sizes.

Energy Pre-heating Layer Thickness Cooling rate α-lath width AM Method Density temperature [µm] [K/s] [µm] [J/mm3] [◦C] SLM 20-150 <100 200-500 103 − 108 <<1 µm (α0)

(a) SLM manufactured AlSi10Mg. (b) Cast and etched AlSi10Mg

Figure 7: The microstructure of (a) SLM AlSi10Mg, (b) cast and etched AlSi10Mg [36].

Figure 7a shows typical microstructure of SLM manufactured AlSi10Mg where a coarse microstructure is usually observed in and around the heat aﬀected zone (HAZ) whilst the surrounding areas shows a ﬁne microstructure. Figure 7b shows cast and etched AlSi10Mg which consist of aluminum dendrites, Al-Si eutectic as well intermetallic phases.

6 Powder manufacturing

Commercial powder manufacturing of Al-alloy powder may be done by several processes but the most common for metal-AM powders are gas atomization (GA), plasma atomization (PA) and plasma rotating electrode process (PREP) [37]. AlSi10Mg alloys are commonly used in industrial applications due to their unique performance as mentioned previously. Recently, the rapid development of additive manufacturing and other metallurgical powder techniques such as powder

17 injection molding, laser forming, warm flow compaction and thermal spraying technologies demand more resilient properties of the AlSi10Mg powder, such as more uniform particle size and higher sphericity flowability. This is a difficult task for commercial AlSi10Mg powder due to its low flowability caused by their irregular shape [38]. The mostly used technique to obtain spherical powder is plasma atomization and by performing the atomization under appropriate feeding rate and flow rate of the carrier gas it may result in a spheridization rate of almost 100%. During the manufacturing, the raw material is firstly inspected and measured in order to set correct parameters for the atomization process [38]. The inspected raw material, in form of a wire is fed into the atomization unit and plasma is used to simultaneously melt the wire and atomize the meld into spherical powder. The plasma torch may reach a temperature up to 10.000◦C and the cooling rate varies between 10-1000◦C. The atomized powder is then collected from the base of the reactor and then sieved to classify the powders particle size distribution. The next step is to blend and homogenize multiple atomized batches into a final powder lot before a final inspection is performed [38]. The plasma atomization process is illustrated in Figure 8.

Figure 8: Illustration of the plasma atomization process [39].

The recommended powder size for SLM machine is a size of 20-60µm but it is highly dependent on the machine and material used. For AM applications the ideal powder should consist of high sphericity, good ﬂowability, no gas-bubble porosity and a pure chemical composition. The particle size is a critical yield parameter and may be controlled by the feed rate and the diameter of the pre-alloyed wire, gas pressure, attack angle of the plasma and the distance between the wire and the plasma torch. Figure 9 shows spherical powders obtained by the plasma atomization process.

18 Figure 9: Spherical AlSi10Mg powder [40].

6.1 Powder characteristics

There are several powder characteristics and their influences on the additive manufacturing process and the component used in service. Appropriate parameters has to be set in order to avoid effects such as a lack of fusion, balling, high porosity and evaporation of alloying elements. The powder material itself has an important influence on the resulting part as well as the surface finish and hence it is not possible to view AM processing separately without considering powder production, powder handling and transport. As a result of Aluminum having a high reflectivity and a high thermal conductivity compared to materials such as stainless steel it is complicated to make process improvements. As mentioned previously, spherical shaped powders are preferred in order to obtain a high flowability which results in a uniform powder bed [43]. If the particle size varies a lot it will result in a different distribution, packing density and also increase the surface roughness of the component. It is also important to control oxygen formation since it may result in oxide residues and hydrogen content can result in hydrogen pores if the melt solidifies faster than the gas evaporates [46]. The influential powder characteristics from article written by S. Dietrich et. al (Table 1) A New Approach For A Flexible Powder Production For Additive Manufacturing is listed below where powder size distribution, flowability, reflectivity and absorptivity, thermal conductivity, oxidation, humidity and hydrogen as well as particle shape and morphology is characteristics are brought up.

6.1.1 Particle size distribution

• The maximum particle size determines the minimal powder layer thickness [45]. • Regarding balanced particle size distribution, however, it has a positive effect on the packing density and powder compatibility as a result of small particles filling small voids [44, 47, 48]. • Small particles are light and get easily thrown out of the process zone, furthermore as a result of their surface to volume ratio they are more likely to inflame or explode if making contact to reactive gas [45].

19 • Small particle size and narrow particle size distribution leads to uniformity in the melt pool, resulting in a higher part density [44, 45, 47]. • The powder bed density inﬂuences the resulting heat transportation and thus the heat balance [45].

6.1.2 Flowability

• The ﬂowability aﬀects the layer deposition and layer quality (e.g. homogeneous layers) [49].

6.1.3 Reﬂectivity and absoptivity

• The laser energy is absorbed or reﬂected in the process chamber by the powder bed [45]. • The absorptivity of a powder depends on the wavelength of the laser and the condition of the powder bed [45]. • High reﬂectivity of the powder bed (>91% for aluminum) requires an increase in laser power [46, 50, 51].

6.1.4 Thermal conductivity

• A high thermal conductivity increases the required laser powder [50, 51] which result in a rapid dissipation of heat away from the melt pool [46].

6.1.5 Oxidation

• Oxides will form during the melting process with an oxygen level between 0.1 to 0.2 % [46]. • The most predominant factor of controlling the ﬂowability is the amount of surface oxides on the particle surface [52].

• The rupture of oxide layer of small powder particles is more diﬃcult [53].

6.1.6 Humidity and hydrogen

• Molecular layers (either single or multiple) can form on the particle surface, leading to hydrogen bonding [54, 55].

20 • Increased interparticular forces result in an decrease in ﬂowability [47, 55, 56] and the metal microstructure of the part [44]. • Hydrogen porosity can occur on the particle surface and in the powder material [54].

6.1.7 Particle shape and morphology

• Common powder defects are through irregular shapes, e.g. elongated particles, hollow or porous particles [43] that affects flowability [47]. Spherical particle morphology is beneficial for powder flowability and helps to form uniform powder layers [43]. • Excessive amounts of large pores or pores with entrapped gas can affect material properties [43]. • Stacking density is a function of powder morphology [45]. • Surface roughness affects the absorptivity [45].

7 Conventional Heat treatment

Post treatments of cast and wrought aluminium alloys are performed to increase the mechanical properties of the alloy. Usual treatments consists of heating and cooling to obtain recrystallization, diﬀerent microstructures as well as get rid of impurities during manufacturing which results in an increase of strength and hardness of the alloy [57].

When a molten metal solidifies the atoms arrange themselves into definite patterns (crystal structures), where body-centered cubic (BCC) and face-centered cubic (FCC) are the most common ones found within metals. These crystal structures grow uniformly in all directions within each developing crystal. As the metal cools these crystals are confined by the adjacent developing crystals, forming grains and intersections within these, known as grain boundaries. Because the grain form independently their crystal structures develop tilted in various directions [58]. All atoms in these crystalline structures are held in place by electromagnetic attraction to neighbouring atoms. If a force or a load is applied to a metal, these electromagnetic bounds stretch, allowing the atoms to move slightly. When the load is removed the bonds pull the atoms back into position. When a high enough force is applied that exceeds the metals yield strength, however, the bonds between these atoms will break and deformation will occur. In order to strengthen the metal and to increase the yield strength a heat treatment is performed on the metal to control the melt and solidification, obtaining desired structure. Alloying elements can also be added to further increase the the strength. The addition of an alloy introduces foreign atoms within the crystal structure of the base metal, disrupting the structural uniformity, resulting in increased strength [58].

The diﬀerent heat treatments can be divided into four categories, where annealing, solution heat treatments, quenching and age hardening can be performed diﬀerently to modify the resulting

21 properties. Non-heat treatable aluminium alloys are dependent on the alloying elements to increase its strength [59]. There are possibilites to further increase the strength for non-heat treatable alloys with post treatments such as cold working or strain hardening. Strain hardening is often followed by heating to stabilize, ensuring that the ﬁnal mechanical properties does not change over time. Heat-treatable alloys on the other hand consists of elements that exhibits increasing solid solubility in the aluminium and it is therefore possible to strengthen the alloy by subjecting them to elevated temperatures, quenching and age hardening [59]. The ten most common designations with these steps are listed in Table 6.

Table 6: Temper designations of post-treated Aluminium alloys.

Index Description T1 Cooled from hot working and naturally aged (at room temperature) T2 Cooled from hot working, cold-worked and naturally aged T3 Solution heat treated and cold worked T4 Solution heat treated and naturally aged T5 Cooled from hot working and artificially aged (at elevated temperature) T6 Solution heat treated and artificially aged T7 Solution heat treated and stabilized T8 Solution heat treated, cold worked and artificially aged T9 Solution heat treated, artificially aged and cold worked T10 Cooled from hot working, cold-worked and artificially aged

7.1 Annealing

Annealing is one of the post-treatments performed on aluminum alloys and is applied to promote softening. After cold working of aluminum, the dislocated structure will be less stable compared to the strain-free, annealed to which it tends to revert. The process is used for both heat-treatable and non heat-treatable alloys to increase the ductility with a slight reduction in strength. Annealing is carried out at elevated temperatures, usually in a range between 300-400◦C for 0.5 to 3 hours. The rate of softening is dependent on the temperature and the annealing time can, however, vary from a few hours at low temperature to a few seconds at high temperature. There are several diﬀerent annealing procedures that can be done where full annealing is the most common one, which produces the softest, the most ductile and the most versatile condition [61]. Stress-relief annealing is used to remove eﬀects obtained during strain hardening and is done on cold-worked alloys. Partial annealing (recovery annealing) is performed on non heat-treatable alloys in order to intermediate the mechanical properties [60].

7.2 Solution Heat Treatment

Solution Heat treatment is performed with the purpose of obtaining dissolution of the maximum amount of soluble elements from the alloy into solid solution. The process consists of heating the

22 alloy at elevated temperatures and keeping it at that temperature for a long period of time in order to achieve a nearly homogeneous solid solution in which all phases have dissolved. The temperature has to be chosen carefully in order to avoid overheating or underheating during the process. If the alloy is overheated, eutectic melting can occur with a corresponding degradation of properties such as tensile strength, ductility and fracture toughness. If the alloy is underheated it will result in lower strength values than expected due to an incomplete solution treatment. This process is applicable to the heat treatable alloys and is followed by a rapid quenching for the solution to be retained [60]. Before solution heat treatments, the relatively large particles around the grain boundaries will obstruct few slip planes. After the treatment, however, large number of particles will be dispersed and these will obstruct more slip planes which results in an overall increase in hardness and strength properties, as illustrated in Figure 10.

Figure 10: Before vs. after solution heat treatment of Al [57].

7.3 Quenching

Quenching in aluminium refers to cooling the hot, heat-treated material in either water och poly (alkylene) at lower temperatures compared to the temperatures used in the heat-treatment. Water or poly (alkylene) is the preferred medium for quenching aluminium in order to avoid precipitation detrimental to mechanical- and corrosion properties. Quenching is performed in order for the solid solution formed during the solution heat treatment to be cooled rapidly enough to produce a supersaturated solution at room temperature, providing an optimal condition for subsequent age hardening. Quenching can be performed in diﬀerent ways which includes hot water immersion, ambient water immersion, water spray, forced air, forced air with mist and poly (alkylene) glycol in water [60]. Quenching is the most critical step in the heat-treatment sequence due to the fact that cooling rate has to be controlled carefully to establish a solid form of the solution obtained in the previous steps. Through immersion quenching, cooling rates can be reduced by increasing the quenchant temperature. This is also known as slow quenching and is performed on complex shaped parts in order to improve the distortion characteristics Four factors are involved in order to

23 minimize the distortion in the aluminium during quenching [60].

• Temperature of the quenchant • Agitation rate of the quenchant • Speed of entry from the previous step to quenchant • Orientation of the aluminium part as it enters the quenchant

7.4 Precipitation Hardening (Age Hardening)

The goal with aging is to cause precipitation dispersion of the alloy solute to occur. A stable equilibrium is achieved for a given function which is dependent on time and temperature. To obtain a stable equilibrium, the microstructure has to recover from an unstable or a metastable phase that has been obtained during the solution heat treatment, quenching or through cold working. Precipitation hardening is when precipitates in the alloy impede the movement of dislocation within the crystal lattice. The precipitates grow in size during time and it results in a stronger material [63]. This is done through an aging process, either at room temperature or at elevated temperatures. Age hardening has an effect on the mechanical properties and this is further accelerated by reheating the quenched material [62]. This is done at temperatures between 150-200◦C, known as artificial aging. A characteristic feature of elevated-temperature aging effects on tensile properties is that the effect of the yield strength is more pronounced compared to the tensile properties. This type of heat-treatment results in higher yield strength but the ductility of the material may decrease [62]. Thus, an alloy that has been artificially aged has a higher strength but lower ductility compared to the the same alloy that has been aged at room temperature. The artificial aging process of an aluminium alloy can be described with the graph shown in Figure 11.

Figure 11: Aging curve and precipitate size [64].

24 When an alloy undergoes an artificial aging at low temperatures (< 1h) an increase in yield strength is observed. This can be seen for alloys undergoing artificial aging at higher temperatures for a longer period of time (4h < t < 8h) with even higher resulting yield strength. At some point, however, the yield strength will start to decrease at a stage where the alloy has been over-aged. When an alloy is over-aged, the precipitates will grow over time and as heat is added to the system the overall size of the precipitates begins to increase and the number of precipitates declines. This is known as the ”Ostwald Ripening effect” which is illustrated in Figure 12 [65]. When the Ostwald Ripening effect occurs, the atoms on the surface of a particle will be less stable compared to those on the interior which results in the overall surface area of the precipitates to decrease and so does the energy of the system. When more heat is added over time, the precipitates will grow even larger and at some point the precipitates will be large enough where they will be less effective at impending the movement of dislocation and hence weakens the strength properties of the alloy. As the precipitates continues to grow, the distance between them increases and eventually they will be far away from one another that diffusion no longer occurs. The goal with this project is to find the most suitable temperature and time for additively manufactured AlSi10Mg to obtain the best possible yield strength [65].

Figure 12: The Ostwald-Ripening eﬀect where (a) shows small and larger precipitates randomly, (b) shows larger precipitates growing and reducing amount of smaller precipitates, (c) shows large precipitates with no small precipitates in between them, resulting in precipiates being far away from each other [65].

7.5 T6 Heat treatment

The T6 heat treatment presents a condition of a temper designation for aluminium alloys in such way that it is solutionized, quenched and artificially aged. When the alloy is solutionized it is heated at elevated temperatures (> 500◦C) which allows dissolution of intermetallic phases into the aluminium metal matrix. This process is diffusion dependent and therefore the control of time and temperature is important. In order for the artificial aging step to respond well, a quenching is performed in order to freeze in the solutionized microstructure. The solubility of Mg and Si is low at room temperature and Mg2Si particles will form homogeneously throughout the metal matrix

25 which yields an increase in the overall strength of the alloy. This process is known as aging. If the aging is performed at room temperature (natural aging) it refers to the condition indexed as T4. It is, however, possible to perform aging at elevated temperatures (150-200◦C) for several hours which results in a better distribution of the formed Mg2Si particles, yielding improved strength properties. This type of aging is known as artiﬁcial aging and the condition is indexed as T6. This process is represented in Figure 13.

Figure 13: The T6 heat treatment process [66].

Figure 14: Stress relieving at diﬀerent heat-treatments of AM AlSi10Mg, (a) 300◦C for 2h, (b) 530◦C for 5h with water quench (T4 treatment), (c) 530◦C for 5h with water quench and artiﬁcial aging at 160◦C for 12h (T6 treatment) [67]

8 Mechanical properties of SLM components

The mechanical properties of a metal component is determined by its microstructure, the amount of defects and its design. Studies have shown that SLM components increases in strength but exhibit an inferior elongation to fracture in comparison to wrought materials. The heterogeneity in

26 the microstructure found in SLM produced components plays thus a major role in the mechanical properties of a SLM component. The influence in tensile properties is dependent on three different boundaries: melt pool boundaries, grain boundaries and the intragranular cell boundaries [68]. It is the cellular segregation network structure that have shown to be the most influential factor on the strength in SLM components. Studies have shown that a high amount of dislocations can act as a strengthening mechanism of the produced component [68]. During the SLM process the cooling rate is very rapid at the surrounding areas of the laser, resulting in a fine microstructure of the metal alloy. This is desired for increasing tensile and hardness properties. If compared to cast material, AM usually exhibits the mechanical properties due to the fine microstructure obtained from the high cooling rate. Other influencing parameters regarding mechanical properties for SLM is the nano-inclusions at the cell boundaries. These occur inside the building chamber where oxygen reacts with elements in the solid solution. The nano-inclusions may have a positive strengthening effect on the material by pinning the dislocations and thus hindering their motions [68]. The building direction may also have an influence in mechanical properties of the SLM produced component. Different building directions can be used where 0◦ and 90◦ directions are the most common ones. Regarding AlSi10Mg components more specifically, however, studies show that tensile and yield strength properties has insignificant differences whilst the elongation value increases for 90◦ produced components. Building direction illustration and the values of the tensile properties for AlSi10Mg are shown in Figure 15.

(b) Results of the tensile properties with regards to building direction (PMC values is post treated (a) Illustration of the usual building directions thatsample with no regards to building direction). can be used for SLM.

Figure 15: Building direction inﬂuence in tensile properties for SLM produced AlSi10Mg [69].

27 9 Materials and Method

9.1 Samples

The AM samples were additively manufactured by the company Lasertech LSH AB in Linköping, Sweden and produced by the SLM procedure and manufactured by using a EOS M290 machine from EOS. The SLM samples were manufactured with a dimension of 120x22x10 mm which is the desired dimensions for samples used for tensile tests according to the ISO 6892-1:2009 standard [70]. The samples were not heat treated after manufacturing and 30 samples with the desired dimensions were delivered in an as-built condition. Samples were cut oﬀ with a dimension of 10x22x10mm from the initial samples and water was used while cutting in order to ensure minimal heat generated on the samples. Figure 16 shows how the samples were cut for the hardness measurements.

Figure 16: The samples delivered from Lasertech and how it was cut for hardness measurements.

The cast samples were cast at Reftele Gjuteri AB and then milled by Alumbra AB, located in the same building as Saab, in order to obtain the same dimensions as the AM parts. All the milling procedures had to be performed carefully in order to control the heat since both the AM and the cast parts were milled as-built and as-cast, respectively. Minimal heat is wanted since the samples will be heat treated afterwards for comparisons.

The samples had to be further processed in order to perform tensile tests. The delivered dimensions were not suitable to yield properties such as elongation as the measurements can not be performed

28 as a result of having no control of the fracture. Both the AM and the cast samples were further milled at Alumbra in order to obtain a neck on the samples with the width being reduced to 10mm at the neck.

9.2 Heat treatment

Heat treatment was conducted on the samples in order to compare the mechanical properties between different heat treatments. The heat treatments were mostly in accordance to T6 but some samples were also heat treated in regards to T4 and T5. The same treatments was done for both AM and cast samples to see whether the manufacturing method would have any influence. For T6 treatments, samples were solution heat treated in a Naber 2804 oven at 520◦C for 3 hours and then quenched in water at room temperature for 5-10 minutes followed by artificial aging that was conducted in a Memmert oven. Different temperatures and times were tested during the artificial aging where the temperature interval was between 150-200◦C and the aging times were 1h, 3h, 6h and 12h. The first batch consisted of AM and cast part that were solution heat treated, quenched and the aged at 150◦C for 1h, 3h, 6h and 12h. The next batch were conducted the same way but with an increase in temperature so that they were aged at 160◦C. This was performed until samples were heat treated and aged at 200◦C for 1h, 3h, 6h and 10h.

For the T4 treated samples, the same procedure for the solution heat treatment part was conducted where the Naber 2804 oven oven was used. The samples were then quenched in water at room temperature and the ”naturally aged” at room temperature for 2-3 weeks. For the T5 treated samples, the samples were aged as-build and as-cast, respectively, in a Naber 2804 oven for 90 minutes at a temperature of 270◦C. Regarding all samples (T4, T5 and T6), they were placed in room temperature after the last heating phase (solution heat treated or artiﬁcial aging) in order to cool down. All the procedures (T4, T5 and T6) are represented with the ﬁgures below, respectively.

T4 Heat treatment procedure

Figure 17: Heat treated in a Naber oven followed by water quenching at room temperature and aged at room temperature.

29 T5 Heat treatment procedure

Figure 18: Delivered as-built from laser tech and then artiﬁcially aged at 270◦C for 90 minutes using a Memmert oven followed by natural aging.

T6 Heat treatment procedure

Figure 19: Heat treated in a Naber oven followed by water quenching at room temperature and artiﬁcially aged using a Memmert oven.

These heat treatments were performed both on additively manufactured and cast samples and was conducted at Saab in Järfälla. Firstly, solution heat treatments (SHT) were conducted in order to see whether there would be any inﬂuence in the SHT time performed. Samples AM-0.1, AM-0.4 and AM-0.10 were solution heat treated for 1 hour, 4 hours and 10 hours, respectively, followed by water quenching. These were also aged at room temperature afterwards and hence made the T4 treatments after all the steps. All the samples used in the project, heat treated or not, are shown in Table 7. The white samples are the ones that were hardness measured and the red ones are the ones that were hardness measured and tensile tested. Microstructure characterization was carried out on selected samples, highlighted in Table 7.

30 Table 7: Type of heat treatment performed on the samples.

Sample Post-treatment Type AM-1 As-built - AM-2 As-built - AM-0.1 SHT 520◦C 1h → quenched T4 AM-0.4 SHT 520◦C 4h → quenched T4 AM-0.10 SHT 520◦C 10h → quenched T4 AM-1.1 SHT → quenched → 1h 150◦C T6 AM-1.3 SHT → quenched → 3h 150◦C T6 AM-1.6 SHT → quenched → 6h 150◦C T6 AM-1.10 SHT → quenched → 10h 150◦C T6 AM-2.1 SHT → quenched → 1h 160◦C T6 AM-2.3 SHT → quenched → 3h 160◦C T6 AM-2.6 SHT → quenched → 6h 160◦C T6 AM-2.10 SHT → quenched → 10h 160◦C T6 AM-3.1 SHT → quenched → 1h 170◦C T6 AM-3.3 SHT → quenched → 3h 170◦C T6 AM-3.6 SHT → quenched → 6h 170◦C T6 AM-3.10 SHT → quenched → 10h 170◦C T6 AM-4.1 SHT → quenched → 1h 180◦C T6 AM-4.3 SHT → quenched → 3h 180◦C T6 AM-4.6 SHT → quenched → 6h 180◦C T6 AM-4.10 SHT → quenched → 10h 180◦C T6 AM-5.1 SHT → quenched → 1h 190◦C T6 AM-5.3 SHT → quenched → 3h 190◦C T6 AM-5.6 SHT → quenched → 6h 190◦C T6 AM-5.10 SHT → quenched → 10h 190◦C T6 AM-6.1 SHT → quenched → 1h 200◦C T6 AM-6.3 SHT → quenched → 3h 200◦C T6 AM-6.6 SHT → quenched → 6h 200◦C T6 AM-6.10 SHT → quenched → 10h 200◦C T6 AM-4.0 270 ◦C 90 minutes → cooled at RT T5 CA-1 As-cast - CA-0.4 SHT 520◦C → quenched T4 CA-1.6 SHT → quenched → 6h 150◦C T6 CA-3.6 SHT → quenched → 6h 170◦C T6 CA-6.6 SHT → quenched → 6h 200◦C T6 CA-4.0 270 ◦C 90 minutes → cooled at RT T5

31 9.3 Hardness measurement

The hardness measurements were conducted at Saab Surveillance in Järfälla. The samples were cut out very carefully with a lot of cold water in order to minimize the generated heat. The dimensions of the cut-oﬀ samples were 10x22x10 mm. After cutting, the samples were polished and carefully washed and mounted on a Leitz-microscope. A 200g weight and the Vicker scale was used for the measurements. Three measurements were done for each samples and they were done randomly throughout the samples. A previous master thesis student conducted hardness measurements on the same material and it was then concluded that it did not matter where on the material the measurements were done. Kappa imagebase was used to measure the edges of the indentation diagonals which gave a hardness value of the sample. The mean value was then taken for the values to obtain the hardness values of the speciﬁc sample. Figure 20a shows the type of Leitz microscope that was used for the experiments and Figure 20b shows one of the hardness measurements performed on the CA-1 (as-cast) sample.

(b) Hardness measurement on sample CA-1. (a) Leitz microscope used for hardness measurements [72].

Figure 20: The set up and program used for hardness measurements.

32 9.4 Microstructure

The microstructure section is divided into two subsections as a result of the necessary sample preparation required for the microstructural analysis.

9.4.1 Sample preparation

The following preparation were conducted for samples used for microstructural analysis. The samples were first cut carefully in order to minimize any heat generated. Water was used in order to keep the samples cooled while cutting them off. Most of the samples were heat treated prior to the next step whilst some were not. The samples were cold mounted using a mixture of Citofix powder and hardener liquid followed by curing that was carried out in a vacuum chamber in order to avoid bubbles in the hardened solution. The next step consisted of polishing the samples, firstly by hand until the samples were plane and then a Struers auto polisher was used to polish the rest. After polishing, the samples were etched using a form of the Keller’s reagent, recommended for Al-Si alloys by a handbook of metal etchants [73]. Table 8 shows the chemicals and part percentage used for the etching procedure.

Table 8: Chemicals used for etching.

Hydroﬂouric acid Hydrochloric acid Nitric acid Chemical Dest. Water (HF ) (HCl) (HNO3) Part (%) 1 1.5 2.5 95

9.4.2 Microstructural analysis

The different manufacturing methods results in different microstructures on the samples. The microstructural analysis was divided into a pre-analysis and post-analysis to compare the difference in microstructure before and after the heat treatment was conducted on the samples. The microstructure was analyzed for as-built, as-cast and a couple of the heat treated alloys and hence not all of the samples represented in Table 7. For example, microstructure was analyzed for T4, T5 and some T6 samples but not all since the microstructure did not vary as much for the different temperatures and times. Images were taken using an optical microscopy, Reichert Polylite 88 in which most of the microstructural analysis was conducted. Images were taken using magnifications between 100x- 500x. The as-built samples were analyzed by using the same optical microscope but a SEM Hitachi SU3500 was also used to analyze the microstructure, where images were taken using magnifications between 700-1600x. All the microstructural analysis were conducted at Saab in Järfälla.

33 9.5 Tensile test

The tensile testing was conducted externally at Elements Materials Technology in Linköping. Eleven samples in total were sent to Linköping for tensile tests (AM-1, AM-0.4, AM-1.6, AM-3.6, AM-6.6, AM-4.0, CA-1, CA-1.6, CA-3.6, CA-6.6 and CA-4.0). This was done in order to compare the ultimate tensile tests values as well as the elongation of the samples before vs. after the heat treatment. In order to control the fracture it is necessary to have a waist on the samples, hence the milling of the samples previously. The rate used for the tensile testing was 0.5 mm/min and the tensile force used was 100 kN. The data was then collected and sent back to Saab together with the fractured samples. As the tensile test were conducted externally, no images were taken during the initial tensile tests. Figure 21 illustrates the tensile test conducted, where the samples gripped using clamps and then pulled with a force of 100kN until fracture was obtained. A computer is used to measure all the data in order to obtain the values of ultimate tensile strength and elongation.

Figure 21: Illustration of the tensile test performed on the samples [74].

34 10 Results

10.1 Hardness measurement

Table 9: Hardness measurements. Table 9 and Figure 22 shows the hardness values obtained from Sample Hardness (HV) the hardness tests where three measurements were done for each AM-1 143 sample. The hardness results show a variation depending on AM-2 136 the type of heat treatment as well as the time and temperature AM-0.1 69 the samples were subjected to. The highest hardness values are AM-0.4 67 found on the as-built samples whilst hardness values are lower AM-0.10 68 for additively manufactured samples after heat treatment. It is AM-4.0 110 possible to observe the decrease in hardness directly after solu- AM-1.1 98 tion heat treatment but a slight increase after being aged, either AM-1.3 106 naturally or artificially. However, none of the T4, T5 nor T6 AM-1.6 108 treatments conducted resulted in an increase in hardness com- AM-1.10 103 pared to the as-built samples. When it comes to the heat treated AM-2.1 104 AM samples, however, the highest hardness value can be found ◦ AM-2.3 105 at AM-3.6 which is the one that was artificially aged at 170 C for AM-2.6 106 3 hours. For cast samples, the results showed the complete oppo- AM-2.10 106 site compared to the AM samples. The as-cast sample measured AM-3.1 108 the lowest hardness value at 56 HV whilst all the heat treated AM-3.3 114 samples showed an increase in hardness. The highest hardness AM-3.6 112 value for the cast ones is the CA-3.6, which is the one that was artificially aged at 170◦C for 6 hours. When it comes to the T6 AM-3.10 109 heat treated it is possible to see that hardness values are above AM-4.1 85 100 HV for AM samples when they are artificially aged between AM-4.3 88 150◦C for 3 hours up until they are aged at 170◦C for 10 hours. AM-4.6 98 After that the values decreases even more and are below 100 HV AM-4.10 88 for all AM-T6 samples. The same trend can be seen for the cast AM-5.1 91 samples where the two highest values are found on samples that AM-5.3 97 were artificially aged at 150◦C and 170◦C whilst a decrease (be- AM-5.6 94 low 100HV) is seen for the sample that was aged at 200◦C. The AM-5.10 90 AM-4.0 sample (T5 heat treated) showed a hardness of 110HV AM-6.1 82 in comparison to the T5 heat treated cast sample, CA-4.0, that AM-6.3 96 showed a hardness of 79 HV. For the additively manufactured AM-6.6 95 T6-treated samples, the values shows a plateau with an interval AM-6.10 82 between 105-114 before decreasing at temperatures at 180◦C and CA-1 56 above. As a result of doing three hardness measurements of each CA-0.4 82 sample, some of the samples showed different hardness values CA-1.6 114 where the most extreme sample had a +/- 10 HV in standard CA-3.6 121 deviation. This is illustrated in Figure 22 where it is possible to CA-6.6 99 see a large scatter for some samples (especially for AM-1.3). CA-4.0 79

35 Figure 22: Results of the hardness measurmenets.

Figure 22 illustrates the values seen in Table 9. As mentioned previously, AM-1 shows the highest hardness value and the sample was measured a week after arriving from Lasertech. AM-2 is also an as-built sample and shows the next highest hardness value. However, that sample was measured approximately 8 weeks after arrival (been placed in room temperature for the time), which shows a decrease in hardness properties. It is still higher than the heat treated samples and the cast samples, but a decrease in comparison to the AM-1 that was measured 7 weeks earlier.

36 10.2 Tensile test

10.2.1 Ultimate tensile strength (UTS)

Figure 23: Results of the UTS values obtained from the tensile test.

Figure 23 shows the result obtained from the tensile testing where the blue bars represents the additively manufactured samples and the red bars represents the cast samples. The same trend can be seen on these results in comparison the previously seen hardness results. AM-1 shows the highest UTS values followed by AM-4.0 (T5 heat treated) and T6 heat treated ones. A very signiﬁcant decrease in UTS is shown after heat treatment on AM samples. When it comes to the cast samples, however, it once again shows the complete opposite in comparison to AM samples. The CA-1 sample (as-cast) shows one of the lowest UTS values, together with CA-4.0. However, the samples that were solution heat treated showed a signiﬁcant increase in UTS. The highest value for the cast samples were once again CA-3.6 as it was for the hardness results. The UTS showed an increase of 171MP a after being T6 heat treated, whilst the next highest AM sample (T5 heat treated) showed a UTS decrease of 138MP a. The T6 heat treated AM showed a UTS decrease of 156MP a in comparison to the as-built sample. As previously mentioned, only eleven samples shown in Table 7 were tensile tested and hence no statistical scatter were obtained for the tensile test.

37 10.2.2 Elongation

Figure 24: Results of the elongation obtained from the tensile test.

Figure 24 shows the elongation values obtained after the tensile tests. These results do not follow the same trend as previously seen with the other results. AM-1 has the next lowest elongation value for the additively manufactured samples. The highest values is seen for AM-0.4 (T4 heat treated) and for AM-4.0 (T5 heat treated) which both showed an elongation value of 10.8%, which is an increase in 5.6% in comparison to the AM-1 sample that showed an elongation value of 5.2%. This means that the T4 and T5 heat treatment almost doubled the elongation in comparison to the as-built condition. For the T6 heat treated samples, however, AM-1.6 and AM-3.6 increased in elongation whilst AM-6.6 decreased in comparison to the as-built sample. The cast samples all showed a elongation that was less or equal to one percent (1%≤). It indicates in a very brittle-like behaviour and it did not matter whether the cast samples were heat treated or not.

38 10.3 Microstructure

10.3.1 As-cast condition

(a) As-cast condition at 100x magniﬁcation.

(b) As-cast condition at 200x magniﬁcation.

Figure 25: Microstructure of the CA-1 sample.

Al-Si-Mg alloys in as-cast condition have a microstructure that primarily consists of aluminium dendrites (α -Al), illustrated as the white regions on the Figure 25. The phase surrounding these white regions is an Al-Si eutectic phase. The dark spots on the microstructure is either Mg-Si rich regions or inter-metallic phases that may form as a result of consisting elements, such as Fe which forms Al-Fe-Mg-Si structures.

39 10.3.2 Cast T6

(a) Cast T6 at 100x magniﬁcation.

(b) Cast T6 at 200x magniﬁcation.

Figure 26: Microstructure of the CA-3 sample showing precipitation dispersion in the Al matrix.

Figure 26 shows the cast sample CA-3 that has been T6 heat treated and artiﬁcially aged for 170◦C for 6 hours. The microstructure resembles the as-cast condition with the white spots being aluminium dendrites (α-Al) surrounded by Al-Si eutectic microstructure. However by taking a closer look inside the aluminium matrix it is possible to see that precipitation dispersion. These precipitates have agglomerated and is homogeneously distributed throughout the aluminium matrix.

40 10.3.3 As-built condition

(a) As-built condition, 100x magniﬁcation.

(b) As-built condition, 100x magniﬁcation.

Figure 27: Microstructure of the AM-1 sample (arrows indicates building direction).

Figure 27 shows the AM-1 sample which is the one in an as-built condition. It it possible to see that it does not resemble any similar pattern as the cast samples. By observing Figure 27a where the building direction is into the picture it is possible to see the meander scanning strategy where each layer has been scanned with a 67◦ diﬀerence compared to the previous layer. Figure 27b is seen from the side of the sample, with the building direction to the left and it is possible to see the melt pools from the laser tracks obtained during manufacturing.

41 (a) As-built condition, 500x magniﬁcation.

(b) As-built condition, 700x magniﬁcation.

Figure 28: Microstructure of the AM-1 sample (arrows indicates building direction).

Figure 28a shows the melt pools with a higher magnification (500x magnification) where microstructure like features begin to appear. By using a scanning electron microscope it is possible to analyze the microstructure even clearer and Figure 28b shows the microstructure of the AM-1 sample using a scanning electron microscope at 700x magnification.

42 (a) As-built condition, 1500x magniﬁcation.

(b) As-built condition, 1600x magniﬁcation.

Figure 29: Microstructure of the AM-1 sample (arrows indicates building direction).

Figure 29 shows sample AM-1 at 1500x and 1600x magnification. The microstructure is seen at the edges of two melt pools and a heat affected zone (HAZ) from the laser is clearly visible. A region of coarse microstructure is observed inside the HAZ whilst the surrounding area shows region of fine microstructure. This pattern is seen throughout the alloy of the as-built sample where the sample mainly consists of fine microstructure whilst the HAZ consists of a coarse microstructure.

43 10.3.4 AM T6

(a) AM-T6 sample (AM-3.3), 100x magniﬁcation.

(b) AM-T6 sample (AM-3.3), 100x magniﬁcation.

Figure 30: Microstructure of AM-3.3 (T6 heat treated).

Figure 30 shows the microstructure of the AM-3.3 sample that was T6 heat treated. It does not resemble anything like the AM-1 (as-built) sample even though it is the exact same material with the same manufacturing method. The ﬁne microstructure is no longer visible as well as the melt pools and the boundaries are no longer seen. Instead the aluminium matrix is seen with dark dots which are Si-particles that are spread homogeneously throughout the alloy.

44 10.3.5 AM T4

(a) AM-T4 sample (AM-3.0), 100x magniﬁcation.

(b) AM-T4 sample (AM-3.0), 200x magniﬁcation.

Figure 31: Microstructure of AM-3.0 (T4 heat treated).

Figure 31 shows the AM-3.0 sample that was T4 heat treated. The microstructure is similar to the previously seen T6 heat treated, AM sample (AM-3.3). The ﬁne microstructure from the as-built is not seen on this sample either and it also shows the aluminium matrix consisting of Si-particles homogeneously spread out throughout the alloy.

45 10.3.6 AM T5

(a) AM-T5 sample (AM-4.0), 100x magniﬁcation.

(b) AM-T5 sample (AM-4.0), 200x magniﬁcation.

Figure 32: Microstructure of AM-4.0 (arrows indicates building direction).

Figure 32 shows the microstructure of the AM-4.0 sample that was T5 heat treated. As seen, the microstructure resembles the as-built microstructure where the melt pool microstructure and boundaries are still intact. However, images were taken for at 100x and 200x magniﬁcation and not as in depth as the as-built sample where images were taken up to 1600x magniﬁcation, hence more investigation has to be done for the AM-4.0 (T5 heat treated) sample to observe the microstructure.

46 11 Discussion

To summarize this work, the mechanical properties have been evaluated as well as the microstructure of additive manufactured and cast AlSi10Mg, before and after conducting different heat treatments. Additive manufacturing shows promising results and the resulting cooling rate obtained during the SLM process yields a fine microstructure which is the cause of an increase in mechanical properties. This is not seen for the cast parts where the microstructure is completely different due to the difference in cooling rate. For the cast aluminium alloy, the microstructure consist of primary aluminium dendrites (α-Al) which form during the initial solidification phase, surrounded by an Al-Si eutectic phase that could be seen in Figure 25. Also, as a result of the much slower cooling rate, inter metallic phases are present in form of Mg-Si rich regions and phases such as Al-Fe-Mg-Si structures as a result of Fe elements being present, among other elements. This is not the case for the additively manufactured samples where the microstructure showed a more homogeneous structure where it was possible to observe a heat affected zone at the boundaries of the melt pool, which is a result of the laser scan during the SLM process. The laser tracks yields a coarse microstructure whilst the surrounding areas, which is also heated and melted, cools down much rapidly and yields a fine microstructure. Something that has to be taken into consideration is the internal stresses obtained during the process as a result of the very rapid cooling. This is why suppliers tend to do a T5 heat treatment after the manufacturing in order to relief the internal stresses. The meander scanning strategy is performed in order to distribute the internal stresses on different building planes rather than one. Little information is reported for this matter and hence more investigation has to be done in order to conclude the amount of internal stresses present in the as-built sample.

The T5 sample do resemble similar microstructure to the as-built sample and it is the same heat treatment as the supplier is using before delivering the samples. That being said, the hardness values and the UTS did decrease quite significantly and hence more investigation has to be done for that heat treatment type. Studies have shown that T5 heat treatment performed at lower temperatures (160◦C) for 4 hours may increase UTS and yield strength values due to obtaining a α-Al cells that a decorated at the boundaries by fine eutectic Si nanoparticles [76]. Also, the experiments performed in this thesis showed an elongation value that doubled compared to the as-built samples which is quite significant.

As for the T6 heat treated samples, the hardness and tensile properties decrease significantly as a result of the solution heat treatment that completely changes the microstructure of the alloys. The fine α − Al cells surrounded fine eutectic Si nanoparticles are no longer seen, but an aluminium matrix consisting of larger, agglomerated Si-partcles can be observed. This is also seen for the T4 heat treatment which indicates that the solution heat treatment is the cause of this. For cast aluminium, a solution heat treatment is necessary to cause a dissolution of the elements on the boundaries to obtain a homogeneous distribution of the elements throughout the alloy. This is not the case for additively manufactured components as the fine microstructure obtained from the SLM process is more desirable due to the increase in mechanical properties. The cast alloys do not have cooling rate as rapid as AM has which makes it impossible to obtain a similar microstructure which makes the solution heat treatment a requirement to increase the mechanical properties. The ageing procedure for the cast aluminium, makes a precipitation dispersion in the alloy which increases the mechanical properties even further, until a point when the Mg2Si particles as well as other

47 intermetallic phases agglomerates too much, causing a so called over ageing which decreases the mechanical properties. This is seen throughout this thesis where the CA-1.6 showed an increase in hardness and UTS values, followed by CA-3.6 that showed an even higher increase in these values, where the values peaked for the cast samples. CA-6.6 however showed an decrease in these properties as a result of being artiﬁcially aged at 200◦C for 6 hours compared to CA-1.6 and CA-3.6 that were artiﬁcially aged at 150◦C for 6 hours and 170◦C for 6 hours, respectively. This indicates that the CA-6.6 sample over aged whilst the CA-1.6 sample resembled under aged samples and the CA-3.6 showed the most promise of a peak-ageing of the sample used in the thesis.

To further discuss the matter of generated heat, the milling procedure may have influenced the results. The supplier was specifically told to print the AM samples with recommended dimensions for tensile testing, whilst the resulting samples were delivered without a neck, which is required to measure tensile data such as elongation and also overall gives a more accurate data from the tensile testing. To achieve this, the samples had to be post-treated by milling which may have generated some heat and may have influenced the results. The same can be said for the cast aluminium, whilst the aluminium were not cast into tensile-testing dies, the samples had to be milled for obtain the same dimensions and the required waist for tensile testing.

48 12 Conclusions

In this work, additively manufactured and cast AlSi10Mg were studied in respect to mechanical properties and inﬂuence of heat treatment. Based on the results, the following conclusions can be made:

• Additive manufacturing offers completely different microstructures compared to cast alloys. This is as a result of the very rapid cooling rate obtained during the SLM manufacturing. • Heat treatment was effective for cast aluminium alloys where an increase in mechanical properties such as tensile strength and hardness was obtained.

• Heat treatment was not effective on additively manufactured samples as a result of the as- built conditions having a very fine microstructure which completely disappears after solution heat treatment. • Heat treatment of additively manufactured samples did increase the elongation significantly, where the T4 and T5 heat treated samples almost doubled in elongation in comparison to the as-built sample.

49 13 Future work

Since the UTS and hardness properties decreased for AM samples quite signiﬁcantly after heat treatment, it would be very interesting to analyze the internal stressed in the samples prior to heat treatment as well as post heat treatment. If minimal internal stresses are present, then the question of ”why heat-treat AM?” arises. The supplier of the AM samples, Lasertech, usually performs a T5 heat treatment after manufacturing before sending them to the customer. Another interesting thing would be to take a look at the fractography of the samples post tensile testing to see how the fracture surface changes before vs. after the heat treatments, as well as for the AM vs. cast samples. Also, this thesis mainly focused on T6 heat treatment for AM AlSi10Mg and more investigations has to be done for the T4 and T5 heat treatments. The T4 and T5 heat treatment did decrease the hardness and UTS values for AM but almost doubled in elongation and it would be interesting to investigate the background behind this and to see whether it is useful information or not.

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56 List of Figures

1 Illustration of the SLM process [10]...... 9

2 SLM process parameters [10]...... 10

3 Materials used for manufacturing within SLM [10]...... 11

4 Phase diagram of Al-Si [29]...... 14

5 Illustration of the microstructure obtained after selective laser melting [32]...... 15

6 Microstructure obtained of cast ingots [28]...... 16

7 The microstructure of (a) SLM AlSi10Mg, (b) cast and etched AlSi10Mg [36]. . . . . 17

8 Illustration of the plasma atomization process [39]...... 18

9 Spherical AlSi10Mg powder [40]...... 19

10 Before vs. after solution heat treatment of Al [57]...... 23

11 Aging curve and precipitate size [64]...... 24

12 The Ostwald-Ripening eﬀect where (a) shows small and larger precipitates randomly, (b) shows larger precipitates growing and reducing amount of smaller precipitates, (c) shows large precipitates with no small precipitates in between them, resulting in precipiates being far away from each other [65]...... 25

13 The T6 heat treatment process [66]...... 26

14 Stress relieving at diﬀerent heat-treatments of AM AlSi10Mg, (a) 300◦C for 2h, (b) 530◦C for 5h with water quench (T4 treatment), (c) 530◦C for 5h with water quench and artiﬁcial aging at 160◦C for 12h (T6 treatment) [67] ...... 26

15 Building direction inﬂuence in tensile properties for SLM produced AlSi10Mg [69]. . 27

16 The samples delivered from Lasertech and how it was cut for hardness measurements. 28

17 Heat treated in a Naber oven followed by water quenching at room temperature and aged at room temperature...... 29

18 Delivered as-built from laser tech and then artiﬁcially aged at 270◦C for 90 minutes using a Memmert oven followed by natural aging...... 30

57 19 Heat treated in a Naber oven followed by water quenching at room temperature and artiﬁcially aged using a Memmert oven...... 30

20 The set up and program used for hardness measurements...... 32

21 Illustration of the tensile test performed on the samples [74]...... 34

22 Results of the hardness measurmenets...... 36

23 Results of the UTS values obtained from the tensile test...... 37

24 Results of the elongation obtained from the tensile test...... 38

25 Microstructure of the CA-1 sample...... 39

26 Microstructure of the CA-3 sample showing precipitation dispersion in the Al matrix. 40

27 Microstructure of the AM-1 sample (arrows indicates building direction)...... 41

28 Microstructure of the AM-1 sample (arrows indicates building direction)...... 42

29 Microstructure of the AM-1 sample (arrows indicates building direction)...... 43

30 Microstructure of AM-3.3 (T6 heat treated)...... 44

31 Microstructure of AM-3.0 (T4 heat treated)...... 45

32 Microstructure of AM-4.0 (arrows indicates building direction)...... 46

List of Tables

2 Feature comparisons between SLM and EBM [8]...... 8

3 Chemical composition of the AlSi10Mg alloy...... 13

4 Properties of as-built AlSi10Mg [41]...... 13

5 Typical conditions selected for SLM with resulting grain sizes...... 17

6 Temper designations of post-treated Aluminium alloys...... 22

7 Type of heat treatment performed on the samples...... 31

58 8 Chemicals used for etching...... 33

9 Hardness measurements...... 35