Optimization of Laser Deposited Rapid Tooling

D. Schwam and Y. WangCase Western Reserve UniversityNADCA DMC, Chicago Feb.17, 2010*

*Outline Objectives-why laser deposit? The POM process Preliminary thermal fatigue results In-plant evaluation of cores at General Die Casters FEA optimization of laser deposited layer Conclusions

*Objectives why laser deposit?Use of high thermal conductivity alloys in cores and inserts can accelerate heat extraction, lower surface temperature reduce soldering and shorten cycle time.

Copper alloys have good thermal conductivity and are cost effective candidates for this application.

However, copper alloys dissolve in molten aluminum. High temperature and high velocity molten metal flow exacerbate dissolution.

A layer of laser deposited H13 over the copper can prevent dissolution.

Washout Damage at the Corners of Cu-Ni-Sn Thermal Fatigue Specimens Copper alloys dissolve in molten aluminum

Metal Mold Material Properties

Rapid Tooling by DMD*-Courtesy POMDirect Metal Deposition of H13 on Copper - the POM Method

H13 / H13 sample as deposited/ finished H13 / Alloy 940 sample as deposited/ finished

Chart1

00

1.60.03

14.130.11

27.6513.86

160.3

2"X2"X7", WC7

H13 / OIL / 51HRC1.5" dia. Cooling Line

Deposited H13 on Cu0.5" dia. Cooling Line

Deposited H13 on Cu

H13 / OIL / 51HRC

Thermal Cycles

Total Crack Area (x 106mm2)

Total Crack Area - Laser Deposited H13 on Cu Core

Sheet2

S (x106mm2)0250050001000015000

H13 / OIL / 51HRC00.030.1113.86160.3

S (x106mm2)050001000015000

Deposited H13 on Cu01.614.1327.65

&A

Page &P

Evaluation of Laser Deposited Cores The core is surrounded by molten aluminum and has a record of overheatingand soldering. Extracting heat more efficiently from the core can lower maximum temperature, prevent soldering and allow shorter cycle times.*

Typical H13 core with cooling line*

Composite Core -- H13 Deposited on CuH13Cu*

*Thermal Profile of Cover Half Steel During SolidificationComposite CoreH-13 CoreAt start of a cycle

Thermal Profile of Section Through Cover Half at Die OpeningComposite CoreH-13 CoreAt 20 seconds Temperature of Composite Core is lower. *

Temperature Advantage of the Copper Core Temperature of Composite Core is lower. *

Creep Failure of in First Composite CoreThe core shows creep damage after 250 cycles due to insufficient stiffness and strength at high temperature.*

Surface Cracks in the H13 Layer*

Remedial ApproachesCaves inBulges outThe distortion of the core seems to originate from insufficient strength and stiffness at the operating temperature. Anviloy and H13 cores do not suffer from this problem. Priority 1 - Increase strength: use core as deposited w/o tempering (downside-lower toughness).Priority 2 - Increase thickness of H13 layer(downside slows down heat transfer).

*Optimization of the Laser Deposited H13 The first core was tempered aggressively. The hardness of the laser deposited H13 was reduced from 51HRC to 40 HRC to improve toughness. However, this reduced the strength and caused excessive distortion. The core had to be removed after 250 shots. A new core was made with the laser deposited H13 in the as-deposited condition (no tempering) at 51HRC.

This core has been in production at General Die Casters. So far it has accumulated 5,000 shots.

FEA optimization of the H13 laser deposited layer thicknessThe 3D Die Casting model was simplified to a 2D axi-symmetric FEA model and analyzed with commercial FEA software Abaqus.

The accuracy of 2D Abaqus analysis was verified by comparing it to a 3D MagmaSoft simulation.

Cyclic heat transfer and stress analysis was applied to study the failure mechanism of the composite core.

Composite cores with different H13 layer thickness and hardness were compared, to optimize the design. *

Finite Element Analysis ModelMold (H13)Casting (A389)Composite Core:Copper (394)Steel (H13)2D Axi-symmetric model*

Boundary ConditionsPreheat (40 sec)Mold close, Metal Injection and Solidification (20 sec)Mold open, Ejection, air cooling (4 sec)Spray cooling (3 sec)Air cooling (13 sec)New cycle begins, each cycle takes 40 seconds.Process flow chart:Material stress-strain property definition:*

Simulation initial conditionsCasting AlloyA389Mold MaterialH13Furnace Temperature1350oFMaterial plastic behaviorstrain rate independent isotropic hardening

Parameters studiedParameter 2 Thickness of the H13 layer:Parameter 1 H13 hardness / strength:( Ref: Philip D. Harvey, Engineering Properties of Steel, American Society for Metals, Metal Parks, Ohio, 1982, P457-462.)*

Label of different designsThickness of H13 in composite coreCore0500.05 inCore1000.1 inCore1500.15 in

Label of different hardnessYield strength at room temperature (ksi)UTS at room temperature (ksi)HR40153183HR50239289

Verification of Axi-symmetric FEA Heat Transfer Analysis* Maximum temperature difference is smaller than 20oC.** Abaqus compared to MagmaSoft

Thermo-mechanical Analysis for Core100 with H13-HR40ABCTemperature andstress fields become stable after five cycles. *

Thermo-mechanical Analysis for Core100BI. Thermal gradientII. Thermal expansion mismatchIII. Thermal gradientIV. Thermal expansion mismatchDominant factors in deformation: The thermal gradient at the surface is a key factor in deformation and failure.

Normal stress along the hoopdirection is the largest. *

Thermo-mechanical analysis findingsThe temperature and stress fields become stable in about five cycles.

The thermal gradient at the surface is the key factor to deformation and failure.

Normal stresses in the hoop direction are largest at the surface , and may cause surface cracks.

The failure mechanism depends on the maximum stress and whether it exceeds the material strength at the operating temperature. *

Type I Failure Mechanism Creep Core100 H13-HR40/ Copper Core after 250 cycles When strength is low (ex. H13-HR40). yield

Type II Failure - Low cycle fatigue When strength is high (ex. H13-HR50).

yield

Factors that determine low cycle fatigue lifeT1T2Fatigue life NStress variation SSurface thermal gradientTemperature variation TCore thermal conductivity Higher conductivity for thinner H13 layerNot linear relationship to H13 thicknessH13 layer thickness

Detailed analysis*

The Effect of layer thickness on the low cycle fatigue life of the composite coreThe design of composite core with 0.05 thick H13-HR50 layer offers:Lower max temperature;Longer fatigue life.*

SummaryA pre-requisite to prevent premature failure is sufficient strength to avoid creep .

A 0.050 thick H13-HR50 layer provides a good balance of heat transfer and thermal fatigue resistance: Maximizes heat flux, decreasing mold temperature, and shortening cycle time.Reduces the thermal gradient, lowering maximum stress and extending core life. *

*****What is the exact location of the nodes*Show coordinates. Change loops to hoop*Please explain *How do we rationalize a higher temp for 0.150 layer that only H13 core*