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双面钻孔组合机床调查报告我毕业设计的题目是：汽车变速箱体孔加工专用机床设计。汽车变速箱体有前后两个端面需要进行孔加工，为了提高效率，我们采用双面钻孔组合机床对它们同时进行加工。为此，我们一组特地于2007年4月3日前往扬州机床厂，对他们车间的双面钻孔组合机床进行调研。我国传统的组合机床及组合机床自动线主要采用机、电、气、液压控制,它的加工对象主要是生产批量比较大的大中型箱体类和轴类零件(近年研制的组合机床加工连杆、板件等也占一定份额) ,完成钻孔、扩孔、铰孔,加工各种螺纹、镗孔、车端面和凸台,在孔内镗各种形状槽,以及铣削平面和成形面等。组合机床的分类繁多,有大型组合机床和小型组合机床,有单面、双面、三面、卧式、立式、倾斜式、复合式,还有多工位回转台式组合机床等。随着技术的不断进步,一种新型的组合机床柔性组合机床越来越受到人们的青睐,它应用多位主轴箱、可换主轴箱、编码随行夹具和刀具的自动更换,配以可编程序控制器( PLC) 、数字控制(NC) 等,能任意改变工作循环控制和驱动系统,并能灵活适应多品种加工的可调可变的组合机床。另外,近年来组合机床加工中心、数控组合机床、机床辅机(清洗机、装配机、综合测量机、试验机、输送线) 等在组合机床行业中所占份额也越来越大。我们主要到扬州柴油机厂的汽缸盖生产车间，主要参观了钻汽缸盖前后端面螺纹孔的专用机床。双面钻孔组合机床是针对在工件两相对表面进行钻孔加工而设计的一种高效自动化专用加工设备，其基本结构是：两个动力滑台对面布置，安装在标准侧底座上，刀具电动机固定在滑台上，中间底座上装有工件定位、夹紧装置。该双面钻孔组合机床采用电动机驱动和液压系统驱动相结合的驱动方式，其控制过程是典型的顺序控制，若采用可编程控制器（PLC）来构成其电气控制系统,则电气系统具有体积小、维修量少、工作可靠、操作简单并能适应控制要求等优点。双面钻孔组合机床由动力滑台提供进给运动,电动机拖动主轴箱的刀具主轴提供切削主运动,机床的动力滑台和工件定位、夹紧装置均由液压系统驱动。 这次的调研使我们加深了对组合机床的感性认识，结合以往所学知识和参考资料，我们将对将要设计的双面钻孔组合机床有更好的理性认识，将对随之而来的设计工作起到很好的实践指导作用。International Journal of Machine Tools & Manufacture 47 (2007) 779784Further studies in selective laser melting of stainless andtool steel powdersM. Badrossamay, T.H.C. Childs?School of Mechanical Engineering, University of Leeds, Leeds, UKAvailable online 7 November 2006AbstractPrevious reported work on the selective laser melting of a H13 tool steel powder bed surface has shown that there is a scan speed rangein which the layer mass increases/fluctuates with increasing speed. This paper expands the investigation towards M2 tool steel and 316Lstainless steel powders to identify if they reveal similar behaviours. Wide ranges of scan spacings and scan speeds have been examined, atselected laser powers. Furthermore, the masses of the layers have been compared with those predicted from an existing finite elementthermal model. It has been found that at a constant laser power, the variation of mass with scan speed is much less than might beexpected from a constant assumed absorptivity into a powder bed.r 2006 Elsevier Ltd. All rights reserved.Keywords: Rapid manufacturing; Selective laser sintering; Absorptance; Simulation1. IntroductionOne current trend in production is the shortening of leadtimes for product development. New processes, especiallythose in the field of layer manufacturing, support thistrend.Inaddition,theyopennewpossibilitiesformanufacturing 1. One group of such processes, selectivelaser sintering or SLS, has become popular for rapidlymanufacturing freeform parts using a wide range ofmaterials 2. Laser sintering of metallic materials showsgreat promise, but significant further research and under-standing are still required for obtaining high-quality parts.This paper is built on previous studies on selective lasermelting (SLM) of single tracks 3 and single layers 4 offerrous alloy powders in the surface of a deep powder bedby a scanning laser beam. SLM is an emerged name for thedirect route of SLS when the complete melting of powderoccurs rather than the sintering or partial melting 5. Theaim is to create a strong part that is usable without furtherpost processing other than surface finishing. In the singletrack work 3, where M2 and H13 tool steel and 314S-HCstainless steel powders were examined, different trackforms were identified, depending on the laser power andscan speed that was used. Explanations for the transitionsbetween these various forms of track were given in terms ofthe melt pool dimensions, temperatures and temperaturegradients that existed. In the single-layer work 4 throughstudying the topography of melted layers from H13 toolsteel, the onset of porosity was addressed.Subsequently the melted masses of single tracks andsingle layers were investigated at various process para-meters 3,4. Since at constant laser power and (for layers)scan spacing, delivered energy density to the bed reduceswith increasing speed, the initial expectation was that forany level of laser power, melted mass would reduce as scanspeed increased. However, the reverse was observed in bothsingle-track and single-layer works. In fact, there existedscan speed ranges in which the mass of melted track orlayer increased (or fluctuated in the case of layer) with scanspeed, at constant laser power and scan spacing. To studythis, the amount of heat required to melt the observed masswas compared to the incident energy. An explanation wasgiven in terms of absorptivity of the laser energy into thebed increasing with speed. But the observation for singletrack was not much emphasized as it was thought that theconditions, in which the increase occurred, being associatedwith the formation of elliptical section (non-flat) tracks,ARTICLE IN PRESS/locate/ijmactool0890-6955/$-see front matter r 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.ijmachtools.2006.09.013?Corresponding author. Tel.: +441133432165; fax: +441133432150.E-mail address: t.h.c.childsleeds.ac.uk (T.H.C. Childs).were not practically useful for processing. However, thesimulated results of single-layer melting have shown thatthe variability of layer mass for the speed range is notpredictable with a constant absorptivity a value. Indeed,understanding the causes of a variation becomes of centralimportance to fundamental studies of single-layer forma-tion. This paper expands the investigation towards M2 toolsteel and 316L and 314S-HC stainless steel powders toidentify these material behaviours in single-layer melting,in conditions in which dense layers have been formed, thatmight form the foundation for the subsequent layers. Thequestions that it addresses are the variation of a in theSLM process and effects of scan spacing and thermalhistory on the melted mass. In an alternative approach forexplanation of mass variations, an idea based on theprocess efficiency is developed later.2. ModellingAn existing finite element thermal model for lasersintering of polymers 6 has been developed to predictthe mass of melted metallic material by the scanning laserbeam. It is a transient heat conduction three-dimensionalmodel in which the temperature rise caused by thetravelling laser beam is calculated 7. The model takesinto account the latent heat and the influence of bothporosity and temperature on the thermal properties of thepowder bed. In the temperature calculation, thermalconductivity k of the powder bed is assumed to vary withporosity as observed experimentally in 8:k ks1 ? ?1 ak?0:78,(1)where ksis the solid conductivity and akis chosen to fit themeasured initial bed conductivity to its measured initialporosity. ksand heat capacity Cpare assumed to varylinearly with temperature:ks k0 k1T; Cp C0 C1T,(2)where k0, k1, C0and C1are material-dependent coefficients.The influence on temperature of phase change with latentheat L between the solidus and liquidus temperatures TStoTLis treated by the temperature recovery method 9.Powder bed density is then assumed to change from itsinitial value to that of solid material as temperatureincreases from TSto TL.In the previous work 3, concerned with melting the firsttrack in a powder bed, the temperature of the powder bedat the start of processing was assumed to be uniform, at apre-determined ambient value. In modelling multi-track(i.e. layer) melting, the temperature existing in the bed atthestartofprocessingatrackisthetemperaturedistribution existing from processing all the previoustracks. Since the current model calculates temperatures ina limited volume surrounding the laser beam, it takes intoaccount this issue (temperature history) approximately.The model presents lower estimates and approximate upperestimates of melted masses for the layer. The lowerestimate assumes the bed to be cooled to ambienttemperaturebeforeeachnewtrackisstarted.Theapproximate upper estimates are based on calculatingmelted masses of raster-scanned tracks each 3mm (fivelaser beam diameters) long, allowed to cool down to thepoint that all molten material has resolidified before thenext track is started. The initial temperature distributionfor melting the next track is taken to be that existing at theinstant that the previous track has just resolidified. This isfelt to give an overestimate to the initial temperaturedistribution around the region of the next track. In thissense, the calculated melted mass of the next track is likelyto be approximately an upper estimate.Thermal conductivity k and absorptivity a of a powderbed are inputs to the thermal model. A calorimetricapproach based on theory in 10 was used in 1 todetermine these values. It was shown that these obtaineddata validated the simulation in a low scan speed region11. An alternative approach to predict the absorptivity,based on heat balance, is used here. Assuming that noevaporation occurs on the surface of the powder bed theoverall energy balance equation is expressed asQabs Qmelt Qloss,(3)where Qabsand Qmeltare the absorbed energy and requiredenergy to melt a layer mass m per unit area, respectively,and Qlossis the rest of energy which disappears through thepowder bed. The heat absorbed per unit area scanned atspeed U by a laser of power P is aP=Us, where s is scanspacing. Qmeltis defined as mCPaveTm? T0 L, whereCPaveis the average specific heat from ambient bedtemperature T0to the metals melting temperature Tmand L is the latent heat of fusion. Energy is lost from themelting zone by conduction, convection and radiation.However, the major source of heat loss through the powderbed is the conduction mode and contributions of convec-tion and radiation are so small as to be neglected withoutany loss of accuracy. By the assumption of negligible heatlosses through the powder bed in the heat balance Eq. (3),the minimum estimate of absorptivity aminof laser energyinto the powder bed can be expressed asamin m Us=P?CPaveTm? T0 L?.(4)Although, Eq. (4) gives only a minimum estimate of a in aheat balance sense, it expresses dependency of absorptivityon the process parameters. This issue is discussed later.Consideration of a values and their reliability is of majorimportance in this paper.3. ExperimentationSingle layers have been produced, by a scanning CO2laser beam, in the surface of beds made from gas-atomizedpowders of composition and size fraction listed in Table 1.All powders were obtained from Osprey Metals Ltd, UKand, were spread and levelled in a flat tray of areaARTICLE IN PRESSM. Badrossamay, T.H.C. Childs / International Journal of Machine Tools & Manufacture 47 (2007) 779784780120mm?150mm to a depth of 5mm. Square areas 15mm?15mm were melted in a research SLS machine. The SLSequipment has been described before 12,13. A 250Wcontinuous wave CO2laser beam, focused to a spotdiameter of 0.6mm, was used to scan the surface of thepowder bed inside a controlled environment processchamber. All experiments were carried out in 99.9% pureargon atmosphere, enough to avoid oxidation. Duringprocessing the gas flow rate through the chamber was 3l/min and the net pressure was kept around 50mbar abovethe atmospheric pressure.For the main tests laser powers of 80, 110, 140 and200W were used, with scan speeds 0.5300mm/s. Theraster scans were parallel to a side of the square area, withscan spacing 0.060.9mm. After processing, layers werelifted from the bed, strongly brushed to remove loosepowder, and weighed. Layer topography was recordedusing optical microscopy equipped with a digital camera.Subsequent experiments were carried out to determinethe influence of bed temperature on the amount of layermelting. Beds were processed in a similar manner to thosein the main tests, but were allowed to cool down beforeeach new track was started. M2 and 316L powders wereexamined in these experiments. The power of 110W andscan spacing 0.6mm, with scan speeds 315mm/s werechosen in these experiments. Each layer was formed by 25tracks and cooling-off periods were set 1590s beforescanning of each new track.The finite-element model has been used to calculate masslayers of H13 in the same condition as the experiments.Scan spacings of 0.2 and 0.4mm were used to matchapproximately the experimental spacing of 0.15 and0.45mm. The mass of a 15mm ?15mm melted layer hasbeen estimated pro rata from the smaller area actually usedin the calculations.4. Results and discussionFig. 1 shows the measured masses of the 15mm?15mm squares of 316L and M2 as function of scanARTICLE IN PRESSTable 1Composition and size ranges of gas atomized powdersMaterialComposition (wt%) (balance Fe)CSiSMnNiCrMoWVSize range (mm)M20.880.270.0040.281.9?53H130.380.930.324.91.71.0?150/+75316L0.0290.230.0091.411.816.92.3?45314S-HC0.441.40.9120.324.7?53010203040012345Mass (gr)Mat: 316L, s= 0.6 mm P= 110 W P= 140 WABCDE010Mass (gr)03040Mass (gr)Mat: 316L, P=110W s= 0.06 mm (90% overlap) s= 0.18 mm (70% overlap) s= 0.30 mm (50% overlap) s= 0.48 mm (20% overlap)010203040500123Mass (gr) Mat: M2, P= 110 W s= 0.06 mm (90% overlap) s= 0.30 mm (50% overlap) s= 0.48 mm (20% overlap) s= 0.90 mm (-50% overlap)2.52.01.51.00.50.010Scan speed (mm/s)2050Scan speed (mm/s)Scan speed (mm/s)302040Scan speed (mm/s)0.54.54.03.53.02.52.01.51.0Mat: M2, s= 0.6 mm P= 110 W P= 140 W(a)(b)(c)(d)Fig. 1. Mass of a 15mm ?15mm single layer of 316L and M2 as a function of scan speed at two power levels and various scan spacings.M. Badrossamay, T.H.C. Childs / International Journal of Machine Tools & Manufacture 47 (2007) 779784781speed. It can be seen that both materials reveal more or lesssimilar trends in variation of layer mass with scan speed. Inall cases depending on the scan speed, four individualregions are distinguished. The first region is a low scanspeed range, marked AB in Fig. 1(a), in which mass reducesrapidly with increasing speed. There is then a range BC inwhich mass increases with speed, the third CD range inwhich mass varies little with speed. Finally DE range inwhich mass reduces again with increasing speed, but moregradually than in the AB range. However, in some cases(typicallyfor316LatP 110Wands 0:06mm,Fig. 1(c) the increasing-mass range BC may be absent. Itcan be observed that for both the materials the increasing-mass range occurs at higher scan speeds with reduced scanspacing. For example for M2 material, the BC range occursat around 3 to 5mm/s when s 0.9mm and 6 to 12mm/swhen s 0.48mm ( Fig. 1(d).The role of the scan spacing in determining the layermass variations was studied by varying the scan spacingfrom 10% to 150% of laser beam diameter as shown inFig. 2(a). This figure presents an alternative view, plottinglayer mass against scan spacing for the M2 powder, at thefour scan speeds of 4, 8, 14 and 30mm/s and P 110W.Here the scan spacing is expressed as a fraction of laserbeam diameter (i.e., s 0.3 equals to 0.18mm scanspacing). In all cases the mass firstly reduces sharply withincreasing scan spacing to obtain its lowest value at aroundscan spacing of 20% to 30%. In two low scanning speedcases (U 4 and 8mm/s) mass then increases graduallywith increasing scan spacing. It is likely that the larger thescan spacing, the more fresh powder is processed. How-ever, in the two further cases (U 14 and 30mm/s) themass initially increases and then decreases with increasingscan spacing. The latter phenomena, decreasing the layermass with increasing scan spacing, may be explained dueto insufficient energy in higher scan speeds to producedense layers.The influence of bed temperature on the amount ofmelted material is shown in Fig. 2(b). The measured layermasses of 316L and M2 powders as functions of scanspeed, for the scan spacing of 0.6mm at the laser power of110W and 30s cooling down period (for M2, 20s) betweeneach track scanning have been compared with masses ofordinary melted layers (i.e. the layer was produced with nodelay time between track scanning). In the case of 316Lpowder compared to the ordinary scanned layers, the masscontinuously decreases with increasing speed. However, inthe case of M2 powder, layer mass is almost independent ofscan speed and slightly reduces with speed. Except forthe 316L powder and speed range of less than 5mm/s, themelted layers in a cooled bed display a lower mass than theordinary scanned ones. For example, for 316L andU 10mm/s, the layer mass decreased from 1.82 to1.17g (?36% reduction) when the layer was produced ina cooled bed. There are two possible explanations. The pre-heating of a powder bed caused by laser processing up tothe time of the current scan, decreases the temperature rise,DT Tm? T0, for melting the powder bed surface. As aresult, the melted material for unit energy input increases.The sensitivity analyses based on the energy balanceequation have been performed in this case. It was seenthat in the same condition as experiments with keeping aconstant at 0.4, the predicted melted layer mass decreasedfrom 1.66 to 1.18g (?29% reduction) when DT waschanged from 800 to 12001C. On the other hand,absorptivity is a function of temperature itself and alsoshows a general trend to increase with increasing tempera-ture for the case of metallic materials 14. Therefore, pre-heating of powder bed caused by laser melting of previoustracks influences the absorptance of a powder bed.Fig. 3 compares experimental and simulated layer massesfor H13 powder. The simulated data in Fig. 3(a) are all fors 0.4mm. The results marked (a) and (b) are respectivelythe lower and approximate upper estimates as described inSection 3. In Fig. 3(b), both (b) and (c) results areapproximate upper estimates, but (b) is for s 0.4mm and(c) is for s 0.2mm. In both figures, values of a 0.25and 1.0 have been chosen, as these are the values that havebeen found to be appropriate for single-track modelling 3.Unlike experiments, the simulated results show that thelayer mass always decreases as the scan speed increases fora constant absorptivity. However, the simulations labelled(a) and (b) in Fig. 3(a) support the decreasing of meltedmaterial in a cooled bed, as was found in the experiments(Fig. 2(b). The results in Fig. 3(b) suggest in addition thatlarger layer masses should have been obtained withARTICLE IN PRESS0.4 0.6 0.00.51.01.52.02.5U= 4 mm/sU= 8 mm/sU= 14 mm/sU= 30 mm/s246810121416Mass (gr)Mass (gr)2.52.01.51.00.5Scan speed (mm/s)Scan spacing (fraction of beam diameter)0.0 0.21.01.4P=110W, s=0.6 mm316L, with cooling off (30 sec)316L, without cooling offM2, with cooling off (20 sec)M2, without cooling off(a)(b)Fig. 2. (a) Variation of layer mass with scan spacing for M2 at P 110W and (b) dependence of layer mass on scan speed and cooling down.M. Badrossamay, T.H.C. Childs / International Journal of Machine Tools & Manufacture 47 (2007) 779784782reduced scan spacing, for a constant absorptivity. On theother hand, the simulated results support the suggestion ofincreasing absorptivity with speed, as proposed previouslyfor single-track formation 3. However, the uncertainty inthe value of absorptivity is found to affect the predictedlayer mass significantly in the simulations.To further study layer mass variations Eq. (4) was usedto obtain the minimum estimates of absorptivity. Thevalues of aminwere derived from CPave 700J=kgK, LM2270kJ=kg and L314S 280kJ=kg. Fig. 4 presents the valuesobtained. The values of aminfor M2 and 314S-HC powdersat the two levels of powers, with scan spacing 0.3mm, arerevealed in Fig. 4(a). The results propose a rapid growth ofabsorptionwithincreasingscanspeed.However,atsufficient high speeds the aminonly slightly increases oreven became constant. In addition, except for a low scanspeed range (p5mm=s), the absorption values are rela-tively larger for the M2 powder. On the other hand, itwould be expected that the absorptivity depends on theincident laser power, because the absorptivity increaseswith increasing temperature that largely depends on theinput laser energy. However, the results indicate that aminisnot significantly affected by the laser power. Fig. 4(b)shows how estimated absorptivity values varied with scanspacing variations. The results presented here are for the316L powder processed at laser power of 110W, butsimilar trends were found for the other process parametersand powders. It can be seen that at a constant scan speed,the larger the scan spacing, the higher absorptance. Asmentioned earlier, this would be because a higher propor-tion of unprocessed powder is being scanned, which has ahigher absorptance than the previously processed material.As evidence, a study based on the temperature measure-ments in a stainless steel powder bed at a power that causedbed melting, showed that absoptivity reduces from 0.25 to0.1 as scan spacing reduced from 75% to 20% of beamdiameter 15.Despite the proposition of increasing absorptivity withscan speed, the reverse trend might be expected from aphysical point of view. For a given laser power and beamdiameter, the lasermaterial intraction time decreases asthe laser beam scanning speed increaces. This reduces thedeposition of laser energy on the surface and concequentlythe temperature of the bed surface decreases as thescanning speed increases. Therefore, the absorptivity ofthe powder bed is expected to be lower at high scanningspeeds for the same laser parameters 16. Therefore, itwould seem quite logical to consider alternative explana-tions for this. In this paper, an idea based on the processefficiency is presented. The presented model by Eq. (4)ignores the heat loss. This is an acceptable assumption aslong as heat losses remain approximately a constantfraction of the absorbed energy. However, some reportedworks on similar issues (such as welding or laser cutting)suggest that heat losses are influenced by the processparameters.ARTICLE IN PRESS01020304050 Experiments for P=77W, s= 0.45 mmSimulation, = 0.25Simulation, = 1.0(a)(b)(a)(b)0Mat: H13, P=143W experiments s= 0.15 mm experiments s= 0.45 mmMass (gr)1.51.00.50.0Scan speed (mm/s)Simulation, = 1.0(c)(b)(c)Simulation, = 0.25Scan speed (mm/s)150200100500.0Mass (gr)2.01.51.00.5(b)Mat: H13,(a)(b)Fig. 3. Comparison between experimental and simulated layer mass for H13, (a) P 77W and (b) P 143W; (a)(c) cases are explained in text.0100(a)314S-HC, P=80 W314S-HC, P= 110WM2, P= 110 WM2, P= 140 Ws= 0.3 mm (50%)M201001250.00.20.40.60.8 s= 0.06 mm s= 0.18 mm s= 0.30 mm s= 0.48 mm s= 0.60 mmScan speed (mm/s)Absorptivity min.0.60.30.0Scan speed (mm/s)255075255075150314S-HCAbsorptivity min.(b)Fig. 4. Estimates of aminvs. scan speed, (a) for M2 and 314S at powers 80, 110 and 140W and s 0.3mm, (b) for 316L at 110W and various scanspacings.M. Badrossamay, T.H.C. Childs / International Journal of Machine Tools & Manufacture 47 (2007) 779784783The Rosenthal model for moving line sources is a well-established solution for simulation of either single passwelding in thin materials or a fully penetrating keyholeweld, as well as laser beam cutting process. The modelassumes that the energy is absorbed uniformly along a linein the depth direction. As the penetration of a laser beam inthe powder bed is much longer than in bulk materials, thismodel could be applied for SLM as well. Swifthook andGick 17 presented an analytic solution to the modeldepending on the scanning speed. From their solution itcan be derived that at high speeds (typically 450mm/s)around 52% of the delivered energy is lost throughconduction 18, while, for low speeds (typically o5mm/s)the loss could be around 95%. On the other hand, Schulzetal.19gaveacorrelatedanalyticalsolutionofconduction loss taking into account the cylindrical heatsourceinlasercuttingprocessasPcon 2rCPDTurt Pe=2?0:7where Pconis the conduction loss, r thematerial density, Cpthe heat capacity, DT the relativeelevated temperature, u the cutting speed, r the radius ofbeam, t the material thickness, and Peis the Peclet number,which equals ru/k (k is the thermal diffusivity). It can beseen that with increasing Peclet number, cutting speed andheat loss decrease.Finally, as discussed so far, it seems the neglecting heatloss assumption cannot be applied in all ranges of processparameters. In fact, in low speed processing heat lossesplay an important role in the process and can not beneglected. The layer mass variations might be addressed bythis route. However, more work is needed to confirm thisissue in a quantitative way.5. ConclusionExperimental studies on melting single layers of M2 highspeed steel and 316L and 314S-HC stainless steels in thesurface of powder beds, by a raster-scanning laser beamhave confirmed that variations of layer mass with scanspeed are not consistent with what is expected from thedelivered energy to the powder bed. Two explanations aresuggested. The first one, which was supported by thesimulation as well, suggests that absorptivity may increasewith increasing scan speed. In the second one the heat lossdecreasing with scan speed is proposed. In addition, effectsof scan spacing on the layer mass and absorptivity wasstudied. Experimental and simulation results also showedthat the thermal history of processing is influential indetermining the amount of melt under a laser beamirradiation.References1 G.N. Levy, R. Schindel, J.P. Kruth, Rapid manufacturing and rapidtooling with layer manufacturing (LM) technologies, state of the artand future perspectives, Annals of the CIRP 52/2 (2003). 525540.2 D.T. Pham, R.S. Gault, A comparison of rapid prototypingtechnologies, International Journal of Machine Tools and Manufac-ture 38 (1997) 12571287.3 T.H.C. Childs, C. Hauser, M. Badrossamay, Selective laser sintering(melting) of stainless and tool steel powders: experiments andmodelling, Proceedings of the Institution. of Mechanical Engineers,Part B: Journal of Engineering Manufacture 219B (2005) 339358.4 T.H.C. Childs, C. Hauser, Raster scan selective laser melting of thesurface layer of a tool steel powder bed, Proceedings of the Institutionof Mechanical Engineers, Part B: Journal of Engineering Manufac-ture 219B (2005) 379384.5 F. Abe, K. Osakada, M. Shiomi, K. Uematsu, M.