BK10A0402 Kandidaatintyö
METALLIEN LISÄÄVÄN VALMISTUKSEN LAATU JA SEN MÄÄRITTELY
QUALITY OF ADDITIVE MANUFACTURING OF METALLIC MATERIALS AND ITS DEFINITION
Lappeenrannassa 22.01.2020 Saku Vesanen
Tarkastaja TkT Heidi Piili Ohjaaja TkT Heidi Piili Ohjaaja DI Markus Korpela
LUT Kone Saku Vesanen
Metallien lisäävän valmistuksen laatu ja sen määrittely Kandidaatintyö
2020
40 sivua, 8 kuvaa ja 4 liitettä
Tarkastajat: Dosentti Heidi Piili, TkT
Nuorempi tutkija Markus Korpela, DI
Hakusanat: Lisäävä valmistus, 3D-tulostus, metallit, jauhepetisulatus, laadunvarmistus, jauheen käsittely, heuristinen analyysi
Tässä työssä tutkitaan metallien lisäävän valmistuksen laatua ja siihen vaikuttavia seikkoja.
Työ suoritettiin 3DStep Oy:lle. Yritys haluaa kehittää sarjavalmistuksen laatua jauhepetisulatuksessa. Laatu on monimutkainen käsite lisäävässä valmistuksessa. Siihen vaikuttaa monia tekijöitä. Yritys käyttää tuotannossaan jauhepetisulatukseen perustuvaa SLM-280HL laitetta. Työn tavoitteena on muodostaa heuristinen kysymyslista, jolla pystytään arvioimaan tuotteen laatua, joka on valmistettu jauhepetisulatuksella. Tätä kysymyslistaa sovelletaan 3DStep Oy:n toiminnan ja toimitilojen arviointiin. Kysymyslista muodostetaan kirjallisuuskatsauksen pohjalta. Kysymyslistaa arvioidaan tulosten perusteella.
Jauheen käsittely on tärkeä jauhepetisulatuksen laatuun vaikuttava seikka. Jauheen oikeanlainen käsittely varmistaa jauheen tasalaatuisuuden, jolloin jauhe vastaa tuottajan antamia ohjearvoja. Laitevalmistaja on kehittänyt automaattisen suljetun kierron jauheensyöttölaitteen ja sensoreita helpottamaan valmistusprosessia ja sen parametrien tarkkailua. Lisäävän valmistuksen standardisointi on kesken. Standardeja on olemassa jo, mutta niitä tullaan täydentämään tulevaisuudessa.
Yrityksellä on kysymyslistan mukaan hyvät periaatteet tuotannossa. Tuotantotilojen järjestely vaatii vielä kehitystä. Jauheiden sekoittumisvaaraa on pienennetty ilman automaattisen jauheensyöttölaitteen hankkimista. Kysymyslista osoittautui päteväksi yleisellä tasolla. Jatkokehitys laadun suhteen vaatii kysymysten tarkennusta ja lisäystä.
Jätejauheen käsittelyä ei tässä työssä huomioitu ollenkaan. Tämä seikka voisi olla tärkeä sisällyttää tulevaisuuden kehitykseen.
LUT Mechanical Engineering Saku Vesanen
Quality of additive manufacturing of metallic materials and its definition
Bachelor thesis 2020
40 pages, 8 figures and 4 appendices
Examiners: Docent Heidi Piili, D. Sc. (Tech.)
Junior Research Scientist Markus Korpela, M. Sc.
Keywords: Additive manufacturing, 3D printing, metals, powder bed fusion, quality management, powder handling, heuristic analysis
This thesis focuses on quality of additive manufacturing of metallic materials and factors affecting to it. This thesis was conducted for 3DStep Oy. The company aims to develop quality of serial production in powder bed fusion. Quality is complicated concept in additive manufacturing. Many factors are affecting to quality. The company uses SLM-280HL system for the production. The system is based on powder bed fusion. The goal of this thesis is to form a heuristic checklist, which can be used for evaluating the quality of product manufactured using powder bed fusion. The checklist will be applied on evaluation of manufacturing process and area of 3DStep Oy. Checklist is formed according to findings of literature review. The checklist is evaluated according to results.
Powder handling is an important factor affecting to quality of powder bed fusion. Proper handling ensures homogeneous powder, then powder matches the description of powder supplier. System manufacturer has developed an automatic closed-loop powder feeding system and sensors to aid manufacturing process and monitoring of parameters.
Standardization of additive manufacturing is under development. Standards exist, but they will be complemented in the future.
According to the checklist, the company has good principle in their production. The organization of the manufacturing area requires still development. The danger of mixing of powders has been reduced without acquiring automatic closed-loop powder feeding system.
The checklist qualified well on the general level. Further development requires detailing the questions. Handling of waste powder was not considered in this study at all. This could be important factor in future development.
I would like to thank my instructors Heidi and Markus for excellent counselling, 3DStep Oy for the topic and counselling, friends and family, who gave support during the thesis making, especially my wife Elizaveta.
Saku Vesanen
Lappeenranta 22.01.2020
TABLE OF CONTENTS
TIIVISTELMÄ ... 1
ABSTRACT ... 2
ACKNOWLEDGEMENTS ... 3
TABLE OF CONTENTS ... 5
LIST OF SYMBOLS AND ABBREVIATIONS (IF NEEDED) ... 7
1 INTRODUCTION ... 8
1.1 Introduction of 3DStep Oy ... 8
1.2 Research problem ... 9
1.3 Research questions: ... 9
1.4 Focus of research ... 9
1.5 Introduction of PBF ... 10
1.6 What is heuristic analysis? ... 11
2 METHODS ... 12
LITERATURE REVIEW ... 13
3 QUALITY RELATED TO POWDER USED IN PBF ... 13
4 POWDER HANDLING IN THE SYSTEMS OF SLM ... 19
5 STANDARDS FOR QUALITY IN AM ... 23
6 CASE STUDY OF QUALITY MANAGEMENT PROCEDURE FOR AM ... 24
7 CHECKLIST BASED ON LITERATURE FOR EXAMINING QUALITY ... 25
7.1 Checklist ... 26
EXPERIMENTAL PART ... 28
8 RESULTS AND DISCUSSION ... 28
8.1 Company goals ... 28
8.2 Discussion of protocols and arrangements ... 28
8.3 Discussion of success of checklist ... 31
8.4 Updated checklist. ... 32
9 CONCLUSION ... 34
10 FURTHER RESEARCH TOPICS ... 36
LIST OF REFERENCES ... 37 APPENDIX
Appendix 1: SLM 280 specifications
Appendix 2: Aluminum powder composition and material properties Appendix 3: Stainless steel powder composition and material properties Appendix 4: Tool steel powder composition and material properties
LIST OF SYMBOLS AND ABBREVIATIONS (IF NEEDED)
AM Additive manufacturing
ASTM American Society for Testing and Materials ISO International Organization for Standardization LCS Layer control system
LPM Laser power monitoring L-PBF Laser powder bed fusion MPM Melt pool monitoring PBF Powder bed fusion PSD Particle size distribution fc Particle circularity fs Particle shape
1 INTRODUCTION
Additive manufacturing (AM) is developing technology for industry. AM has changed the way, how design and production can be performed. Every manufacturing method is always studied concerning the quality of the finished product. The achieved quality determinates the usefulness of every process in industry.
1.1 Introduction of 3DStep Oy
3DStep is additive manufacturing company, which business is based on AM by powder bed fusion (PBF) of metals (Selective laser melting (SLM)) and plastics (Jet Fusion). 3DStep factory is located in Ylöjärvi, near Tampere. (3DStep, 2019) The company was founded in 2016. (3DStep Company information) 3DStep has conducted already numerous important serial additive manufacturing projects for the industry. In those projects, a product of another company is brought to them, in order to manufacture it by PBF. This is to gain more functionality or other properties necessary to the product. 3DStep helps companies to utilize AM with specialized scouting services and training. (3DStep, 2019)
3DStep has manufacturing machines for metallic and plastic products. Machine for metal print is based on laser powder bed fusion (L-PBF) and machine for plastic printing is based on binder jetting process. (3DStep 2019, HP Jet fusion 3D 4200, 4210 printing solution 2018) For metallic products, used machine is SLM 280HL Twin and for plastic products HP Jet Fusion 4200. SLM-280HL Twin contains two 400W lasers. Material used for these machines are:
• Stainless steel 317L
• Aluminum AlSi10Mg
• Tool steel Maraging 1.2709
Material details for metallic materials are given in appendix 1. These details include chemical composition of powders and mechanical properties of manufactured specimen by SLM Solutions. (3D Metals – Discover the variety of metal powders)
1.2 Research problem
Key piece of information is missing from quality analysing toolkit of a company of 3DStep, whose business is partly in the field of additive manufacturing of metallic materials. The company has tools for estimating the quality of products manufactured in large scale using additive manufacturing, however more efficient utilizing of these tools is in the interests of the company. The quality of finished product is important in production. This means, that analysing the results of process and production process itself, is important to be able to further develop it. (Ketola 2018, 3DStep 2019)
1.3 Research questions:
Research questions in this thesis are:
1. What is heuristic analysis?
2. How heuristic analysis can be applied into quality analysis of AM process of metallic materials?
3. Which factors create variation of quality in series production?
4. Which criteria are important for evaluation of process?
5. What is quality of specific AM process? (Evaluation of 3Dstep Company’s process) 6. How to develop the specific AM process for decreasing the variation between the
manufactured products?
1.4 Focus of research
This research focuses on a company, 3DStep, using AM of metallic materials in production.
This process is L-PBF of metallic materials. L-PBF process is studied for improving the quality of products in series production. The equipment used by the company is SLM-280 by SLM Solution. Materials used in the production are aluminum, tool steel and stainless steel. (Ketola, 2018, 3DStep 2019)
The production process has number of factors, which affect the quality of finished product.
These factors are for example powder quality, powder properties and maintenance of equipment. As it can be seen from Figure 1, there are many factors in L-PBF process.
(Brandt, 2017, p. 56)
Figure 1. Process parameters for PBF. (Brandt, 2017, p. 56)
Main focus in this thesis is on series production and factors, which occur from moment the manufacturing begins. This moment is defined in this thesis on following way; model of product is ready and input to machine and powder is input to machine. In other words, the machine is waiting the start command.
1.5 Introduction of PBF
3DStep Company uses SLM-280 manufacturing machine in production. This machine is based on L-PBF process. (Ketola, 2018). Figure 2 presents basic working principle of L- PBF process. As it can be seen from Figure 2, the process itself works in cycles. First, powder is spread using recoater on building platform. After this, in case of SLM-280, laser beam is used to melt material layer on desired locations. Then material is spread again. This cycle is repeated until the component is ready. (Brandt, 2017, pp. 5556)
Figure 2. PBF process principle. (Brandt, 2017, p. 55)
The SLM-280 uses fibre laser. More specifications of SLM-280 can be found in Appendix 1. Powder tank is located on top of the building platform in the system presented in Figure 2. The powder tank can be located next to building platform. Powder tank location depends on the system manufacturer. (Milewski, 2017, pp. 134135)
1.6 What is heuristic analysis?
Heuristic analysis is a problem solving method, which uses rules, estimates and educated approximations to find a solution for a problem. Heuristic derives from Ancient Greek word meaning ‘to discover’. (What is heuristic analysis?) Heuristic approach is curious, since it does not guarantee any solution at all. Solutions gained utilizing heuristics are good enough to find suitable solution for a problem. The solutions are satisfactory, not optimal. Heuristics are means to reduce effort or cost of finding a solution for complex problems. Clear advantages for heuristics are low cost and absence of formal application restrictions. It should be always remembered that heuristics do not guarantee any results or solutions.
(Gruning & Kuhn, 2017 pp. 37 - 40)
2 METHODS
This research is divided into two main parts, literature review and experimental part.
Research questions are answered in this thesis by conducting a literature review. The results of literature review are then applied to evaluate the manufacturing process of the company.
As a results of literature review, a checklist of different factors is created. This checklist is then given for engineers of company for reviewing it. The engineers give comments and proposals for improving the checklist. These improvements will be made for checklist before applying it on the process of the company in the experimental part.
Scopus, Google Scholar and LUT-Finna are used for finding literature. LUT-Finna is database of LUT-University library for material, which the library has access to. Sources were evaluated according to available metrics in each database. For example in Scopus, the available metrics are Plum-metrics and field-weight factor. Both of them are basing on the amount citations in other articles found in Scopus.
LITERATURE REVIEW
3 QUALITY RELATED TO POWDER USED IN PBF
The powder quality has several variating factors. First factor is the manner, how powder is placed into the powder feed chamber of machine. This must be considered, because it can happen that more dense powder ends up on the bottom of powder feed chamber, while less dense stays on the top of the powder feed chamber. This creates variation in quality for high builds. (Gibson, Rosen & Strucker. 2015, p. 51)
Metallic parts must be connected rigidly to building platform, otherwise the crated structure will warp during the cooling due to cooling and large residual stresses. Warping and deformation will create uneven spread of following layers. (Gibson et al. 2015, p. 53)
Recycled powder affects to quality of finished product. As an example, a build, where recycled powder is used, can be spoiled by impurities from cleaning the previous builds.
(Gibson et al. 2015 p. 55)
Powder oxidation is significant factor in L-PBF. The research conducted by Leung et al.
(2018, pp. 294305) studied the effect of virgin, stored and reused powders on the quality of product manufactured by laser additive manufacturing. Studied technologies were overhang conditions and L-PBF. It was found that re-melting the layers decreased size and density of pores in L-PBF. Keyholing or liquid flow can trigger gas release from pores via direct release. Keyholing and liquid flow are partially or completely filling the pores existing under the surface. Keyholing and liquid flow can cause negative effects such as droplet spatter or open pore. The negative effects may occur, when re-melting the surface results in bursting surface due to pores under the surface. (Leung et al. 2018, pp. 294305) However, the research did not use the same materials as 3DStep Oy does. There could be similar effects in material behaviour under melting during L-PBF.
While using oxidised powder, the amount of spatter is significant. Using low oxygen content powder can minimise defects in L-PBF. (Leung et al. 2018, pp. 294305)
Meier et al. (2018) describe in their research the effect of powder flowability. Powder flowability affects into post flow effect after the recoater has spread the powder. This means that powder particles move after the recoater has passed powder. Powder flowability is affected by powder particle size. This is because cohesiveness decreases less than gravity forces. Cohesiveness is opposite value of flowability; cohesiveness describes how easily particle sticks to each other. Layer thickness also affects into post flow effect. (Meier et al.
2018)
L-PBF is sensitive for changing parameters due to vast amount of variables. Changing powder layer thickness must be done carefully. A study, conducted by Jelis et al. (2018), shows that increasing powder layers thickness with particle size range can result in same level quality as lower powder layer thickness. Ductility was material property, which experienced reduction from increased powder layer thickness. Other material properties remained on comparable level to lower powder layer thickness. Jelis et al. conducted experiments for the specimen after heat treatment as post-processing operation. In addition, Jelis et al. states that maintenance of recoater blade is vital, since poorly maintained recoater blade caused reduction in the part strength. (Jelis et al. 2018)
Meier et al. (2018) states that oxidation or other type of contamination of powder affects greatly on surface quality by affecting layer thickness of each layer. Figure 3 presents the effect of different layer thickness in relation to powder particle size. (Meier et al. 2018)
Figure 3. The effect of flowability presented from the side of the layer. Also different layer thickness in relation to powder particle size is presented. (Meier et al. 2018)
As it can be seen from Figure 3, powder flowability affects the surface of spread layer of powder. The surface levelness variates depending on the powder flowability and layer thickness. (Meier et al. 2018) Similar matter is discussed in other research as well. Particle size and shape affect into flowability of powder and density of spread powder layer. (Jokinen
& Riipinen, 2016, p. 5)
Porosity is a defect, where the gas gets trapped inside the material during melting of powder layer. Pores decrease density of manufactured product and increase residual stresses. These pores may be caused by gas-atomized powder, moisture on powder particles or dissolved hydrogen in powder. These are main reasons of porosity in aluminum powder products.
Laser power affects into creation of pores. These are called key-hole pores. Too high laser power causes collapsing of the melt pool and creates pore to surface, which is later covered by powder on next spread layer. Density can be increased by second melting of layer.
(Brandt, 2017, pp. 6567) Handling the powders in correct way, may increase the quality of manufactured products.
Elevated temperature affects the material. Material around the melted material is affected.
Material can fuse, react with gases in build chamber or change powder properties otherwise.
For this reason, recycling and reusing particles must be done carefully. Mixing different powders affects into properties of finished product. It may happen so that different parts of product have different properties due to differences in powder composition. (Gibson et al.
2015, pp. 129130)
Reusing material affects into properties of powder by altering oxidation, flowability and particle size distribution. AlSi10Mg is particularly affected by reuse after six times of reusing powder. This is related to the chemical behaviour of aluminum, which is reacting relatively easily with oxygen. Virgin powder has lesser flowability due to lightweight of powder and including finer particles, while reused powder has greater flowability with larger particles. Reusing AlSi10Mg powder creates changes in shapes of particles. The changes in shapes of the particles lead to rougher surface, increase in the number of satellites and deformation towards a teardrop shape. (Cordova, Campos & Tinga, 2019, pp. 10651069)
Cordova et al. (2019) presented reusability decision diagram based on their findings and powder properties. This diagram is presented in Figure 4.
Figure 4. Reusability decision diagram according to Cordova et al. (Cordova et al. 2019, p.
1070)
As Figure 4 presents, diagram is divided into evaluations (diamond-shape) and properties (rectangular boxes). The diagram has two steps with option to process powder into range of reusability, particle size distribution (PSD) and shape of particles (fc and fs). After next evaluations, if powder does not meet the requirements, powder is not advised to use anymore. Evaluations of powder include powder oxidation, flowability and packing density of powder. In case of AlSi10Mg, the flowability increases and powder packing density remains constant during reuse. (Cordova et al. 2019, pp. 1069 1071
Sieving of powder should always be adjusted according the material, which is currently in use. Sieving controls the PSD, however it must be remembered that each material behaves differently in reuse. (Cordova et al. 2019, pp. 10691071
For stainless steel, 174 PH, similar study conducted consisting 11 builds. The builds were combination of virgin, reused and recycled powder. The properties of powder and the product were evaluated through different methods, which include dynamic digital image analysis, X-ray diffraction. It was found that properties of powder and product did not change vastly. Changes were found in carbon content of product. Particle size had tendency to decrease slightly. PSD remained nearly same. Powder flowability increased with the times recycled. The increased flowability was also linked into increasing powder bed density, which increased similar manner as powder flowability. Surface quality did not alter between builds significantly, only between build directions. Alto conclude all, the minimum property values were reached with each build. Minimum property values were given by the powder supplier. The most important factor according to Jacob et al. (2017) is exposure time under laser. Heat is recognized as the most important factor altering powder and material properties. (Jacob et al. 2017, pp. 4647)
According to literature (Nguyen et al. 2017, Powder handling 2018) there is tendency for importance of powder handling. If powder contaminates or is reused too many times, properties of the powder increase or decrease into negative direction. It is vital to handle powder correctly. (Nguyen et al. 2017, Powder handling, 2018)
4 POWDER HANDLING IN THE SYSTEMS OF SLM
This section handles only machinery operated by 3DStep Company and comparable SLM Solutions Company machinery. Machinery is developed and manufactured by SLM Solutions Company. For solving powder handling issues, SLM Solutions Company has developed its own system for clearing used powder for sieving after the build. This system is presented in Figure 5. In the system powder is fed after sieving process. The feeding is performed through pipeline highlighted in Figure 5.
Figure 5. Automatic powder sieving system by SLM Solutions. (SLM Solutions 2018)
Sieving system is shown in Figure 5. These systems are available for SLM-280 and SLM- 500. The presented system is automated with powder storage of 90 litres. The automated system ensures that individual powder bottles are not required for filling. The powder is always sieved before powder is input to building process. (SLM Solutions 2018)
Another property in this system, is the possibility for recirculation of overflown powder.
Recovering overflown powder is presented Figure 6.
Figure 6. Method for recovering powder from the build chamber for recirculation. (SLM Solutions 2018)
As it can be seen in Figure 6, the overflown powder is sieved and then re-input to build process. In Figure 6 and Figure 5, there are lines in the middle of set up. The lines are for vacuum cleaning the powder after build is completed. The recovered powder is sieved and afterwards reused. (SLM Solutions 2018)
Chemical composition and finished product properties contain variation according to specimen information provided by SLM Solutions. Material information can be found from appendices 2, 3 and 4.
SLM Solutions Company has developed several systems for monitoring quality of parameters during PBF process. Laser power monitoring (LPM) is used to monitor and collect data from whole build process concerning laser power. This means that target power is set and LPM records the actual laser power. The results then can be compared with set target parameter. The deviation between actual laser power and target power is obtained by comparing the values. Melt pool monitoring (MPM) system is watching over the melt pool by detecting heat radiation from the melt pool. This makes possible to notice defects caused in melting the powder. The MPM collects data from entire build process. Finally, layer control system (LCS) monitors powder bed itself. LCS is detecting defects during spreading the powder layer. If a defect is detected, LCS responds before damage occurs.
(Additive.quality 2019)
Inserting powder to SLM-280HL without PSV is conducted by connecting powder bottle to pipe located on top of the machine. This position is highlighted in Figure 7.
Figure 7. SLM-125HL machine to demonstrate the powder input to SLM-280HL (SLM Solution SLM-125HL 2018)
As it can be seen in Figure 7, the way of inputting powder remains the same between SLM- 125HL and SLM-280. Only difference between machines is the size of the build chamber.
Above presented figures help to understand the powder circulation even without PSV.
Overflown powder is collected then lead into powder bottle placed under the build platform.
(SLM Solution SLM-125HL 2018, SLM Process, 2016, PSV Powder handling unit, 2017).
This can be seen also in Figure 6. Instead of pipeline to PSV, the powder bottle is placed there. After this, powder should be sieved again before it can be reused. (SLM Solution 2018)
Powder storage condition can vary. Parameters varying can be for example, humidity and heat. These parameters can alter powder composition, if time under conditions is long enough. Increased humidity can affect powder properties such as flowability. In research conducted by Vluttert (2016), increased humidity in storage conditions affected only little to powder properties. Little effect was noticed only due to short time spent on research. Short time span questions the relevance of the research. Main idea in this research is that conditions for powder storage must be carefully established and maintained. (Vluttert, 2016) Storage conditions, such as humidity, heat and containers, have an effect on powder flowability.
Powders are especially affected by storage conditions. A difference between dry and humid condition creates variation in surface roughness of build product. Powder, which had been storage in dry conditions, caused low surface roughness, when product was manufactured using AM. (Mitterlehner et al. 2017)
5 STANDARDS FOR QUALITY IN AM
A standard is a document to guide, in example designing and manufacturing. A standard is designed by all interested and related parties for this certain product, process, service, or material. Since the desired quality is agreed together with all parties, everyone benefits standardization. (What is a standard? 2019)
International organization for standardization (ISO) has created standards for testing the quality of parts manufactured utilizing AM methods. SFS-EN ISO 17296 covers the AM methods from terminology to data collecting and processing and is divided into three parts.
SFS-EN ISO 17296-3 consists testing principles for parts manufactured utilizing AM methods. As main quality characteristics, SFS-EN ISO 17296-3 defines for feedstock and products. For example, the feedstock parameters are powder particle size, morphology and flowability, among others. For products, SFS-EN ISO 17296-3 defines more parameters, which are divided into more groups; surface requirements, geometric requirements, mechanical requirements and build material requirements. As an important note, there will be more characteristics added in versions to come. The new versions will be covering i.e.
thermal properties and electrical requirements. (SFSEN ISO 17296-3, 2016, p. 7)
SFS-EN ISO 17296-3 gives classification for planning testing of manufactured part.
Classification consists three levels with highly engineered parts, which are safety critical, functional parts, which are not safety critical, and design and prototype parts. For different material groups, the standard gives recommendations, which material property should be tested, depending on the classification of the part, respectively. Following the standard gives good basic plan for testing the manufactured part. (SFSEN ISO 17296-3, 2016, pp. 79)
Other standardization organizations, such as American society for testing and materials (ASTM), have also developed standards for AM-methods. For example, ASTM-standards cover topics from terminology to testing. (ASTM, 2019) As a future development of standards, ASTM and ISO are working together in developing AM standards. For instance, upcoming standard named ISO/ASTM 52950 replaces ISO 17296-4:2014. (International organization for standardization, 2019)
6 CASE STUDY OF QUALITY MANAGEMENT PROCEDURE FOR AM
A study conducted by Schmid & Levy (2012) discusses on quality management procedure for AM. The focused area is L-PBF. The study gives recommendations according the materials. Division of materials is between plastic and metallic powders. The SLM process quality is divided into five different categories; equipment, material, production, batch and part. These categories affect to the quality of finished parts. For each category there is table.
Each table contains recommendations for improving or maintaining the quality of parts.
(Schmid & Levy, 2012)
In order to achieve high quality parts, it is recommended to clean the equipment on daily basis. Another recommendation for the equipment is to have periodical maintenance program. The maintenance program should be more testing and checking the equipment, such as laser, gas supply and temperature control. After cleaning and periodical maintenance, a specimen should be manufactured. This specimen should be examined and documented.
Documentation of quality of specimen can help following the progress of entire production.
According to findings from the specimen, the parameters or equipment should be adjusted.
(Schmid & Levy, 2012)
For metal powders, there are three actions for following the quality. First is conducting a material log book. Logbook is for following how material is used. Second action is inspecting the data sheet of delivered powder material. However, the delivered quality of powder is the responsibility of powder supplier. Third possible action is sieving the powder.
Documentation is vital for sieving; what kind of mesh size is used for sieving, for instance.
(Schmid & Levy, 2012)
For final three categories, production, batch and part, it is highly recommended to document parameters of the process, manufacture specimen time to time and perform testing of finished part with non-destructive testing. (Schmid & Levy, 2012)
7 CHECKLIST BASED ON LITERATURE FOR EXAMINING QUALITY
The checklist is created on the mentality of reducing the variables in manufacturing process, which could lead to failure of quality of the finished product. The questions are based on literature. First questions are broad and do not give any accurate answers. The first questions are only to narrow down the options and possible outcomes. Following questions then relate to answers from previous questions. Following questions must always be adjusted according to answers of first questions. Otherwise the following questions do not give answer or solutions, which could give any valuable information regarding the testing and quality of a part.
Consulting companies use heuristics for designing and improving customer services and products. Nielsen (Nielsen, 1994) presents 10 usability heuristics for designing software and interface. These rules of thumb can applied on any field, with modification respectively. All of these rules could be applied to AM-methods, but for this study other rules are more essential than others. The most important rules for AM-methods are:
• #4: Consistency and standards
• #5 Error preventing
• #6 Recognition rather than recall
• #9 Help users recognize, diagnose and recover from errors
• #10 Help and documentation.(Nielsen, 1994)
7.1 Checklist
List below contains checklist questions based on literature:
Is PSV used in production?
• If not:
1. How powder is handled, sieved and reused?
2. How is powder reuse history followed?
How build chamber cleaning is performed?
What are the storage conditions?
• How are the storage conditions followed?
How history of powder used is followed?
Are process parameters written down every time?
Are defects and their origin followed frequently?
• If yes:
1. When defect is found, is a study made to find out what caused the defect?
2. How detailed process is defect trailing?
How is the testing procedure designed?
• Is testing procedure constant for all products? Or basing case by case?
• What has been the problem in the original component? (How has the component, manufactured using traditional method, failed?
• Does the new component have the same danger to fail?
• What defects are identified to be the most dangerous for the new component in the application?
• In what location of component the defect is the most dangerous?
• What testing methods can found detrimental defects in the critical area of component?
• Are test results recorded? (Idea is to follow, which test revealed a defect. This helps future designing of testing.)
• Are test methods chosen according to history of recorded tests, found defects and application?
Is any AM standard in use?
Do customers require manufacturing specimen or following AM standards?
Are specimen manufactured along the product?
EXPERIMENTAL PART
8 RESULTS AND DISCUSSION
Visit to 3DStep Oy was conducted on 25.11.2019. Guide for the visit was Vesa Kananen, CTO of 3DStep Oy. Agenda of visit based on the literature review and checklist done based on literature. First, a general presentation of factory, then introduction for the powder handling and storage methods, which company follows. Finally, introduction to methods of company concerning the laboratory usage and testing of sample and products. The methodologies are also evaluated and considered, what could be done differently to reach more efficient utilization of laboratory and time.
8.1 Company goals
Company mentioned during the visit that there are several desires concerning process and protocol development. The first one is, that company would invest to new manufacturing machine for metal powders. This would be important, because then aluminum and steel materials could be manufactured in separate machines. In other words, need for changing material for machine disappears. Second essential idea for the company is measuring the moisture content of powder before manufacturing.
3DStep Oy also presented that three the most quality affecting factors are moisture and purity of powder, design and orientation of product. The orientation of product is a term for built direction in relation to products own geometry. Design and orientation affect immensely on required amount of support in product manufactured using AM-methods. SLM-280HL, used by 3DStep Oy, produces data for each layer, in order to follow the successfulness of each product. This data has been stored and revised only if defects occurred or test results did not meet the required levels.
8.2 Discussion of protocols and arrangements
3DStep Oy uses good practises in many arrangements, such as having different equipment and tools for each material used. But even though production area tidiness is well maintained, the company could arrange the production area differently. Different
organization of area could result in lower risk of contaminating powders, process chamber or other critical phases of manufacturing.
An example from previous experience in factory is concerning plastic extrusion plant. There all equipment and materials were sorted and organized according to 5S method. 5S-method is used to reach more organized working environment. Each tool has a place, to where it is returned after use. Excessive materials and equipment are discarded. 5S-methods aims to retain only necessary items in locations, where they are necessary. Another idea behind 5S- method is to look good. Since it is customary to invite customers to visit factory, it has an important effect on thoughts of customer about the company and manufactured goods by the company. The customer might not be an expert nor familiar with quality aspects of AM. This way quality could be shown otherwise.
Figure 8. Example of the facilities at Amexci AB. (Amexci AB, 2020)
A good example of organizing of AM facilities is Swedish AM-company Amexci AB. In webpage of Amexci, pictures of well organized, laboratory-like conditions are presented. An example can be seen in Figure 8. Only manufacturing machines are seen in manufacturing area. Powders are stored in separate rooms according to Amexci AB webpage. However, concerning the 5S-method, the reception of ideas on the employee level was not as good as
desired. Typical resistance of changes was recognized in the earlier experience. In order to inspire employees to follow strictly 5S or similar methods, it is vital to give grounds in understandable way. This could occur as explaining importance using quality meters and money. Field related quality meters and money are probably the two most common measurements that employees understand.
The most important notification is that, there was not excessive powder on the floor. Strict cleaning protocols are followed. More organized production area could help minimizing the risk and reducing the time and effort spent on removing powder from the floor. Good example is that stored powder is laying on the floor in stacks. The best possible option for this, could be separate room with shelves and lockers for each material separately. Another suggestion for the storage room is the reduced exposure for sunlight. However, sunlight exposure is more problematic with plastic materials.
. In addition to desire of investing fully new manufacturing machines, company could also consider closed-loop system for powder feeding. An example of this is presented in literature review in chapter 4 Figure 5. This example is PSV system designed and produced by SLM Solutions. This could lead to reducing the manual powder handling and risks of contamination.
Testing is designed and recommended for each customer separately. The test design and recommendations are based on the product and its requirements. The company does good work on this part
Company addressed issue of standardization in AM field. Company said that it is difficult to reach perfect quality, since there has not been developed complete standards for AM-method quality and testing aspects. For most of the products, which are planned to be manufactured using AM-methods, testing sequence must be individually designed. Standards do not give full coverage for the required material properties or end quality requirements for possible aspects. 3DStep follows development of AM-standards. In addition, 3DStep desires to be developing the quality of AM.
One of 3DStep Oy main concerns during visit was, that every manufactured batch of a single products tends to have some variation of quality. This variation can be seen coming simply from variation of powder properties. The powder properties are given with certain ranges.
These ranges then can occur differently to each batch of products. In addition to powder properties, factors found in literature review affect as well.
8.3 Discussion of success of checklist
The checklist worked well for evaluation. The questions covered on the general level the whole manufacturing process of the company. Additional questions could be included to the checklist. These questions should involve data collecting and analyzing. The company collects data from each manufacturing batch. This data should be then utilized. The existing questions in the checklist are broad. More detailed approach could produce answers, which then could be used for improving the manufacturing process. Developing the questions into more detailed and specific is therefore important. Especially in powder handling section.
Powder handling section questions do not have sub-questions to detail possible outcome more.
Idea with the sub-questions would be breaking each work phase into small pieces and evaluating those. Adding very simple questions, for example concerning work order in removal of finished product. This action would be broken into phase-by-phase evaluation.
This could end up tedious evaluation. This evaluation could be conducted with the employees of the company. Opening each action with all parties could provide point of views, which were not considered in this study.
An important addition for the checklist could be the handling of waste powder. Waste powder is important to removed and managed carefully. The company presented their method for handling waste powder. The method is safe, since the powder is combined with water. Water removes the danger of fire and makes particle size increase. Additional questions and development for this area could increase the safety and quality of the entire process.
8.4 Updated checklist.
How is the testing procedure designed?
• Is testing procedure constant for all products? Or basing case by case?
• What has been the problem in the original component? (How has the component, manufactured using traditional method, failed?
• Does the new component have the same danger to fail?
• What defects are identified to be the most dangerous for the new component in the application?
• In what location of component the defect is the most dangerous?
• What testing methods can found detrimental defects in the critical area of component?
• Are test results recorded? (Idea is to follow, which test revealed a defect. This helps future designing of testing.)
• Are test methods chosen according to history of recorded tests, found defects and application?
Is PSV used in production?
• If not:
1. How powder is handled, sieved and reused?
2. How is powder reuse history followed?
3. Is it necessary to follow reuse of powder due to high consumption of powder?
How build chamber cleaning is performed?
• What kind of protective gear is worn during operation?
• How excessive powder is removed?
• Is the build chamber fully cleaned before removing the product?
• Is the build chamber inspected after removing the product?
• Is the cleaning operation standardized inside the company to be fully same each time regardless of performer?
What are the storage conditions?
• How are the storage conditions monitored and maintained?
• Are the powder containers stored separately from the manufacturing area?
• Are there detailed instructions for operating in the storage area?
1. Are materials clearly divided to different locations?
2. Is there inspections for the storage conditions?
How is history of used powder followed?
Are process parameters written down every time?
• Is data used for studying tendencies in quality?
Are defects and their origin followed frequently?
• If yes:
1. When defect is found, is a study made to find out what caused the defect?
2. How detailed process is defect trailing?
Is any AM standard in use?
• Is the development of the standards frequently followed?
Do customers require manufacturing specimen or following AM standards?
Are specimen manufactured along the product?
How waste powder is managed?
• Is the spread of waste powder prevented?
• How is the waste powder stored?
• How often is the waste powder delivered to recycling from the factory?
9 CONCLUSION
Quality in AM is complex subject. It has not been defined fully in standards. Standardization in AM is still developing. For this reason 3DStep Oy wanted to understand, what could be done for improving quality in their processes. Information for improving quality of series products manufactured using AM-methods was desired. These needs are driven by 3DSteps Oy goal to lead development of quality in L-PBF process, especially for series production using AM-methods.
In this study, a literature review is conducted and then a heuristic checklist is formed. This checklist is then used for reviewing the quality of manufacturing process and protocols of 3DStep Oy. The results from literature review and evaluation of quality of the company are discussed. The experimental part of this research is basing in the literature review and visit to the factory of the company. The visit of the company is an introduction and evaluation of the factory.
AM is developing manufacturing field. There are plenty of literature on the topic. Topics handle variety of aspects of AM. In this research the focus was on the quality of AM of metallic materials using L-PBF. The most important factors governing the quality are related to powder handling and properties of powder. Powder handling affects to particle size distribution, which then affects to density of finished product.
SLM Solutions have developed different secondary systems for aiding the L-PBF process.
The secondary systems record manufacturing parameters and conditions, and handle powder input automatically, therefore reducing the risk of failure. Recorded parameters and conditions are laser power, state of melt pool and state of powder bed. These parameters and conditions can give good idea on possible failures and their reasons.
For examining quality in production of 3DStep Oy, a heuristic question list is formed according to findings of literature review. Heuristics are simple and basic ideas for maintaining quality. Heuristics produce solutions, which are good enough, but might not give any result at all.
3DStep Oy uses L-PBF in the production. Used system is SLM-280HL by SLM Solutions.
3DStep oy does not use the PSV system. The company uses other quality recording sensors, which are already intact in SLM-280HL system. These sensors record some data for the company, which is saved. The collected data is not really put in use. Unless failure occurs, this data is not reviewed.
Standardization in AM is developing, especially in quality wise. This acknowledged by the company as well. This is one of the motives of company to research the topic of quality in AM. The company follows development of standards, but also desires to be ahead of the curve in quality. The company has the general level of protocols on good level. Only few suggestions are made in protocols. Mainly these focuses on using quality management process, in example 5S, which is part of LEAN-management tools. Other suggestions given for the company are concerning powder handling and storage of powder
All in all, the company has good idea about the factors affecting to quality. The company manufactures series products and therefore the variation of properties of powder and manufacturing system, such as laser power and shielding gas flow. For the company goals, it is important to invest for the machinery. Additional machinery could be then used with one machine for each material. This results in lower risk of contamination of final product.
The checklist provided general answers. The general answers are good way to start examining the quality. More detailed questions could benefit especially in the powder handling section. Powder handling is an important factor, which affects to the quality of finished product. The company visit presented a view for the waste powder handling. Waste powder handling was not covered in the checklist at all. Therefore adding questions concerning waste powder handling could increase the value of the checklist.
10 FURTHER RESEARCH TOPICS
For 3DStep Company, a good topic to research is the data history, which they have produced during operating. There could be drawn all the failed products and information relating those failures. Company could then investigate, what are the trends in their failures. The company could conduct a study, what actions led to failures and how these failures could be avoided in future. The company could then adjust their manufacturing protocols and learn new relations in quality aspects of AM.
More topics could be covering the waste powder handling. The waste powder recycling and handling are problems, which the company addressed. The company wished that there would be better solutions than they have at the moment. More research could be made on finding better ways for handling waste powder.
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APPENDIX 1 SLM 280 specifications SLM Solutions 2019. Available: https://www.slm- solutions.com/en/products/machines/slmr280-production-series/
Build envelope volume(x/y/z) 280x280x365
IPG Fibre laser power Single 400W or 700W Twin 2x 400W or 2x 700W
Build rate 88 ccma/h 400W twin
Layer thickness 20 – 75µm
Min. feature size 150 µm
Beam focus diameter 80 – 115µm
Scan speed 10 m/s
Inert gas consumption in operation Ar/N2 2.5 l/min Inert gas consumption in purging Ar/N2 70 l/min
E-connection/consumption 400 V 3NPE, 63 A, 50/60 Hz, 3.5-5.5 kW Compressed air requirement ISO 8573-1:2010 [1:4:1], 50 l/min @ 6 bar Dimensions in mm (L x W x H) 2600 x 1200 x 2700
Weight 1300 kg without powder
1800 kg with powder
Appendix 2 Aluminum powder composition and material properties SLM Solutions 2019. Available: https://www.slm- solutions.com/fileadmin/user_upload/200EN180924-02-POWDER_WEB.pdf
AlSi10Mg Chemical composition (Nominal) %
Element %
Al Bal.
Si 9,00 – 11,00
Mg 0,20 – 0,45
Cu 0,05
Fe 0,55
Mn 0,45
Zn 0,10
Ti 0,15
Ni 0,05
Pb 0,05
Sn 0,05
Others 0,05
Total others 0,15
AlSi10Mg mechanical properties according to SLM Solutions.
Mechanical data* Formula symbol and unit AlSi10Mg**
Tensile strength Rm [MPa] 386 ± 42
Offset yield stress Rp0,2 [MPa] 268 ± 8
Break strain A [%] 6 ± 1
Reduction of area Z [%] 7 ± 1
EModul E [GPa] 61 ± 9
Hardness by Vickers [HV10] 122 ± 2
Surface roughness Ra [µm] 8 ± 1
Surface roughness Rz [µm] 63 ± 10
*Process conditions and parameters according to SLM Solutions standards
**Layer thickness 50µm, as built
Appendix 3 Stainless steel powder composition and material properties SLM Solutions 2019. Available: https://www.slm- solutions.com/fileadmin/user_upload/200EN180924-02-POWDER_WEB.pdf
316L (1.4404) Chemical composition (Nominal) %
Element %
Fe Bal.
Cr 16,00 18,00
Ni 10,00 14,00
Mo 2,00 3,00
Si 1,00
Mn 2,00
C 0,030
N 0,10
P 0,045
S 0,030
O 0,10
Mechanical data* Formula symbol and unit 316L (1.4404)**
Tensile strength Rm [MPa] 633 ± 28
Offset yield stress Rp0,2 [MPa] 519 ± 25
Break strain A [%] 31 ± 6
Reduction of area Z [%] 49 ± 11
EModul E [GPa] 184 ± 20
Hardness by Vickers [HV10] 209 ± 2
Surface roughness Ra [µm] 10 ± 2
Surface roughness Rz [µm] 50 ± 12
*Process conditions and parameters according to SLM Solutions standards
**Layer thickness 50µm, as built
Appendix 4 Tool steel powder composition and material properties SLM Solutions 2019. Available: https://www.slm- solutions.com/fileadmin/user_upload/200EN180924-02-POWDER_WEB.pdf
1.2709 Chemical composition (Nominal) %
Element %
Fe Bal.
Ni 18,00 19,00
Mo 4,70 5,20
Ti 0,50 0,80
Co 8,50 9,50
Al 0,05 0,15
Mn 0,10
C 0,03
P 0,01
S 0,01
Mechanical data* Formula symbol and unit
1.2709** 1.2709***
Tensile strength Rm [MPa] 1135 ± 29 1784 ± 313 Offset yield stress Rp0,2 [MPa] 987 ± 15 1920 ± 12
Break strain A [%] 11 ± 1 3 ± 1
Reduction of area Z [%] 44 ± 2 10 ± 0
EModul E [GPa] 113 ± 8 125 ± 5
Hardness by Vickers [HV10] 373 ± 2
Surface roughness Ra [µm] 9 ± 1
Surface roughness Rz [µm] 67 ± 5
*Process conditions and parameters according to SLM Solutions standards
**Layer thickness 50µm, as built
***Layer thickness 50µm, heat treated
