APPLICATION OF LASER ADDITIVE MANUFACTURING TO PRODUCE DIES FOR ALUMINIUM HIGH PRESSURE DIE-CASTING #

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1 APPLICATION OF LASER ADDITIVE MANUFACTURING TO PRODUCE DIES FOR ALUMINIUM HIGH PRESSURE DIE-CASTING # 1 M.F.V.T. Pereir 1 *, M. Willims 2 & W. B. du Preez 3 Metls nd Metls Processes CSIR Mterils Science & Mnufcturing, South Afric 1 mpereir@csir.co.z, 2 mwillim@csir.co.z, 3 wdupreez@csir.co.z ABSTRACT A number of lser dditive mnufcturing (LAM) technologies cn produce fully-dense metl components tht potentilly offer opportunities to pply the technology in die or mould mking (known s rpid tooling). From these LAM technologies, three were selected for evlution of their suitbility s die cvity inserts in the mnufcture of high pressure diecsting (HPDC) dies. Aprt from compring the different LAM inserts with one nother, their performnce ws lso compred with components mnufctured in stndrd hot work steel. In the HPDC process, the die is unique to ech component to be produced. Die cvities in prticulr re subjected to demnding conditions, such s cyclic heting cused by the introduction of molten luminium t over 650 C, followed by cooling in wter-bsed die relese medium t tempertures round 25 C. Besides cyclic heting, the die cvities re lso exposed to pressures exceeding 1500 MP during the injection of molten luminium into the cvities. This ppliction of LAM, therefore, poses extreme chllenges to the technology. The results of this study confirmed tht the metls used in some of the LAM technologies did indeed meet the demnding requirements of the current ppliction, nd cn led to significnt time- nd cost-sving during product or process developments. Bsed on these findings, number of recommendtions re mde for users interested in the ppliction of LAM to produce die cvities. OPSOMMING n Antl lser lgvervrdigingstegnologieë (LLV tegnolgieë) is in stt om volledig digte metlkomponente te lewer, wt tot gevolg het dt hierdie tegnologie potensieel vir vormingsmtrys- of gietmtrysvervrdiging gebruik kn word (bekend s snelgereedskp). Drie vn hierdie LLV tegnologieë is gekies vir evlusie vn hulle geskiktheid s gietvorminsetsels tydens die vervrdiging vn gietmtryse vir hoë druk mtrysgiet (HDMG). Benewens die onderlinge vergelyking vn die verskillende LLV insetsels met mekr, is hulle werkverrigting ook vergelyk met komponente wt in stndrd wrmwerkstl vervrdig is. In die HDMG proses is die gietmtrys uniek n elke komponent wt gevorm word. Die mtrysholtes word verl n uiterste toestnde onderwerp, soos sikliese tempertuurverskille, veroorsk deur die blootstelling n gesmolte luminium by meer s 650 C, direk gevolg deur fkoeling in wtergebseerde smeermiddel by temperture vn tipies 25 C. Buiten sikliese verhitting, word die mtrysholtes ook tydens die inspuit vn gesmolte luminium in die holtes blootgestel n drukke hoër s 1500 MP. Hierdie toepssing vn LLV stel dus uiters hoë uitdgings n die tegnologie. Die resultte vn hierdie studie het bevestig dt die metle wt in sommige vn die gekose LLV tegnologieë gebruik word, wel voldoen n die besondere vereistes vn die huidige toepssing, en dt die gebruik hiervn kn lei tot beduidende tyd- en kostebespring tydens produk- of prosesontwikkeling. Gebseer op hierdie bevindings word n ntl nbevelings gemk vir gebruikers wt sou belngstel in die toepssing vn LLV vir die vervrdiging vn gietmtryse. 1 # This rticle is n extended version of pper presented t the 2011 RAPDASA conference. South Africn Journl of Industril Engineering, July 2012, Vol 23 (2): pp

2 1. INTRODUCTION Aluminium lloy components for number of utomotive pplictions re mss produced using production process clled high pressure die-csting (HPDC). This mnufcturing process imposes severe stresses on the dies forming during processing. These stresses re cused when molten luminium is injected t high pressure into the die cvities to fill them nd then to solidify. Furthermore, the die mnufcturing cost is significnt component determining the economic fesibility of the die-csting process. Dies re mnufctured from steels normlly referred to s hot work steels tht cn withstnd the process conditions for n undetermined period of time. The terms die life nd expected minimum die life before filure follow from the process-induced wer nd ter. Dies must be ble to produce components repetitively, with prescribed dimensions nd good surfce finish tht comply with specifiction. After some use, die cvity wer nd ter is noticeble to such n extent tht the die becomes unusble, nd new die is required to meet the dimensionl nd surfce requirements of the component being produced. The most importnt die wer nd ter filures re clssified under two ctegories: wshout nd therml ftigue. Wshout dmges re direct result of the flow of the luminium melt impinging on nd rubbing ginst the die cvity surfces. Corrosion is ttributed to friction wer tht is cused when the melt solidifies round the core surfces, nd when the csting is ejected. Therml ftigue, the most influentil filure mode in die-csting, revels itself in two wys: het checks nd stress crcks. Therml ftigue crcks occur s the die cvity surfces re plced under tension when the cold (~25 C) wter-bsed relese gent impinges on the hot surfces previously exposed to the luminium melt. The cooling effect of the die lubricnt spry on the underlying hot mteril cuses tensile stress in the hot die surfce, cusing surfce crcks. This cycle is repeted ech time csting is mde. The chrcteristic feture of het checks is the ppernce of fine crcking lines on surfces tht look like spider s web. Stress crcks pper minly in corners, nd pper s individul nd clerly defined crcks, sometimes filled with luminium. Becuse die mnufcture is n expensive nd time-consuming process, it is often the limiting fctor in the drive towrds reduced time-to-mrket for new product development. Should criticl prts of die or mould hve to be redesigned nd replced, development costs could significntly increse, ccompnied by undesirble time delys. For this reson, the cost sving nd time reduction tht could result from using dditive mnufcturing (AM) to produce complex prts of die hve become strong incentives for exploring the integrtion of AM in the mnufcturing chin, especilly during product development. Applying this pproch to the prticulrly chllenging high temperture process of HPDC hs formed the core of reserch progrmme pursued by the uthors in recent yers. No similr ttempts hve been published to dte. In the reserch reported here, smples grown with the lser dditive mnufcturing (LAM) technology pltforms LserCUSING, EOS, nd LENS were compred with ech other under typicl opertionl conditions. The im ws to ssess the bility of the vilble AM metls to withstnd the HPDC processing conditions. The AM cores were evluted ginst stndrd hot work steel counterprts. 2. LASER ADDITIVE MANUFACTURING TECHNOLOGIES INVESTIGATED In this reserch, two types of LAM technologies suitble for the mnufcture of die components were investigted. One of the types uses the technology commonly referred to s powder bed lyer deposition, with EOS nd Concept Lser s LserCUSING being the two selected leding technologies. Another pproch to building prt from lyers of mteril is to deliver powder by nozzles directly to the point where focused lser melts the powder, fusing it into prt, line-by-line nd lyer-by-lyer. Clled powder deposition [2], the technique typiclly offers lrger working envelopes, nd the bility either to mke prts or to repir existing prts. LENS, leding technology in this field ws selected for the 148

3 current reserch. In this reserch, test coupons mde with the mterils given in Tble 1 were evluted. Tble 1: Metl powders used by service providers to mnufcture test coupons Number Mnufcturer System Mterils 1 Concept Lser M3 Liner CL 50 WS (Hot work steel) 2 Lens Lens 750 Model H13 (Din Hot work steel) 3 EOS EOSINTM250X Direct Steel DS20 (steel)* * EOS equipment suppliers were not ble to provide test coupons for evlution in more suitble mteril for die components. 3. EXPERIMENTAL METHOD A test rig ws designed nd mnufctured for the reserch. The rig is ble to subject the LAM grown, nd the stndrd steel mnufctured specimens, to cyclic heting nd cooling through immersion in liquid luminium. The dipping cycle closely resembles the heting nd cooling cycle of typicl luminium die under csting conditions. The suitbility of ny LAM technology tht cn produce fully dense metllic components cceptble for die-csting cn be quickly ssessed with the test rig. With economic fesibility in mind, nd knowing tht on stndrd hot work steel, evidence of het checking dmge ppers fter few thousnd cycles, 10,000 cycles ws set s the experimentl benchmrk. The ssessment of dmge inflicted on the cycled specimens ws performed through opticl microscopy of both fces nd shrp corners, to nlyse the extent of het checking crcks, s well s the possible presence of corrosion pits. Furthermore, impct toughness nd hrdness vlues of cycled nd un-cycled specimens were evluted, in order to ssess the extent of mteril property vrition. The initil experimentl pln ws for ech LAM system supplier to fbricte four test coupons: three solids, nd one with n open blind-hole pssge. (Refer to the digrm in Figure 1.) Figure 1: Test coupon with squre cross section of 10mmx10mm, length 100mm, with blind hole dimeter 6mm up to 2mm from the bottom 149

4 Figure 2: Test coupon hole spect for the three systems The results of initil visul nd geometricl evlution of the test coupons re shown in Tble 2. Tble 2: Results of visul exmintion Description System Concept M3 Liner LENS 750 Model EOS M 250 X Totl shpe Complete Complete Complete Dimension chnge mm ± 0.1 ± 1.5 ± 0.1 Fine detils Very ccurte Poor Very ccurte Holes Shrp Correct depth Circulr Deformed; required 1mm per side mchining Shrp Correct depth Circulr Shrp edges Shrp Blunt Shrp Shrp corners Shrp Blunt Shrp Surfce roughness (finished surfces) Smooth Rą 3.2 µm Rough Rą 6.4 µm Smooth Rą 3.2 µm Lyer coherence Complete coherence (Lyers re not visible) Complete coherence (Lyers re not visible) Complete coherence (Lyers re not visible) Crcks [2] No crcks No crcks Internl crcks Exmintion result Very Good Averge Good The comprison of costs nd finishing is shown in Tble 3. Tble 3: Processing nd finishing costs of test coupons Description (1) System Concept M3 Liner LENS 750 Model EOS M 250 X Lyer thickness (mm) Totl process time (hrs) Finishing time(hrs) 1 + (het tretment) 4 1 Mterils cost USD / kg [1] Coupon benchmrk cost vs wrought mnufctured 5.2 1, Averge cost Very high Medium High 150

5 3.1 Cyclic immersion in molten luminium Figure 3 below shows the testing pprtus tht ws developed to simulte therml cycling conditions tht occur inside the cvity surfces of the die when in contct with the luminium melt. The specimens were mounted in this rig, with two in ech rm end. At the sme time, specimens mounted on both sides of the rotting rm were immersed either in luminium t C or in cooling bth t C. Then the two opposite sides of the rotting rm were lifted nd the immersion order ws reversed. An verge cycle time of 20 seconds ws chieved. The heting nd cooling cycle mesured on the specimens rnged between 80 nd 540 C. A typicl die-csting therml shock cycle experiences rnges between 110 nd 480 C. This mens tht the specimens were subjected to n even more severe het shock cycle thn during norml csting conditions. The time of immersion in either luminium melt or cooling medium ws five seconds. Pneumtic Cylinder Arm Specimens Cooling bth Crucible Figure 3: Aluminium melt cyclic dipping test rig 4. EXPERIMENTAL RESULTS AND DISCUSSION The dt nd results obtined re described, discussed, nd summrised in the following sections. 4.1 Specimen preprtion The LAM smples used in the experiment hd mesured degree of verge surfce roughness of Rą of 3.2 µm. The preferred stndrd die cvity finish lies between Rą0.80 nd 0.20µm. Surfce grinding, conventionl mteril removl process, ws used to bring the finish in line with the ccepted norm. The grinding opertion took cre of both the roughness nd the distortion induced by het tretment of the LAM specimens. A mximum distortion of 0.2mm ws mesured on the specimens. The LAM smples mesured between 9.98 nd 10.10mm on the requested 10.0x10.0mm cross section dimensions, fter grinding to n cceptble finish free of distortion. The cross sections mesured between 9.50 nd 9.80mm. The mesured deposition rte ws 4cm³/h, which trnsltes to 2.5h for growing specimen, nd nother hlf-n-hour for removl from the support plte. 151

6 Specimen HWS DIN Tble 4: Comprison of mnufcturing times Specimen EOS Specimen Lens Specimen Concept Process Time (h) Process Time (h) Process Time (h) Process Time (h) Milling 2.5 Lser sinter 10.5 Lser weld 2.5 Lser weld 12 Grinding 1 Grinding 1 Grinding 4 Grinding 1 Jig bore Jig bore Jig bore Jig bore Het tretment 4 Het tretment Het tretment 4 Het tretment F grind F grind F grind F grind Polish Polish Polish Polish Fitting Fitting Fitting Fitting Totl Het checking nd corrosion checks (Note: Times re bsed on four specimens.) The results fter cyclic immersion show tht some luminium welding occurred on cores grown with the LAM pltforms, nd to lesser extent on the H13 core (Figure 4). Closer investigtion showed evidence of crcks nd pitting, minly t the corners. The specimens were clened in NCl solution before conducting closer investigtion Figure 4: Specimens before nd fter 10,000 luminium melt dipping cycles Figure 5 shows these conditions t 5x mplifiction of the cores. 4.3 Mechnicl integrity nd microscopy The toughness nd hrdness of the specimens ws determined before dipping nd fterwrds. A Vickers nd Rockwell notch hrdness testing mchine ws used to mesure the hrdness of the cores. Tble 5 gives the results, using 20kg nd 10kg lod. A cler softening effect ws noticed on ll specimens. It ws less pronounced for the lredy soft EOS grown specimens.

7 Specimen H13 Specimen H13 Typicl corner spect before dipping Specimen Concept Corner spect fter 10,000 dips: chipping wshout, pitting & slight Al welding Specimen Concept Typicl corner spect before dipping Specimen LENS Al welding, pitting & wsh (post 10,000 dips) Specimen LENS Typicl corner spect before dipping Specimen EOS Al welding, & crcks (post 5,000 dips) Specimen EOS Typicl corner spect before dipping Al welding, pitting & crcks (post 5,000 dips) Figure 5: Opticl microgrphs of the specimen corners nd notches t 5x mgnifiction 153

8 H13 Hrdness (HRC) (20kg) Tble 5: Results of Vickers nd Rockwell notch hrdness tests LENS Hrdness (HRC) (20kg) EOS Hrdness (HV) (10kg) Concept Hrdness (HRC) (20kg) Before: Before: Before: Before: After: After: After: After: Tble 6 gives the chnges in impct toughness found with the number of immersion cycles. H13 (Din1.2344) Impct energy (Joules) Tble 6: Results of Chrpy impct tests LENS Impct energy (Joules) EOS Impct energy (Joules) Concept Impct energy (Joules) Not dipped 11.8 Not dipped 7.8 Not dipped 7.8 Not dipped 18.3 Dipped 8.8 Dipped 8.8 Dipped 7.8 Dipped 8.0 The H13 cores showed decline in impct toughness the result of the nneling effect experienced during the het cyclic dipping test. Surprisingly, the LENS cores showed slight increse in toughness, which cn be ttributed to less brittle, more tempered core fter experiencing the het cycling tretment. The EOS cores showed no chnge in impct toughness. The Concept LserCUSING cores showed steep decrese in toughness. This cn be ttributed to the fst nneling, ccompnied by loss of ductility or lower elongtion properties of the cores, compred with their condition before cyclic dipping. Metllogrphic investigtion reveled some microstructure modifiction on the H13 nd LserCUSING specimens. However, the H13 core showed slightly less crck growth thn the LAM cores, which clerly reveled some degree of chnge. Only n extended dipping cycle experiment (20,000 dips) would revel whether the difference in the size of crcks, nd the crck density between the test coupons, is significnt. The prtil loss of hrdness of the H13 (Din ) test coupon (composition C = 0.4%, Si = 0%, Cr = 5.3%, Mo = 1.40%, nd V = 1.0% with high red strength) when compred with the Concept CL 50 WS equivlent to Din powder (composition C = 0.02%, Si = 0.1%, Mn = 0.1%, Ni = 18.5%, Ti = 0.7%, Mo = 5.0% nd Co = 8.8%) ws surprising. The greter loss of hrdness cn be ttributed to the high Ni (bove 18%) content of the Concept lloy. Ni content in lloy steels normlly lowers the mrtensite to ustenite trnsformtion temperture (which tends to be lower thn the Al melt temperture), resulting in rpid softening of the steel. The slower softening rte of the conventionl (H13) nd LENS Din specimens clerly indictes tht the het tretments were done in non-protective tmosphere. Further metllogrphic investigtion did not revel significnt microstructure chnges in the Concept, LENS, nd H13 specimens. However, the EOS specimen clerly showed some degree of chnge. The microgrphs of the microscopic nlysis of the EOS specimens in Figure 6 indicte n incresed presence of gs pores, ccompnied by crck formtion. 154

9 Typicl surfce ppernce of EOS specimen before dipping (20x) Pore re in corner conducive to crck formtion in EOS specimen before dipping (50x) Chipped corner on dipped EOS specimen (50x) Typicl spect of shrp corner of H13 specimen before dipping (20x) H13 microstructure before dipping(100x) hrdness 50HRC. Even diffusion of lloying crbide elements in ferrite mtrix Evidence of some crcking nd corrosion on dipped H13 specimen (100x) LENS microstructure very coherent due to the complete melting of powders during processing (200x) hrdness 52 HRC LENS specimen microstructure fter dipping (200x) hrdness 50HRC. No significnt vrince of mtrix with very few porosities nd inclusions Slight evidence of crcking on dipped LENS specimen (100x) Concept specimen melt deposition trcks visible; nucletion nd ggregtion of intermetllic lloys (100x) Concept coupon fter dipping: notice gglomertion nd segregtion of intermetllic lloys coupled with incresed number of smll pores (200x) Concept coupon fter dipping: chipped, nd wer on corner visible (100x) Figure 6: Opticl microgrphs of H13, LENS, Concept Lser, nd EOS smples 155

10 4.4 Discussion nd recommendtions 156 Previous findings on EOS grown cores [3,4] showed tht this technology ws dequte to produce: good cndidtes for time compression of product development or production of prototype production runs; cores tht will withstnd industril-stndrd HPDC processing conditions to produce smll btches i.e. <5,000 shots. Lrger btches could be produced if it ws economiclly fesible to prepre spre cores for replcement fter predetermined number of shots; good ner-net cpbility tht cn reproduce geometry with n llowed mteril tolernce of 0.25mm, but t slow rte of deposition (typiclly 2-10cm³/hour); significntly reduced time to mnufcture dies if due considertion is given to size nd volume constrints of the LAM pltform. From the current investigtion, cores grown with Direct Steel 20 hve shown low core nd surfce hrdness, s well some degree of porosity. Low core hrdness nd porosity limit extensive use under molten luminium processing conditions. Fst crck propgtion, resulting in surfce degrdtion, is the min limiting fctor. It ws interesting to note tht the degree of porosity evident in the LAM grown cores reveled some positive results under processing conditions. When such core is exposed to het, the ir present in the pores expnds nd cretes film tht llows for longer melt flow distnce nd fster solidifiction cooling rte. The performnce of the LENS LAM grown cores [5] under these prticulr experimentl conditions ws remrkble: They showed n bility to withstnd industril-stndrd HPDC processing conditions, to produce cstings equivlent to stndrd die mterils. The time to mnufcture dies cn be significntly reduced due to the vilbility of mchines with build envelope size equivlent to NC die mchining cpbility (motion 150cm x 90cm x 90cm (z-xis)). The LENS multi-nozzle cpbility opens opportunities for further development of: o Strtegies of fst rtes generlly, nd lower rtes of mteril deposition where geometricl ccurcy is required. o Multi-mteril deposition. o Full die mnufcture nd die repir, due to the vilbility of pltforms with multi-xis rnge of motion (up to seven). The 3D model genertion of cores before lser growing for the LENS system should include n overll surfce mteril llownce of between 1.5 nd 2.5mm. The current work gives cler evidence tht specimens mnufctured from powders equivlent to hot work steel DIN hve core nd surfce hrdness equivlent to the stndrd die mteril. The loss of hrdness experienced under cyclic het testing is significntly less thn tht experienced with the stndrd hot work steel counterprts used by industry. This finding is supported by microscopy, which revels finer nd welldistributed metl phses nd constituent lloy element diffusion in the steel mtrix when compred with the industry counterprt. The performnce of the LserCUSING LAM grown components [6] cn be considered quite remrkble: They re ble to produce die components for stndrd industril production btches. Their good ner-net cpbility is ble to reproduce geometry with n llowed mteril tolernce of 0.3mm. A resonble turnround time, idel for complex geometry cvity inserts requiring integrted cooling tht is prticulrly difficult or impossible to ttin with norml mechnicl metl removl techniques.

11 Avilbility of steel powders with similr chrcteristics to stndrd used hot work steels, e.g. Din As fr s core nd surfce hrdness properties re concerned, the current reserch yielded results tht compre very fvourbly with the stndrd counterprt. The bsic metl qulity ws found to be better thn the industry stndrd hot work steel. From these nd previously-published reserch results, it is cler tht the following spects hve to be considered in respect of the use of lser dditive mnufcturing to produce die or mould components: Component geometry is very complex i.e. in order to mnufcture conventionlly, multiple mteril removl technologies would be required, such s electric dischrge mchining (EDM), milling, nd grinding. Hot work steel cn be deposited through LAM with cceptble qulity. Cost must be competitive. A rule of thumb is to look t the mount of mteril tht needs to be removed from billet or piece of wrought mteril to mnufcture die cvity. If the mount of mteril removed is bove 75%, there is good likelihood tht the component is cndidte for LAM. As finl recommendtion bsed on the overll outcome of the reserch, number of die mnufcturing strtegies should be pplied. To minimise deposition nd die mnufcture time, consider: growing the core over comptible mteril substrte; in the design of cores or die structures fixtures, llowing for the required holding nd setting up when finishing prts by conventionl mchining methodologies; designing hollow cores, with the opportunity to include cooling in the components; plnning for components to be grown t the required finl hrdness, free of distortion. 5. CONCLUSION The ccelerted testing of cores mnufctured through the EOS-DMLS, Concept Lser s LserCUSING, nd the LENS blown powder LAM technology under typicl production conditions provided vluble insight into the pplicbility of these technologies. Comprtive nlyses of both the LserCUSING nd the LENS specimens with the stndrd die steel showed tht both would be ble to fulfil this role in very similr wys. The ner-net shpe mnufcturing cpbility of powder bed LAM technology mkes systems such s EOS the choice for developmentl, prototype, or short production run dies. Concept Lser s LserCUSING technology is good choice for complex, smll-to-medium die inserts tht must be ble to withstnd stndrd luminium production rtes. The LENS blown powder LAM process is the choice for the mnufcture of lrger dies nd the repir of production dies. More extensive mechnicl removl type of finishing opertions will be required when compred with the powder bed technology LAM pltforms. LAM technology hs mtured enough for its widespred use by industry. Economic fesibility will be the key to more rpid doption of the technology by industry. The rnge of pplictions could multiply exponentilly if the gp is reduced between the cost of LAM technology using steel powders, versus wrought steels nd llied mechnicl removl technologies. 157

12 REFERENCES [1] Abdel Ghny, K Benchmrk comprison to evlute five 3D printers nd rpid prototyping systems, 7 th Annul RAPDASA Interntionl Conference, Cpe Town, South Afric. [2] Wohlers, T Rpid prototyping, tooling & mnufcturing stte of the industry, Annul Worldwide Progress Report, Wohlers Assocition Inc, USA. [3] Pereir, M.F.V.T., Willims, M. & Bruwer, R Rpid die mnufcturing using direct lser metl deposition, Journl for New Genertion Sciences, 7(3), [4] Pereir, M.F.V.T., Willims, M. & du Preez, W.B Chrcteriztion of metl powder bsed rpid prototyping components under luminium high pressure die csting process conditions, 9 th Annul RAPDASA Interntionl Conference, Bloemfontein, South Afric. [5] Pereir, M.F.V.T., Willims, M. & du Preez, W.B Chrcteriztion of metl powder bsed rpid prototyping components with respect to luminium high pressure die csting process conditions, Journl for New Genertion Sciences, 8(2), [6] Pereir, M.F.V.T., Kunene, G., Spiering, A. & du Preez, W.B Performnce of LserCUSING metl powder specimens under luminium high pressure die-csting process conditions, 10 th Annul RAPDASA Interntionl Conference, Est London, South Afric. 158

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